Plum pox virus

Excerpt of: Genetic engineering of Plum pox virus resistance: ‘HoneySweet’ plum—from concept to product, Scorza et al. 
Sharka disease caused by Plum pox virus (PPV) was first observed in Bulgaria during the First World War in 1917 . By the late 1970’s the disease had spread throughout Europe and despite the considerable efforts made in respect to sharka containment, the disease has been reported in all stone fruit producing countries worldwide except Australia, New Zealand and South Africa.
PPV is classified in the family Potyviridae and the genus Potyvirus.  PPV hosts include cultivated, ornamental and wild Prunus species . Non-Prunus species, in at least nine plant families, have been infected artificially with one or more strains of the virus, and in some cases have been found naturally infected in the field. 
Generally, PPV affects the fruits of sensitive cultivars of the commercially cultivated stone fruits such as plums, apricots , peaches and nectarines. Cherries and almonds can also be infected.  Symptoms of sharka disease vary widely depending on the crop, the cultivar, the virus strain, and the environment. Affected plum fruits are deformed and show deeply engraved rings, irregular lines, and fruit malformation. The flesh turns brownish red and is saturated by gum and is unsuitable for marketing.  Up to 90% of fruit fall prematurely and these fruit are unsuitable both for direct consumption and for industrial processing. Similar symptoms of PPV infection are common on apricot, peach, nectarine, and other stone fruits.  Most stone fruit varieties show leaf symptoms which appear as pale or yellow green rings, diffuse spots or leaf mottling, chlorotic oak-leaf patterns, shallow depressions, and shoots may split and die back.   Peach and plum blossoms of PPV-infected trees can also show symptoms, consisting of darker, pink colored speckles on the pink colored petals of the peach. For more information on symptoms, click here.
Highly sensitive plum cultivars such as ‘Ortenauer’, in addition to fruit and leaf symptoms, show bark splitting and cankers on the shoots that become dry and brittle and die. Infected highly susceptible trees decline within only a few years .
The main pathway of long-distance PPV spread is through the use of infected propagative material and the distribution of infected trees. Once sharka has become established in an orchard aphids transmit the virus locally. More than twenty aphid species are known to transmit PPV.  PPV is a non-persistently transmittable virus. Aphids can acquire the virus from infected material in as short as 30 seconds and can transmit it for up to 2-3 h after acquisition.  These short periods are sufficient for spreading the virus within nurseries or orchards. PPV can be spread in orchards by transient aphids as efficiently as aphids colonizing Prunus species.  The speed of natural spread in orchards depends on the distance of the healthy trees to the infection sources. It has been shown that within a 100 m diameter from an infected individual tree, 48-100% of the previously healthy trees have become infected within 10 years. Systemic spread of the virus within a tree may take several years .  The virus may be detected in seed coats and cotyledons, but embryonic tissue and seedlings obtained from germinated seeds have not shown infection, therefore, PPV is not considered to be seed transmitted.
The reduced fruit quality and premature fruit drop caused by sharka disease make stone fruit production in some European countries problematic. Production for European plums was 3 million metric tons (mt) and losses were 1.5 million mt per year with an approximate value of €5400 M over a 30 year period.  The loss of apricot fruit was 0.6 million mt per year with an approximate value of €3600 M for a 30 year period.  Sharka has changed the landscape of stone fruit cultivars. For example, as early apricot cultivars are much more sensitive in terms of symptom expression on fruits than the late cultivars, many early local cultivars have progressively been substituted by later ripening tolerant varieties that show few or no symptoms on fruit.
In countries not affected by PPV or with localized and quarantined infection zones primary focus is placed on preventing the introduction of PPV into uninfected fruit-growing areas. The development and enlargement of the European Union with increased movement and trade between countries with different levels of PPV infection and with different strains of the virus is likely responsible for the spread of PPV in general, and the wider distribution of the various PPV strains. The movement of PPV infected plant material from infected to non-infected or only partially-infected countries in Europe, along with the potential for spread of the more severe strains of PPV have been causes of great concern.  Over the past 25 years over €33 M has been spent in Europe on programs to conduct research on sharka control (Cambra et al. 2006) including the recent SHARCO project funded at over €3.5 M that specifically addressed “Sharka containment in view of EU-expansion”.
To prevent the long-distance spread of PPV within a state, country, or region, quarantine has been the first line of defense. Another element of the strategy in preventing virus introduction to a new area is the use of certified planting stock that has been tested and verified to be free of PPV.  Unfortunately, visual inspections carried out on PPV tolerant cultivars which remain symptomless while infected have little value in terms of quarantine or certification programs.  Once PPV is introduced into a new area the next control measure is the elimination of virus infected plants as quickly as possible before the disease spreads. Since the infected trees cannot be cured or the virus eliminated, infected trees must be eradicated.