Plum pox (Sharka)

Symptoms

The disease attacks plum, apricot and peach, along with other species of the genus Prunus. Pale, chlorotic rings, spots and streaks appear on the leaves along the veins, most clearly visible in spring during cool weather. Depressions, discolouration and deformations form on the fruit, and the flesh beneath is often discoloured and gummy with an impaired flavour. Infected fruit frequently drop prematurely, so the crop loses its market and eating value. The leaf symptoms also take the form of vein clearing along the veins and irregular mottling; they are strongest in spring, often fade (become masked) during hot summers and return the following season, which makes diagnosis difficult. The severity of symptoms depends on the strain of the virus and the susceptibility of the cultivar - ranging from almost symptomless forms to severe deformations. On plum fruit, dark, bluish rings and spots as well as depressions are visible; in apricot, characteristic diagnostic rings and spots also appear on the stone (endocarp), which is one of the more reliable identifying features. In peach, colour breaking of the flower petals (paler streaks) is additionally seen. Freshly infected trees can remain symptomless throughout the season, so reliable confirmation is provided only by laboratory testing (the ELISA serological test or molecular RT-PCR).

Treatment

Sharka is an incurable viral disease - there is no effective treatment, and the only way to limit its spread is to grub out and destroy infected trees together with their roots. At the same time, aphids that transmit the virus non-persistently from tree to tree should be controlled. New plantings are not established in the immediate vicinity of infected orchards. Grubbing out must also include root suckers - the virus persists in the roots, and any suckers left behind regrow already infected and become a new source of infection. The felled material should be destroyed (burned or chipped) and not used for grafts or cuttings. An important note on aphid control: sharka is transmitted non-persistently - an aphid acquires and passes on the virus within a few seconds to minutes of probing - so insecticide sprays do NOT prevent infection, because the aphid infects the tree before it dies. Controlling aphids only reduces the overall vector pressure and is supportive rather than decisive. Sharka is subject to phytosanitary supervision, virus-free material is mandatory in nursery production, and any suspected infection in a commercial crop should be reported to the relevant plant protection inspectorate.

Prevention

The foundation is establishing the orchard from certified, virus-free nursery material and choosing resistant or tolerant cultivars. Regular inspection of the trees and prompt removal of the first infected specimens, before they become a source of infection for the whole plantation, are equally important. Trees should not be propagated (grafted or budded) from one's own or uncertain sources - infected nursery material is the most common route by which the virus is introduced to a new plantation. Among plum cultivars there are resistant forms that react with hypersensitivity, e.g. 'Jojo', as well as tolerant cultivars, which are worth choosing in areas where the disease occurs. New plantings are located away from old, infected orchards and from wild reservoirs of the virus - blackthorn (Prunus spinosa) and other wild-growing plums, which can be a source of both the virus and the aphids that carry it; such reservoirs in the vicinity are worth removing. Inspection is carried out mainly in spring, when the leaf symptoms are most distinct, and in case of doubt laboratory testing is commissioned. Reducing aphid populations across the plantation lowers the rate of spread, although - as above - it does not prevent individual infections.

How to tell it apart from look-alikes

Control — organic and chemical methods

We give active-ingredient groups and modes of action, not brand names — specific products and their approval vary by country and are sometimes withdrawn. Always check and follow the current label.