Bacterial canker on apricot: what Romagna showed

Photograph 1. An unripe apricot carrying extensive dark necrotic spots that have run together into continuous lesions, with a droplet of gum at the edge of one of them. This is exactly the picture the Italians describe as downgrading of the crop: the tree has not died, but the fruit has already lost its marketability. The lesions appear early and go on enlarging until colouring [1]. Photograph: Savvas Pastopoulos.

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

In the Romagna region of Italy, whole young apricot orchards dried up within a few years, killed by a bacterium that until then had been regarded as a problem of sweet cherry. The cause was not a new pathogen; it was the change in the cultivar base. The new, early and vigorous cultivars planted from the late 1990s onwards proved barely tolerant of Pseudomonas syringae. That same plant material is being planted in Greece today.

1. What happened in Romagna, and why it matters here

The first reports of rapid and severe dieback of apricot trees came from the district of Imola and concerned the cultivar Aurora. The trees were dying within a few years, taking the productivity of entire young orchards with them. The disease then spread into the hill zone of Faenza, Forlì and Cesena [1] [9], and was later recorded in the Marche, in Trentino and in Piedmont [7].

The dominant picture was the appearance and the very rapid development of deep cankers on the main scaffolds or on the trunk. The cankers spread quickly both upwards and downwards, driving the wood of the tree into complete loss of function within a few months [1].

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2. The pathogen, two pathovars, one dominant

The species Pseudomonas syringae contains more than fifty pathovars. Two of them matter on apricot [1] [3]:

The isolations yielded many different strains, with their own biochemical, metabolic and pathogenic characteristics [2]. This is not a uniform enemy but a mosaic of populations, which explains why the same strategy gives different results on neighbouring holdings.

3. The symptoms on apricot

The economic damage arrives in two stages, and the first is usually overlooked.

First stage: downgrading of the crop

Before the trees start to die, the fruit loses its marketability. Scabby blisters and spotting develop on the fruit. These lesions appear early and go on enlarging until colouring [1]. On the leaves it produces spotting, while bud death and wilting of the current season's shoots are also seen.

Photograph 2. The same attack in the field. The canopy looks healthy and the problem shows only when a fruit is taken in the hand, a spotted apricot can be made out at bottom right. This is why the disease is picked up late: by the time scaffolds start drying, the bacterium has been at work in the orchard for years. Photograph: Savvas Pastopoulos.

Second stage: the canker and the death of the tree

The canker on the scaffolds appears very extended lengthways. Over part of its length it is marked by a sunken bark, while elsewhere it opens right down to the heartwood. Characteristically, the Italian descriptions note that bacterial exudate was not always present, that is, the absence of gum does not rule out a bacterial infection [1].

With the bark cut away, the tissue underneath is necrotic, wet and without a clear margin, and gives off a smell of fermenting material. That smell is among the most reliable practical indicators in the field.

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4. Why a bacterium makes ice

This is the most misunderstood mechanism of the disease, and the reason why the choice of planting site counts for more than any spray.

P. syringae has the ability to nucleate ice, that is, to make the water around it crystallise at temperatures already close to zero [4]. In this way the pathogen generates micro-frost injuries in the susceptible tissues, in the buds and the flowers, and kills them [1].

5. The cultivars, the susceptibility list

There is no resistant apricot cultivar today. There are, however, clear differences of degree, and the Italian list is the single most useful part of the whole study for anyone planning a planting [1].

The second category does not mean resistance. It means that fruit lesions appeared less often under the particular conditions of Romagna. Behaviour under another climate and another inoculum pressure may differ.

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6. Where it gets in: plant material, pruning, site

Where it overwinters

On apricot the commonest overwintering site is inside the bud tissues and at the base of the bud itself, near the leaf abscission scar. The second overwintering site is the woody cankers on branches and scaffolds that were not removed carefully at pruning [1].

Primary inoculum may be present epiphytically, that is, on the outside, on wild herbaceous species as well as on tree crops. As an endophyte it is found in symptomatic and asymptomatic host tissue [1]. From this follows the recommendation for good weed management, with mowings that keep the weeds from growing away.

The propagating material

In modern nurseries the material is produced mainly by micropropagation and is phytosanitarily safe. The risk lies in the acclimatisation phase and in the growing-on of the young trees outdoors, where the nursery adjoins potentially infected orchards [1]. Italian law requires traded material to be free of P. syringae pv. syringae and pv. morsprunorum; the two pathovars are not quarantine organisms, but they are among the diseases that bear on quality.

Pruning, and which cuts are the most dangerous

Secondary spread is driven by rain and by spring and summer storms. Establishing new plantings in districts of intensive tree fruit growing, where plant pathogenic pseudomonads are endemic, is in itself a risk factor [1].

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7. Copper: rates, timing and the 28-kilo ceiling

Despite the existence of strains resistant to the copper ion, copper remains the only approved plant protection product with established bactericidal action [1].

The Italian programme

The adjustment according to the symptom: where the symptoms concerned mainly the fruit, the autumn applications are reduced and the emphasis moves to spring, but at the reduced rate described above, not at 125 g/hL [1].

The calculation that has to come before the programme

The figures are indicative and come from Italian trials under different soil and climate conditions. The metallic copper content of each product differs and is stated on the label. The final rate and the number of applications are set after an on-site assessment by an agronomist, and always within the approvals in force [10].

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8. What was tried as an alternative, and what worked

Under the Emilia-Romagna rural development programme 2014–2020, three field trials were carried out in the seasons 2016–2018 at Riolo Terme and Faenza, on the cultivars Primaya, Rubista and Pricia. The products were applied during the growing season, from the end of flowering to pit hardening, at intervals of 7 to 12 days, at a spray volume of 700 to 1,000 litres per hectare [1].

The results

2016 was hot and dry and allowed no distinction at all between treatments. The two seasons 2017 and 2018 gave conditions favourable to the pathogen [1].

The phytotoxicity of copper

The researchers are categorical: phytotoxicity is inherent in the use of copper, and mixing it with substances of a different nature, carboxylic acids and oligo-polysaccharides, can amplify that unwanted effect [1]. The trials recorded very severe phytotoxicity symptoms on unripe Lady Cot fruit, attributed to technical errors in the use of the products, and phytotoxicity on leaves and fruit of Robada from the caustic action of a copper concentration the cultivar would not tolerate.

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9. Which of this applies in Greece

Sources: 1 5 6 10

10. The programme for a Greek apricot orchard

11. Sources

The numbers in the text refer to the list below.

• [1] Fagioli L., Franceschelli F., Ceredi G., Bugiani R., Tommasini M.G. and Stefani E. (2019). Cancro batterico dell'albicocco: dalla diagnosi alla difesa. L'Informatore Agrario 8/2019:31–35. In Italian. The principal source of this article: the history of the epidemic in Romagna, the cultivar susceptibility list, the overwintering sites, the part played by pruning cuts on trees of 3 to 4 years, the 125 g/hL copper programme, the protocol and results of the 2016–2018 trials, the phytotoxicity records, and the reddish russeting of Farbaly that is not bacterial.
• [2] Giovanardi D., Ferrante P., Scortichini M. and Stefani E. (2018). Characterisation of Pseudomonas syringae isolates from apricot orchards in north-eastern Italy. European Journal of Plant Pathology 151(4):901–917. Source for the identification of multiple genotypes and for the prevalence of pv. syringae over pv. morsprunorum in the apricot orchards of Romagna.
• [3] Bultreys A. and Kaluzna M. (2010). Bacterial cankers caused by Pseudomonas syringae on stone fruit species with special emphasis on pathovars syringae and morsprunorum race 1 and race 2. Journal of Plant Pathology 92(1 supplement):S1.21–S1.33. Source for the distinction between the two pathovars and for pv. syringae being far more frequent on stone fruit than pv. morsprunorum.
• [4] Hirano S.S. and Upper C.D. (2000). Bacteria in the leaf ecosystem with emphasis on Pseudomonas syringae, a pathogen, ice nucleus, and epiphyte. Microbiology and Molecular Biology Reviews 64:624–653. Source for ice nucleation at temperatures close to zero and for the epiphytic life of the bacterium.
• [5] Commission Implementing Regulation (EU) 2018/1981 of 13 December 2018 renewing the approval of the copper compounds as candidates for substitution. Source for the ceiling of 28 kg of copper per hectare over a period of seven years.
• [6] Commission Implementing Regulation (EU) 2020/2087 of 14 December 2020 concerning the non-renewal of approval of the active substance mancozeb. Source for the ban on mancozeb: approval expiring 4 January 2021, authorisations withdrawn by 4 July 2021, disposal of stocks ending 4 January 2022.
• [7] Morone C., Stefani E. and Vittone G. (2005). Il deperimento batterico dell'albicocco: una minaccia per gli impianti del Nord Italia. Rivista di Frutticoltura e Orticoltura 67(6):28–32. In Italian. Source for the spread of the disease to the Marche, Trentino and Piedmont.
• [8] Chen X.H. et al. (2009). Difficidin and bacilysin produced by plant-associated Bacillus amyloliquefaciens are efficient in controlling fire blight disease. Journal of Biotechnology 140:38–44. Source for the documented action of the bacteriocins against fire blight, which was not confirmed against the pseudomonads.
• [9] Bugiani R. et al. (2014). Come gestire il deperimento batterico dell'albicocco. L'Informatore Agrario 15:2–6. In Italian. Source for the spread from the district of Imola into the hill zone of Faenza, Forlì and Cesena.
• [10] Greek Ministry of Rural Development and Food, database of approved plant protection products. In Greek. The only authoritative source for what is permitted on apricot in Greece, at what rate and with what pre-harvest interval. It does not apply outside Greece.
• [11] Bacterial canker of stone fruit, Pseudomonas syringae, plantprotect.gr. The differential diagnosis with the knife, the nematode and bacterium complex, the difference between copper hydroxide and copper oxide, and the four reasons copper fails.
• [12] Lecigne P. et al. (2000). Mancozèbe, cuivre et bactérioses. Résultats sur tomate, melon, noyer, vigne. Phytoma, la Défense des Végétaux 531:13–16. In French. Source for the synergy of mancozeb and copper against bacterial diseases, to which the Italian paper refers.
• [13] Penn State Extension, Dormant copper sprays and orchard sanitation and University of Maryland Extension, Fungicides for fruit. Sources for copper being applied in the early period and avoided after leaf emergence, and for copper products causing leaf drop on peach, plum and apricot.