Blotch Continued from Page 6
Red leaf blotch is widespread through- out Europe’s Mediterranean basin and the Middle East and is considered one of the most important foliar almond diseases. It has not been found in Central or South America or Australia. Trouillas said the disease likely entered California on imported plant material, as that is a common pathway for invasive spe- cies and diseases. Based on when symp- toms were discovered, he said the disease probably was introduced recently. Known scientifically as Polystigma amygdalinum, the causative fungus is spe- cific to almonds and does not infect related species such as peaches, nectarines and plums, he said. The organism overwinters in fallen leaf litter, and during spring rains, it releases ascospores, specialized spores that infect newly emerged leaflets. The infection remains asymptomatic or latent for 35 to 40 days—about late April to mid-May—after which small, yellow splotches begin to form on each side of the leaf. During this early stage, it could be confused with other common almond problems such as herbicide injury. As spring progresses, the blotches typically expand and turn orange to red- dish-brown. “When you see them, they’re really
Red leaf blotch produces unique symptoms that progress from small, yellow spots to orange or reddish-brown blotches on both sides of the leaves. The disease, which affects almond trees, is believed to have entered California on imported plant material.
bright orange-colored spots and blotch- es on leaves,” Trouillas said. “It’s really unique and cannot be confused with anything else.” A severe infection may cause premature leaf loss, according to reports from Spain. In addition, the disease is weather-de- pendent, Trouillas said. Rain coupled with mild spring temperatures are conducive to disease development, while leaf infection drastically decreases with high summer- time temperatures and low humidity. The good news is many of the same fungicides used to manage spring al- mond diseases such as anthracnose and
shot hole also appear effective against red leaf blotch, Trouillas said, citing studies conducted in Spain. Research from that country also found one fungicide applica- tion at petal fall followed by two additional ones at two and five weeks after petal fall, if rains continue, were effective at controlling red leaf blotch. The sprays must be applied as a preven- tive treatment and aren’t effective once symptoms are present, he said. Trouillas and his colleagues plan to conduct field trials next spring to determine fungicide efficacy under California conditions. They also plan to work with
chemical manufacturers to add red leaf blotch to fungicide labels so growers can use them to manage the disease. Since red leaf blotch has only one in- fection cycle per season, researchers in Spain have looked at reducing the prima- ry source of overwintering inoculum by removing leaf litter, spraying fallen leaves with fungicides or applying urea to speed decomposition. To be effective, Trouillas said those practices should be done on an area-wide basis and would be difficult to implement in California. (Vicky Boyd is a reporter in Modesto. She may be contacted at vlboyd@att.net.)
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August 14, 2024 Ag Alert 7
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