By: Kevin Ong and Corinne Rhodes
Phytoplasmas are a specialized group of bacteria that infect plants. The phytoplasma pathogen ‘Candidatus Phytoplasma prunorum’ causes several stone fruit (Prunus spp.) diseases in Europe, collectively known as European stone fruit yellows (ESFY).
The species most severely affected by ESFY are apricot and Japanese plum. ESFY diseases on these hosts are apricot chlorotic leaf roll, plum leptonecrosis, and plum decline. In parts of Europe, ESFY can limit production in an orchard for years. Currently, ESFY is not present in the United States.
ESFY phytoplasma cause devastating diseases on peaches, flowering cherries, almonds, and European plums, often resulting in serious economic losses. On peaches, the disease is known as peach decline and peach yellows (not the same as the peach yellows phytoplasma present in the southeastern United States). This pathogen infects stone fruit and a few other Hosts.
Symptoms vary depending on the affected host species and cultivar as well as the strain of the pathogen. The most common symptoms on peaches include chlorotic (yellowing) to necrotic (dead) leaf lesions in early summer, leaf reddening and leaf curling in summer or early fall, and premature defoliation (dropping of leaves). Where leaf lesions develop, necrotic areas will fall out, leaving a shot hole appearance. Other symptoms on peaches include:
- Corky and enlarged leaf veins which go from light to dark brown
- Brittle leaves that develop late in the season
- Premature budbreak
- Growth during usually dormant periods
Common symptoms among other stone fruit affected by ESFY include:
- Reddening or yellowing of leaves
- Leaf curling and rolling (Fig. 1)
- Brittle leaves
- Premature budbreak and defoliation
- Off-season growth
Apricots and plums suffer many of these as well as more severe symptoms:
- Reduced terminal growth
- Phloem (carries nutrients throughout the plant) necrosis (Fig. 2)
- Gradual decline of trees
The most important means of disease transmissionis by the psyllid, Cacopsylla pruni. This insect feeds on the phloem of Prunus spp. and transmits the phytoplasma to the plant. After 2 to 3 weeks of being infected with the phytoplasma, the psyllid can infect another plant and remains capable of transmitting the disease for the rest of its life. Wild Prunus hosts serve as a reservoir for both the psyllid and the pathogen. These wild hosts often remain asymptomatic and harbor more insect vectors than do cultivated varieties. Grafting also transmits the pathogen.
Current strategies for managing ESFY include:
- Using certified stock material
- Applying insecticides for psyllid control
- Removing infected orchard trees and nearby wild Prunus trees
- Using cultivars that are resistant to the pathogen and/or attract fewer psyllids
- Using biological controls (There is evidence that plants with avirulent strains of the pathogen have cross-protection against infection by virulent strains.)
- Timing insecticide sprays to target critical psyllid life stages, including emerging young adults and egg-laying females
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