Economic importance and cycle of Polystigma ochraceum, causing red leaf blotch disease of almond, in Lebanon

Abstract

The red leaf blotch disease, incited by the pathogen Polystigma ochraceum, is a major disease of almonds in Lebanon. An investigation was conducted to study the severity of the disease in almond‐growing areas of Lebanon and the cycle and developmental stages of the pathogen in an almond orchard in the coastal area. The disease was found to be widespread and severe in the coastal and mountainous areas, moderate in the north, and mild in the interior Bekaa plain. Ascospore discharge began in early February, just before flowering, and continued until mid‐May. Maximum ascospore discharge occurred about one month after flowering. Pycnidia started to form in the stromata when turning red in colour and acquiring a crusty appearance. Perithecial initials appeared later during the month of July, but continued their development and maturation during autumn and winter to start discharging their ascospores in February. The pathogen could not be grown in vitro on culture media. No secondary infections by pycnidiopores were noticed.