Peach diseases and pests: identification from symptoms, with a diagnostic tool

Savvas Pastopoulos, Agronomist MSc — Pastopoulos Agronomics, Neos Mylotopos, Pella, Greece

In peach, damage rarely comes from a single cause. Brown rot, scab, peach twig borer and oriental fruit moth work on the same fruit, one attack often opens the door for the next, and a single sign — a drop of gum, a sunken lesion, a wilted shoot — is produced by four different things. What follows covers the pests and diseases of the crop with the emphasis on telling them apart: not what exists, but how one is distinguished from another in the field, with a diagnostic tool that compares observed symptoms against the profiles of thirty-three causes.

1. What makes peach a special case

Peach is a deciduous tree with very large annual growth, thin bark, and fruit that ripens through the summer. Each of these three traits changes the way an attack is read.

First, the inoculum overwinters on the tree, not in the soil. Peach leaf curl overwinters as conidia on the surface of the twigs and on bud scales [1]. Scab overwinters in lesions on the shoots, and those lesions are the only source of the conidia that will infect the fruit in spring [2]. Brown rot overwinters in mummified fruit and in twig cankers [1]. Peach twig borer overwinters as a first- or second-instar larva inside a tiny cell in crotches and pruning wounds [6]. In practice this means that winter clean-up of the canopy is not an accessory to plant protection but a part of it: whatever remains on the tree in February is the inoculum of May.

Second, the time between infection and symptom is enormous in certain diseases. With scab, symptoms on the fruit become visible 40 to 70 days after the conidia are deposited [2] — roughly six to seven weeks after petal fall [15]. By the time the black velvety spots appear on the fruit, the spray that would have prevented them was due six weeks earlier. Identification in peach is never only "what can be seen today" but "what the weather did in April".

Third, the same visible sign is produced by different causes, and the distinction is decided at the scale of millimetres. The larva of the peach twig borer enters the fruit at the stem end or along the suture and feeds just under the skin. The larva of the oriental fruit moth also generally enters at the stem end, but bores to the centre and feeds around the pit [5] [6]. One cut with a knife separates the two insects, and the separation matters because their flights do not coincide.

Fourth — and this is the most underrated point — many of the most serious problems of peach are not primary pathogens. Bacterial canker is a weak pathogen that requires predisposing factors, the most important being ring nematode [1]. Fungal gummosis attacks trees predisposed by stress [1]. Capnodis prefers weakened trees [10]. Wood-decay fungi are an indicator, not a killer. In all these cases, "what do I spray" is the wrong question; the right one is "what is stressing this tree".

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2. The identification tool

The tool compares the selected symptoms against the profiles of thirty-three pathogens, pests and physiological disorders of peach. It does not work by simple tallying of matches: each symptom carries a different weight according to how distinctive it is. White mycelial fans under the bark count for far more than general chlorosis, because the latter fits twelve causes.

Certain symptoms also count negatively. Damage on scattered individual trees, unrelated to the low spots of the block, speaks for Verticillium wilt and Armillaria and against Phytophthora, which works in patches where water stands. Without negative weights the tool does not diagnose — it merely agrees with whatever is selected.

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3. Fungi of the blossoms, foliage and fruit

Brown rot

The most serious fungal disease of stone fruit worldwide. In Europe Monilinia fructigena predominates, while M. laxa is also widespread; M. fructicola is mainly Asian and American in distribution but has spread [1]. The fungus overwinters in mummified fruit — on the tree or on the orchard floor — and in twig cankers [1].

Three figures govern management. Blossom infection takes place within 6 to 12 hours at 15 to 20 °C; after 3 to 5 days the blossom is blighted [2]. The temperature range permitting infection is wide, with an optimum of 22.5 to 25 °C, and the minimum wetness duration is 3 to 5 hours at 20 °C, whereas at 10 °C 18 hours are required [2]. On ripe fruit under optimum conditions, decay becomes visible within 48 hours of infection [2].

The diagnostic feature on the blossoms is that they remain firmly attached to the spur instead of falling, with a gum drop where the peduncle joins the stem [2]. This separates brown rot from frost, which drops the killed blossoms. On the fruit the decay is firm and expands rapidly, with greyish sporodochia arranged in concentric rings.

Brown rot is covered in full in the separate article on brown rot of stone fruit, with the critical spray windows and resistance management.

Peach leaf curl

Taphrina deformans produces the most recognisable picture in the crop: leaves thickened, wrinkled and distorted, reddish or yellowish in colour and finally carrying a subtle white layer of asci on the surface [2]. Infections occur at 10 to 21 °C and are favoured by cool wet weather during bud development; when temperatures at early leaf development are high, infection rarely becomes established [2].

Shot hole

Wilsonomyces carpophilus produces small circular leaf spots whose centres become necrotic, drop out and leave a hole — hence the name. At 20 to 25 °C leaf infection requires 8 to 12 hours of wetness, while twig infection requires at least 24 hours of continuous wetness [2]. This difference explains why in years with short rain events only foliar symptoms appear, whereas in years with prolonged rain the problem moves into the wood and becomes perennial.

In semi-arid climates the basic treatment is a dormant one, after leaf fall and before the rains, to protect the twigs and buds [2].

Scab — the disease that shows up when nothing can be done

Venturia carpophila is important in areas with abundant rainfall but also in orchards under overhead irrigation, which keeps the foliage wet [2]. The fungus overwinters in superficial oval twig lesions measuring 3 × 5 to 5 × 8 mm at the end of the season; at 70 to 100% relative humidity it produces abundant conidia within 20 to 30 hours, mainly from petal fall until mid-spring [2].

Lesions on the fruit appear on half- to full-sized fruit approximately 6 to 8 weeks after petal fall, measure 5 to 10 mm in diameter and are first green, then olive to black, often with a green to yellow halo [2]. The earliest, smaller spots reach 2 to 3 mm and appear as a rule near the stem end, with a raised corky appearance [19].

The same disease on apricot, with its own peculiarities, is covered in a separate article, available in Greek only.

Furthermore, when infection is severe the skin cracks, and the crack becomes an entry point for brown rot [19]. That sequence — scab in May, brown rot in July — is frequently attributed in its entirety to brown rot, and the following season is met with the wrong programme.

Powdery mildew

Podosphaera pannosa causes damage mainly on green fruit [1]. Conidia germinate between 2 and 37 °C with an optimum of 21 °C and — unlike every other fungus in this list — prolonged wetness kills powdery mildew conidia [2]. In practice, powdery mildew is the disease of the dry warm spring, while all the others are diseases of the wet one.

One identification trap: the "rusty spot" lesion on peach may be caused by Podosphaera leucotricha, that is by the powdery mildew of apple, where neighbouring apple trees are present [1]. In mixed plantings, the neighbourhood is part of the diagnosis.

Rust

Tranzschelia discolor produces angular yellow lesions on the upper leaf surface and corresponding rusty-brown pustules on the lower one [2]. Urediniospores germinate over a range of 8 to 38 °C, with an optimum of 13 to 26 °C, and require wetness or a saturated atmosphere. At 20 °C, 18 hours of wetness are needed for adequate leaf infection [2]. Spore germination begins within 4 hours, but appressoria form over the stomata 18 hours after inoculation — which is why short rain events are not enough [2].

An important detail for timing: the fungus overwinters as mycelium in stem lesions, and the emergence of sporulating stem lesions in spring has been used as the starting date for the spray programme [2]. Watching the wood, not the leaves, gives the timing.

The cost of rust is not this year's crop but next year's. Premature defoliation forces the tree to live on reserves, depletes the resources available for bud differentiation, and defoliated trees often flower in autumn, wasting flower buds. In addition, bare scaffolds are exposed to direct solar radiation, become sunscalded and are then colonised by wood-rotting fungi [13]. That chain is the reason why rust, without causing visible damage to the current crop, empties orchards over three years.

Anthracnose

Caused by Colletotrichum species — mainly of the C. gloeosporioides and C. acutatum complexes — and appearing in areas and seasons with high rainfall and warm temperatures during ripening [2]. Lesions are circular, brown or salmon-pink, firm and slightly sunken.

The genuinely diagnostic element is the cut: the lesion is cone-shaped in cross-section, hardened, and separates easily from the healthy mesocarp [2]. On the surface the acervuli are arranged in concentric rings. Brown rot, by contrast, neither separates nor forms a cone.

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4. Bacterial diseases

Bacterial spot

Xanthomonas arboricola pv. pruni overwinters in cankers, buds, bark cracks and leaf scars [1]. It is favoured by rain and humidity, but the factor that makes it destructive is wind: heavy rainfall with wind drives the bacterium into leaves, fruit and stems, and the disease is distinctly more severe on the windward side of the tree [2]. The uneven distribution within a single tree is itself diagnostic.

It is favoured by temperatures of 19 to 28 °C and high humidity, while hot dry conditions inhibit spread [2]. On leaves the lesions are angular, delimited by the veins, water-soaked at first and later dark; on fruit they form small sunken lesions that crack and exude gum.

Two practical criteria for separation, from the classic illustrated identification guide for the crop [16]. On the leaves: bacterial spot lesions are angular and do not cross the larger veins, unlike chemical injury, which does. On the fruit: bacterial spot lesions are deep cracks, whereas scab lesions are strictly superficial. These two observations resolve, in the field, two of the commonest confusions in the crop.

A critical point: resistance of peach cultivars to Xap is polygenic, leaf and fruit responses are controlled by different genes, and the alleles conferring susceptibility are the more common ones [2]. In other words there is no "resistant cultivar" in the absolute sense, and a cultivar with clean leaves may have susceptible fruit.

Bacterial canker

Caused by Pseudomonas syringae pv. syringae and pv. morsprunorum. The primary entry point is assumed to be the leaf abscission scars in autumn, and secondarily pruning cuts [2]. Mild temperatures, 12 to 18 °C, with wet conditions favour infection, and the disease is worse in mild wet winters than in consistently cold ones [2].

The most useful field feature is the sour odour of the bark in the canker area in spring [1]. No other cause of gummosis produces it.

Two things change the management. First, the bacterium is a weak pathogen and requires predisposing factors; the most important is the ring nematode, Mesocriconema xenoplax [1] [2]. Second, the pathogen's tolerance to copper makes the classic copper approach questionable, and the focus shifts to limiting stress: avoiding low soil pH, avoiding nutrient deficiency, selecting the pruning date, tolerant rootstocks and management of ring nematodes [2].

Bacterial canker of stone fruit is covered in detail, with its annual cycle and entry points, in the corresponding article.

Peach tree short life

Not one disease but a complex. The canopy of a tree collapses suddenly before, during or just after bloom, usually 3 to 6 years after planting; sap oozes from scaffolds and trunk, the tissue beneath the bark is discoloured and has a sour odour, and — most characteristically — the tree is killed only to the soil line, with shoots later emerging from the rootstock [2].

The immediate causes of death are late-winter cold injury, bacterial canker, or a combination of the two. The predisposing factors are ring nematode, improper rootstock selection, the time of pruning, root injury, soil pH and structure, and replanting on land with a history of stone fruit [2]. The disease is most common in sandy soils; in clay soils, replanting gives fewer problems [2].

Crown gall

Agrobacterium tumefaciens produces galls on the crown and roots. Gall development is faster at temperatures above 20 °C, with visible galls in 2 to 4 weeks [2]. Survival of the bacterium is negatively affected by soil temperatures above 34 °C [2]. The disease is favoured by nematode injury [1] and by wounds made at planting — which is why it appears almost exclusively in young blocks and in nursery material.

Sources: 1 2 16

5. Diseases of the wood and scaffolds

Fungal gummosis

Fungal gummosis is caused mainly by Botryosphaeria dothidea and related species, and appears on trees predisposed by stress [1]. The visible symptom is multiple points of gum exudation on woody shoots and scaffolds.

Fungal gummosis of peach is analysed in detail in the separate article, with the three stages of the attack and the cost in yield. That article is available in Greek only.

Constriction canker — Diaporthe amygdali

The pathogen is known by three names, and that alone creates confusion in the literature: Diaporthe amygdali is the current name, with an anamorph formerly reported as Phomopsis amygdali and, earlier still, as Fusicoccum amygdali. It causes constriction canker and shoot blight on peach, nectarine, almond, apricot, walnut, pear and grapevine [20].

Its distinctive feature is the entry point. Conidia infect through leaf scars in autumn, and in spring through buds, bud scale scars, blossoms and fruit scars [20]. These infections can affect up to 50% of the shoots [1]. The canker forms exactly at the node, constricts the shoot and causes it to break — hence the name.

The figures recorded on peach, as summarised in the recent review of the pathogen's biology [20]:

• α-conidia germinate on moist surfaces over a range of 5 to 36 °C, with an optimum of 27 to 29 °C.
• Infection, however, is most common at 5 to 15 °C — that is, at temperatures far below the optimum for germination.
• The incubation period, from infection to the first visible canker, is approximately one month.
• Optimum mycelial growth lies around 24 to 25 °C, with an upper threshold of 40 °C and a lower one between 5 and 7 °C.
• Cirrus production from the pycnidia occurs over a range of 1 to 38 °C, with the greatest proportion of pycnidia producing cirri at 17 °C, while the maximum production of conidia per pycnidium is slightly warmer, at 24 °C.
• Canker size and the number of pycnidia per canker follow a sinusoidal pattern through the year: lowest values in late winter and early spring, maximum values in late summer and early autumn.

On almond, under controlled conditions, the highest infection was observed after 72 hours of wetness and the lowest after 6 hours, with the ranges for conidial germination and infection extending from 5 to 35 °C; the optimum temperature for α-conidia production within the pycnidia was approximately 22 °C [20]. These values are for almond and are given here as an order of magnitude — the same work notes explicitly that knowledge of this pathogen has come mainly from peach.

Management rests on preventative treatments before flowering and before leaf fall, based on the assessed risk of the specific orchard and local weather, and on removal of affected shoots at summer and winter pruning to reduce inoculum sources [20]. Registered active substances are few, which is why pruning is not a supplementary measure but a basic one.

Constriction canker will be covered in a separate, detailed article. It is included here only to the extent needed to separate it from the other causes of shoot dieback and gummosis.

The point of immediate interest to the grower is that autumn, not spring, is the period of risk — as it is for bacterial canker. The two diseases share the same entry point and the same window, and an orchard with no autumn treatment leaves both of them free.

Silver leaf

The fungus grows over a wide temperature range with an optimum of 25 °C, and wood-exposing wounds are most susceptible in the first week after injury [2]. The impossibility of protecting every wound and of eradicating established infections from the trunk make prevention the only real option: clean nursery stock, minimising large wood-exposing cuts, and pruning technique that promotes rapid wound healing [2].

Wood-decay fungi

The woody perennial fruiting bodies that appear on old wounds are not the cause of the dieback but its indicator: these are fungi that are not strongly pathogenic, but cause serious decay in trees already suffering from other causes.

What their appearance means and what should be checked next is analysed in the article on wood-decay fungi on peach.

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6. Diseases of the root and crown

Armillaria — the disease that is decided before planting

Two species matter. Armillaria mellea produces rhizomorphs, black cords like string that actively explore the soil and reach new roots. Desarmillaria tabescens produces no rhizomorphs and spreads only by root-to-root contact — but it is a primary pathogen and does not need a stressed tree in order to kill [1]. In Europe A. mellea and A. ostoyae predominate [2].

The three diagnostic signs are absolute and resemble nothing else:

• White to yellowish mycelial fans between the bark and the wood, at the crown and on the large roots [2] [9].
• Dark brown to black rhizomorphs on the root surfaces [9].
• Honey-coloured mushrooms at the base of the trunk after heavy rain [1] [2].

Two practical observations separate root rot from peach tree short life: the affected tree dies slowly, with sparse new growth, very short shoots and light green, yellow or reddish leaves, while most of the above-ground wood remains alive and it is the roots and the below-ground trunk that are dead; and to find the dead roots in spring it may be necessary to dig 15 to 45 cm below the surface [16]. The disease is also more severe in low wet areas and on newly cleared land [16].

The fungus survives in the soil for many years as mycelium within infected tissue and as rhizomorphs [2] [9], and is most successful at invading healthy tissue once it has first become established saprophytically — which is why very young trees in soil full of roots from a previous crop are the most exposed [2].

Two measures with documented results. First, a trench about 1.2 m deep, lined with plastic, around the infection centre, which prevents spread to healthy roots [9]. Second, root collar excavation: removing the soil around the crown and upper root zone to a depth of 23 to 30 cm in spring, keeping the area dry through the growing season and periodically clearing debris from it [9]. The mechanism is continuous mild heating and drying, which prevents the fungus from colonising the exposed crown [8].

A development of the same idea is planting the trees approximately 40 cm higher than normal, in open-bottom fabric pots, and excavating the collar eight months later. In five-year trials in two orchards in South Carolina, control trees showed 30 to 70% decline, while trees under this treatment showed only 0 to 10% [8].

Phytophthora crown and root rot

The species involved are Phytophthora cactorum, P. cambivora, P. megasperma and P. cryptogea [1]. Zoospores are attracted by root exudates, encyst on the root surface and infect the fine feeder roots. Crowns subjected to prolonged wetness from flood irrigation or heavy rain develop crown rot, and after each rain or irrigation a new generation of sporangia restarts the cycle [2].

The feature that separates Phytophthora from every other cause of dieback is the distribution in the field: it appears in patches, where water stands, in low spots, or along the drip line. Armillaria and Verticillium wilt strike scattered individual trees. This single observation, which costs nothing and is made from the edge of the block, separates three diseases without a laboratory.

The full analysis, with the stages of the attack and the point at which it is too late, is in the article on Phytophthora and crown rots in fruit trees, available in Greek only.

Verticillium wilt

Verticillium dahliae produces microsclerotia that survive in the soil for many years without a host, when soil temperature is 5 to 15 °C and moisture-holding capacity is 50 to 75% [2]. Their highest concentration is at a depth of 15 to 30 cm, but they are found as deep as 105 cm [2].

The first symptom is sudden wilting of the leaves on one or more scaffolds during hot weather in summer; the leaves turn yellow, then brown, and curl [2]. On young trees the whole tree may defoliate, while on older ones only individual scaffolds are affected. A longitudinal cut shows brown to black streaks in the sapwood; a cross-section shows a discoloured ring in the outer xylem [2].

The history of the block is again critical: the disease often appears where peach has been planted after years of cotton, tomato or pepper [2]. No resistant rootstocks are known [2].

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7. Viruses and the fruit deformation disease

Twenty identified viruses and one viroid have been reported on peach, apricot and plum [4]. The first thing to be said is negative: diagnosis of stone fruit virus diseases cannot be based on symptomatology, because symptoms are affected by species and cultivar, by the strain of the pathogen, by host age, by environmental conditions and by the presence of mixed infections with a second and third virus. Diagnosis rests on laboratory methods [4].

Sharka — plum pox

Plum pox virus is the most destructive virus disease of stone fruit. In Greece it is very widely distributed, mainly in the Peloponnese on apricot and in Macedonia on peach, and has severely damaged both the crop and the juicing and canning industry [4].

On peach, the leaves show chlorosis of the veins or diffuse chlorotic bands around them; a characteristic symptom is the shortening of the veins in their chlorotic portion, which distorts the blade [4]. On apricot, yellow rings with a dark centre on the stone are a pathognomonic symptom, since they appear even when leaves and fruit look healthy [4].

The main route of long-distance spread is infected propagating material; aphid vectors are responsible for gradual spread within the nursery or orchard and between neighbouring orchards [4]. An important reservoir is blackthorn (Prunus spinosa), a common shrub in Greece which shows no symptoms and acts as a latent host [4].

Tolerant cultivars have been evaluated in practice against the strongly pathogenic Greek strains. Among them, for the fresh market, Spring Time, Dixired, Cardinal, Red Heaven, Sun Crest, Blake, Red Cap, Honey Dew Hale and Shipper's Late Red, and for canning the clingstone cultivars Cortez, Loadel, Andros and Fortuna [4].

Plum pox is a regulated harmful organism. Diagnosis, notification and management of outbreaks are the responsibility of the competent plant health service.

Necrotic ringspot and prune dwarf

Prunus necrotic ringspot virus and Prune dwarf virus infect peach, plum, sweet and sour cherry and almond [4]. On peach, PDV causes two diseases: "peach stunt" in single infection and "peach rosette and decline" in mixed infection with PNRSV [4]. One practical element changes the management: diseased trees are more sensitive to low temperatures [4] — a virus can therefore present as frost damage.

In the orchard the disease spreads by pollen, and peach latent mosaic viroid (PLMVd) spreads by pruning shears [4]. The latter is one of the few points at which disinfecting tools between trees has demonstrated value.

The fruit deformation disease

A separate case, because it resembles a virus disease without having been identified as one in samples from the area. The fruit carries deep folds and furrows, dark sunken areas and sections that never colour, while the surrounding foliage remains apparently healthy.

The history of the case, the virological analyses that were carried out, what they excluded and the current picture from the international literature are analysed in the article on peach fruit deformation disease. The cause is included in the tool because in practice it is confused with plum pox and with mechanical damage, and the distinction has consequences: one is a regulated organism, the other is not.

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8. The Lepidoptera: twig borer, oriental fruit moth, tortrix

These three insects are the centre of gravity of peach crop protection in Central Macedonia, and for once local data exist instead of imported models. A study in eight peach orchards in the prefecture of Pella recorded, with pheromone traps from April to October in 2018 and 2020, the seasonal occurrence of Anarsia lineatella, Grapholita molesta and Adoxophyes orana, and used 2021 data to validate the predictions [3].

The degree-days of Pella

The lower developmental thresholds used are 11.4 °C for the peach twig borer, 9.5 °C for the oriental fruit moth and 7.2 °C for the summer fruit tortrix, and in every case the starting point of accumulation — the biofix — is 1 January [3].

All values from the published Pella study [3], with degree-days accumulated from 1 January and with the lower thresholds in the second column. The values are specific to each species and are not interchangeable.

For the start of the second flight, the values are 365 degree-days for the peach twig borer, 133 for the oriental fruit moth and 1,028 for the summer fruit tortrix [3].

Peach twig borer

Anarsia lineatella overwinters as a first- or second-instar larva inside a tiny cell, the hibernaculum, in crotches, pruning wounds and bark cracks [6]. Larvae emerge in early spring, during bloom, and move up to the new growth.

Damage is of two kinds. On the growth, larvae mine inside the elongating shoots and cause terminal dieback — the so-called shoot strikes. On the fruit, they generally enter at the stem end or along the suture and feed just under the skin [6].

The treatment threshold based on growth is three shoot strikes per tree [6]. As to timing, American guidelines place treatment at 300 to 400 degree-days from first capture, with thresholds of 10 and 31 °C — 400 for green fruit, 300 for colouring fruit [6]. These values are not the same as the Pella degree-days, because both the starting point and the lower threshold differ.

Oriental fruit moth

Grapholita molesta overwinters as a mature diapausing larva inside a tightly woven cocoon, in protected places on the tree or in debris near its base [5]. In California it completes five generations per year, and a sixth has been observed in years with warm early spring and late autumn [5].

The early generations produce shoot strikes, as the twig borer does. Later ones turn to the shoot terminals and the green fruit: the mature larva generally enters at the stem end and bores to the centre, feeding around the pit [5].

The treatment threshold is likewise an average of three shoot strikes per tree in heavy infestations [5]. Fruit sampling is weekly, with emphasis on the final four weeks before harvest [5].

Two points with practical value. First, mating disruption is the preferred strategy where it is applied over sufficient area [5]. Second, the parasitoid Macrocentrus ancylivorus can reach 80 to 90% parasitism by August and September in orchards that avoid broad-spectrum insecticides [5]. A figure of that size is not decorative: it is control of the second and third generation at no cost, and it is lost with a single unsuitable spray.

Summer fruit tortrix

Adoxophyes orana webs leaves and shoot terminals with silken threads and feeds inside that protected environment. Damage to the fruit is superficial: scarring of the skin, characteristically where a leaf touches the fruit.

In the Pella data the tortrix differs from the other two on two counts: it has the lowest developmental threshold (7.2 °C) but the largest degree-day requirement — 362 against 70 and 33 for first capture, and 1,239.5 against 511.5 and 204.8 for the peak of the second flight [3]. In practice this means the tortrix appears later in the season despite its lower threshold, and that its second generation falls at a stage when the fruit is already susceptible.

Sources: 3 5 6

9. Sap-sucking pests

Spider mites

On peach the species of interest are mainly Tetranychus urticae and Panonychus ulmi. They overwinter as adult females in protected places on the tree or in litter and weeds, with a characteristic reddish-orange colour in diapause [7].

The figures explain why mites escape so fast: in summer heat a generation is completed in ten days, and depending on temperature 8 to 18 generations per year are completed [7].

Damage is stippling and bronzing of the leaves, and in heavy infestations severe leaf drop. The cost is not only the foliage: premature defoliation prevents the fruit from sizing and exposes fruit and scaffolds to sunburn [7].

Green peach aphid

Myzus persicae has peach as its primary host: it overwinters as wingless females or as eggs beneath the bud scales. After hatching, several generations of wingless adults follow, and in June the winged forms appear; all aphids leave the peach trees during June and July [17].

The treatment thresholds differ greatly between the two fruit types: one colony per tree on nectarines, five or more colonies per tree on peaches, with larger trees tolerating more. Inspection is weekly from petal fall until the terminals harden off [17].

Beyond the direct damage — curling of the terminals, honeydew, sooty mould — the green peach aphid is a vector of plum pox virus [17]. In a region where the epidemic PPV-M strain predominates, which is readily aphid-transmitted [4], the aphid threshold concerns more than this year's damage.

Comstock mealybug

Pseudococcus comstocki is an invasive species of Asian origin, established in northern Italy and southern France since the early 2000s [12]. A survey in north-eastern Italy recorded twenty-one host species, among them peach, apple, pear, mulberry and a range of ornamentals [11].

The biology that governs management: three generations per year and overwintering in the egg stage [11]. First-generation hatching occurs during April, the nymphs settle on the lower leaf surface, adult females appear in June and early July, and egg-laying runs from mid-June to mid-July. The second generation hatches in early to mid-July and the third from late August to early October [11].

The economic damage arises less from feeding than from the honeydew and sooty mould that downgrade marketability. In infested orchards in southern France, with high infestation rates and severe sooty mould, rejection of up to 80% of the harvested fruit is reported [12]. That figure concerns pome fruit and is given in the publication explicitly as a personal communication, not as a measured experiment — it is cited here as an order of magnitude of the risk, not as an expected value for peach.

For biological control, the two parasitoids selected in a European programme are Allotropa burrelli and Acerophagus malinus; by contrast, Clausenia purpurea was rejected as non-specific, since it also parasitises other mealybug species [12].

The species has already been recorded on persimmon fruit in Macedonia, as described in the related article, available in Greek only.

San Jose scale

San Jose scale has a complex life cycle with overlapping stages, and can produce up to four generations of crawlers per season [13]. The overwintering nymphs are found on twigs, scaffolds and trunks, and in spring develop into winged males and stationary females [13].

The most useful field test is simple: rubbing the scale covers with the hand. If the insect is dead, the cover flakes off clean; if it is alive, rubbing leaves a yellow to reddish deposit on the hand [13]. That distinction decides whether treatment is needed or whether the infestation is old and already dead.

On the fruit, the crawlers settle and produce the characteristic red spots around each cover. In the wood, heavy infestations weaken and finally kill twigs and scaffolds [13]. Because scale development is closely tied to temperature, monitoring the stages determines the timing of treatment; the classic basis remains the dormant oil application.

Thrips

Thrips damage on stone fruit appears at two points in time. At flowering and fruit set, larval activity causes scarring and russeting of the skin, which expands and turns brown as the fruit grows. At the final fruit swell stage, damage appears as patches of silvering on the skin, particularly where it has been protected by leaves or by fruit touching [13].

The monitoring recommendation is sampling of buds and flowers plus sticky traps, fortnightly, with particular attention at the pink and full bloom stages [13]. Ground cover management has a direct effect: broadleaf weeds and flowering plants sustain the populations, and replacing them with grasses reduces movement into the trees, since some thrips species do not feed on grasses [13]. Mowing is done after shuck fall is complete and not during flowering, because otherwise the thrips move up into the trees [13].

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10. Wood borers and fruit feeders

The flat-headed root borer

Capnodis tenebrionis is the pest that kills trees without producing a symptom at eye level. Adults emerge in spring and feed on young shoots, twigs, buds and leaf petioles; they live for more than a year and may overwinter twice [10].

Females lay eggs in summer in dry soil near the trunk and prefer weakened trees. Egg-laying begins at 23 °C and peaks in the optimum range of 30 to 34 °C, typically in July and August; one female can produce more than 1,000 eggs per year under favourable conditions [10].

Neonate larvae penetrate the roots and feed on the cortical tissue. Development lasts 6 to 18 months depending on temperature and rootstock. The most serious damage comes from galleries in the roots and the lower part of the trunk, and a few larvae are enough to kill a large tree within two years [10].

As to control, entomopathogenic nematodes and fungi have given 75 to 97% control in field trials [10]. Resistant rootstock options are limited. Cultural measures — hand collection of adults, irrigation management, physical barriers at the trunk base — remain the backbone of the strategy [10].

In detail, with the field diagnostic signs, in the article on Capnodis tenebrionis.

Goat moth

Cossus cossus belongs to the wood-boring Lepidoptera of the family Cossidae, and is the best known of the twenty-six species of that family reported as pests of commercial woody plants [21]. Its host range is enormous and includes almost every fruit tree of the region: peach and other Prunus, apple, pear, quince, cherry, apricot, walnut, chestnut, persimmon, mulberry, olive and grapevine, along with willow, poplar, oak, elm and maple [21].

The biology explains why an infestation goes unnoticed for years. The adults are nocturnal and poor fliers, with limited dispersal ability [21]. Females lay eggs in bark cracks, usually in the lower parts of the trunk, in groups of 12 to 70 [22]. The cycle lasts two to four years depending on region and food quality, and most of it is spent in the larval stage [21] [22]; the larva overwinters the first year in the phloem and then feeds actively inside the wood [22].

The mature larva reaches 7 to 10 cm, is pink to pinkish-brown with strong mandibles, and cuts a gallery about 15 mm wide and up to one metre long, with dark walls [22]. When the galleries multiply, the trunk takes on the characteristic honey-combed appearance — and this happens because the females prefer to lay eggs in areas that are already infested [21].

The damage is not only the loss of tissue. The galleries make large scaffolds and trunks prone to wind breakage and lead to tree decline and mortality, while reducing growth, yield and fruit quality [21]. As to control, the review of the family stresses the importance of pheromone traps and the need for an integrated approach; no single intervention is sufficient [21].

In detail, with the field symptoms, in the article on the goat moth.

Mediterranean fruit fly

Ceratitis capitata attacks ripe and ripening fruit. The diagnostic sign is the oviposition puncture with a dark halo around it and the presence of larvae inside the flesh. Because the attack coincides with harvest, the options are limited and the strategy rests on trap monitoring, clearing fallen fruit and managing neighbouring late hosts. In detail in the article on the Mediterranean fruit fly, available in Greek only.

Sources: 10 21 22

11. Physiological disorders mistaken for diseases

Split pit

A serious physiological disorder of stone fruit and of peach in particular. Splitting at the early stages occurs typically about twenty days after full bloom, while the pit is still soft, but it can also occur after the pit hardens — and then the damage is greater [14]. The pit splits along the suture under pressure from the flesh, which continues to expand while still attached to the pit; in extreme cases the pit shatters [14].

There is also a genetic component, with clear practical value: early-maturing cultivars — for example June Gold and Spring Gold — are more affected, because final swell and pit hardening coincide in time. In late cultivars the two phases are separated, and the bond between flesh and pit has time to loosen before the flesh swells [14]. The practical consequence: thinning an early cultivar is not done on the same criteria as thinning a late one.

One useful identification feature: split-pit fruit are heavier than normal ones, owing to a heavier mesocarp and exocarp [14].

Chilling injury in storage

Chilling injury appears in stone fruit exposed to temperatures below 15 °C, with the critical value depending on maturity at harvest and cultivar susceptibility [14]. The typical symptoms are pitting of the skin, from collapse of the cells beneath it, and browning of the flesh around the vascular bundles [14].

"Mealy" or "woolly" flesh is likewise an expression of chilling injury, and appears in peaches and nectarines held at low temperature for long periods: altered cell wall enzyme activity gives a dry, woolly texture [14]. Cultivars with non-melting, firm flesh are less susceptible than melting-flesh ones [14].

Chilling injury is not infectious and is not addressed by plant protection. It is included in the tool because at crate level it is confused with decay, and because the distinction completely changes what needs correcting.

Sunburn

Not only fruit damage. Scaffolds left bare after premature defoliation — from rust, from spider mites, from bacterial spot — are exposed to direct radiation, become sunscalded, and the killed areas are then colonised by wood-rotting fungi; those scaffolds weaken, become less productive and break under the weight of a crop [13]. Painting trunks with acrylic paint is a documented measure against sun injury and indirectly against perennial canker [2].

Sources: 2 13 14

12. The look-alike table

Each row is a pair that is confused in practice, and the third column is the one feature that resolves it without a laboratory.

13. The annual monitoring calendar

The table is not a spray programme. It shows when the observation is made that decides whether treatment is needed, and which phenological stage corresponds to the critical window for each cause.

The annual cultural operations that frame the above — pruning, thinning, irrigation, fertilisation — are described in the article on annual cultural operations in peach, available in Greek only, while water management during pit hardening is analysed in the article on regulated deficit irrigation in peach and nectarine. Postharvest nutrition, which determines the following year, is covered in postharvest nutrition in stone fruit.

Sources: 1 2 5 6 7 8 11 13 14 17

14. Sources

The numbers in the text refer to the list below. For each source it is stated explicitly whether the full text or only the abstract was used.

• [1] Luo C.-X., Schnabel G., Hu M., De Cal A. (2022). Global distribution and management of peach diseases. Phytopathology Research 4:30. Full text, open access, peer-reviewed. Source for the ranking of the most important diseases by country, the Monilinia species, the overwintering of peach leaf curl on the twig surface, the distinction between Armillaria mellea and Desarmillaria tabescens, the sour odour in bacterial canker and the role of ring nematode, the 50% of shoots figure for Diaporthe, and the "rusty spot" from Podosphaera leucotricha.
• [2] Adaskaveg J.E., Schnabel G., Ritchie D.F., Förster H. — Common Preharvest Diseases of Peach and Nectarine Caused by Fungi and Bacteria: Biology, Epidemiology and Management. Chapter 12, CABI. Full chapter text. The main quantitative source of this article: wetness hours and infection temperatures for brown rot, shot hole, scab, rust, powdery mildew and leaf curl; the 40–70 day scab symptom delay; the biology of Verticillium wilt and Armillaria; peach tree short life and its ten-point management programme; bacterial spot and the role of wind; crown gall.
• [3] Damos P.T., Soulopoulou P., Gkouderis D., Monastiridis D., Vrettou M., Sakellariou D., Thomidis T. (2022). Degree-day risk thresholds for predicting the occurrence of Anarsia lineatella, Grapholita molesta and Adoxophyes orana in northern Greece peach orchards. Plant Protection Science 58(3):234–244. Open access, peer-reviewed; the full published abstract and the reference list were used. The only local source: eight peach orchards in the prefecture of Pella, years 2018 and 2020 with validation in 2021. Source for every degree-day value in the table, the lower thresholds of 11.4, 9.5 and 7.2 °C and the 1 January biofix.
• [4] Bem F. (2007). The most serious virus diseases of apricot, peach and plum. Georgia – Ktinotrofia, issue 6/2007 (in Greek). Full text. The author is a virologist and former Director of the Department of Phytopathology of the Benaki Phytopathological Institute. Source for the Greek picture of plum pox: the predominance of the PPV-M strain of Greek origin, its distribution on peach in Macedonia, blackthorn as a latent host, the list of tolerant cultivars, and the transmission of PLMVd by pruning shears.
• [5] UC IPM — Oriental Fruit Moth, Peach. UC Statewide Integrated Pest Management Program. Full guideline text. Source for the five generations of the oriental fruit moth, overwintering as a diapausing mature larva in a cocoon, boring to the pit, the three-shoot-strike threshold, the final four weeks of fruit sampling, and the 80–90% parasitism by Macrocentrus ancylivorus.
• [6] UC IPM — Peach Twig Borer, Peach. UC Statewide Integrated Pest Management Program. Full guideline text. Source for the overwintering of the peach twig borer as a first- or second-instar larva in a hibernaculum, feeding just under the skin, the three-shoot-strike threshold, and the 300–400 degree-days from first capture with thresholds of 10 and 31 °C.
• [7] UC IPM — Webspinning Spider Mites, Peach. UC Statewide Integrated Pest Management Program. Full guideline text. Source for the ten-day generation, the 8–18 generations per year, the overwintering of the females, the role of dust and water stress, and the seven-day generation of the predator Galendromus occidentalis.
• [8] Clemson University Cooperative Extension — Armillaria Root Rot (peach). Full page text. Source for the above-ground root collar excavation technique — planting approximately 40 cm higher in open-bottom pots and excavating the collar eight months later — and for the five-year results in two orchards: 30–70% decline in the controls against 0–10% in the treatment.
• [9] UC IPM — Armillaria Root Rot (Oak Root Fungus), Peach. Full guideline text. Source for the white to yellowish mycelial fans, the dark rhizomorphs, survival in the soil, the plastic-lined trench around the infection centre, and root collar excavation to a depth of 23–30 cm in spring.
• [10] Nasouri H. (2024). Capnodis tenebrionis (Coleoptera: Buprestidae). Journal of Integrated Pest Management 15(1):20. Peer-reviewed review; the main body of the text was used. Source for egg-laying in dry soil near the trunk beginning at 23 °C with an optimum of 30–34 °C, the more than 1,000 eggs per female, larval development in 6–18 months, the death of a large tree from a few larvae within two years, the role of drip irrigation, and the 75–97% control with entomopathogens.
• [11] Pellizzari G., Duso C., Rainato A., Pozzebon A., Zanini G. (2012). Phenology, ethology and distribution of Pseudococcus comstocki, an invasive pest in northeastern Italy. Bulletin of Insectology 65:209–215. Full text. Source for the three generations per year, overwintering in the egg stage, the hatching and egg-laying dates of each generation, the twenty-one host species including peach, and the movement of the females towards the scaffolds and into the stem cavity of peaches.
• [12] Malausa T. et al. (2016). Investigating Biological Control Agents for Controlling Invasive Populations of the Mealybug Pseudococcus comstocki in France. PLoS ONE 11(6):e0157965. Full text, open access, peer-reviewed. Source for the establishment of the species in southern France and northern Italy, for the selection of the parasitoids Allotropa burrelli and Acerophagus malinus and the rejection of Clausenia purpurea as non-specific, and for the report of up to 80% of fruit discarded in infested orchards — which the publication gives explicitly as a personal communication and which concerns pome fruit.
• [13] NSW Department of Primary Industries — Integrated Pest and Disease Management for Australian Summerfruit, fact sheets on rust, San Jose scale and thrips. Full fact sheet texts. Source for the consequences of premature defoliation from rust — autumn flowering, sunscald of the bare scaffolds, colonisation by wood-rotting fungi —, for the up to four generations of scale crawlers and the rubbing test on the covers, and for the two timings of thrips damage and the management of the ground cover.
• [14] Malik A.R., Raja R.H.S., Javaid R. — Physiological Disorders in Stone Fruits, chapter 7. Full chapter text. Source for split pit about twenty days after full bloom, the role of excessive thinning and irrigation, the greater susceptibility of early cultivars, the greater weight of split-pit fruit, and for chilling injury below 15 °C with pitting and browning around the vascular bundles.
• [15] Hartman J.R. — Peach Fruit Diseases. University of Kentucky, Plant Pathology Fact Sheet PPFS-FR-T-09. Full text. Source for the appearance of scab six to seven weeks after petal fall and for the fact that the skin of infected fruit does not slip off easily in hot water before processing.
• [16] Clemson University Cooperative Extension Service, CE 16 (reprinted 1997) — Peach Diseases: An Aid to Identification and Control. Full text. Illustrated identification guide. Source for three field criteria: that bacterial spot fruit lesions are deep cracks while scab lesions are strictly superficial; that the angular lesions of bacterial spot do not cross the larger veins, unlike chemical injury; and that in root rot the tree dies slowly with the above-ground wood alive, the dead roots being found at a depth of 15 to 45 cm.
• [17] Penn State Extension — Tree Fruit Insect Pest: Green Peach Aphid. Full text. Source for overwintering beneath the bud scales, the migration of the winged forms in June and July, the thresholds of one colony per tree on nectarines and five or more on peaches, and for the species' role as a vector of plum pox.
• [18] Greek Ministry of Rural Development and Food — database of authorised plant protection products. The only valid source for what is permitted on peach in Greece today, at what rate and with what pre-harvest interval. No active substance is proposed in this article; every choice is checked here and on the product label. These authorisations are valid in Greece only.
• [19] Whidden A. — Peach Scab of Florida Peaches. UF/IFAS Peach Newsletter, Spring 2012, Central Florida Edition. Full text. Source for the size of the first scab lesions (2–3 mm), the yellowish halo, the position near the stem end, the raised corky appearance, and the skin cracking that opens the way for brown rot.
• [20] Francia C., Lázaro E., Novellón M., Ramón-Albalat A., Beluzán F., Vicent A., Berbegal M., Armengol J. (2025). Biology and epidemiology of Diaporthe amygdali: understanding how environmental factors influence fungal growth, sporulation, infection and lesion development on almond. Frontiers in Plant Science 16:1717223. Full text, open access, peer-reviewed. Source for the current nomenclature of the pathogen, its host range, the entry points (leaf scars in autumn, buds and blossoms in spring), and for the values the same work summarises from earlier studies on peach: conidial germination 5–36 °C with an optimum of 27–29 °C, infection most common at 5–15 °C, incubation of about one month, optimum mycelial growth 24–25 °C, cirrus production 1–38 °C with the greatest proportion at 17 °C and maximum conidium production at 24 °C, and the sinusoidal annual pattern of inoculum peaking in late summer. The experimental results of that work itself (72 hours of wetness, 22 °C for conidium production) concern almond.
• [21] Choochuen T., Foit J. (2025). Cossid moths (Lepidoptera: Cossidae) as pests of woody plants – A review. Agricultural and Forest Entomology. Full text, open access, peer-reviewed. Source for the twenty-six Cossidae species reported as pests of commercial woody plants, the one- to three-year life cycle with most of it in the larval stage, the host range of Cossus cossus, the honey-combed appearance of infested trunks and the preference of females for already infested areas, the nocturnal habit and low dispersal ability of the adults, wind breakage of scaffolds, and the importance of pheromone traps.
• [22] Forest Pests — Cossus cossus (goat moth). Full page text. Source for egg-laying in bark cracks in the lower parts of the trunk in groups of 12 to 70, the two-year development with the first year overwintered in the phloem, the size and colour of the mature larva (7 to 10 cm), the gallery 15 mm wide and up to one metre long, the coarse wood chips mixed with frass and the fermenting liquids at the tree foot, and the strong vinegar smell.