PNRSV Prunus necrotic ringspot virus
Symptoms can vary from very severe, to mild, to symptomless, depending on the virus involved, the species or variety of tree and the environmental conditions. They can appear on leaves, buds, blossoms, shoots or fruits and are usually characterized by the discoloration or localized necrosis of these tissues. Individual branches show delayed budbreak and leaf development and can show terminal dieback. Chlorotic or bright yellow blotching, chlorotic to yellow mottling, ringspots, lines and/or "oak leaf" patterns appear on developing leaves. In severe cases, the chlorotic areas become necrotic and fall out, resulting in shredding, deformation or "shot-hole" aspects. Fruit maturity may be delayed, and fruit may be marked, rendering them unmarketable.
PNRSV causes necrotic ringspot in many species of Prunus, often with subsequent recovery from symptoms. The virus can be transmitted through plant propagation methods, making the spread of tree nursery stocks and root grafting in orchards problematic. The virus has also been shown to infect pollen and seeds and to be transmitted in this way to pollinated plants. Bees or other insects carrying infected pollen into orchards can thus serve as vectors. The virus can also be transmitted by thrips, however, the contribution and importance of thrips in its propagation is unknown. PNRSV infects all flower parts, therefore infection of seeds can occur from an infected pollen grain, an ovule, or both. It also seems that the virus can infect the fruits.
Combatting Stecklenberger disease is very difficult. Thermotheraphy of seeds with hot air (24-32 days at 38 °C) or hot water can ensure healthy stocks. The generation of new trees through a method called apical meristem culture (free of virus) has also been effective.
Always consider an integrated approach with preventive measures together with biological treatments if available. Viral diseases in general cannot be controlled by pesticides. As thrips may serve as a possible vector of disease spreading, their control may reduce infection cases.