Occurrence of powdery mildew (Sphaerotheca pannosa) in Japanese plum in Northern Israel and its control

Powdery mildew, caused by the fungus Sphaerotheca pannosa (Wallr.) Lev., is a widespread and destructive disease of nectarine in the northern region of Israel. Powdery mildew has not previously been observed on Japanese plum (Prunus salicina Lindl.) in this region, and we describe here the first occurrence of powdery mildew on plum trees in the largest growing area in Northern Israel. The first symptoms and signs were observed on 22 April 2000 as single colonies on young plum fruits, and these expanded during the season and covered large parts of the fruit. The disease caused browning and russeting of the fruit surface, and chlorotic spots on mature and coloured fruits until harvest. The disease on leaves was less severe, characterized by chlorotic (bright yellowish) lesions of 5-10 mm diameter on the upper surface and "oil spot" type lesions on the lower surface. Microscopic examinations revealed that the mycelium, conidiophores and conidia were similar to those of S. pannosa that attacks nectarine trees in this region. Cleistothecia were not found during the growing season. Disease rating in two orchards showed that fruits were more infected than leaves. The most severly affected cultivar was 'Red Beaut' with 27% and 97%, respectively, of leaves and fruits infected. Field experiments conducted in 2000, 2002 and 2004 showed that foliar applications of DMI (demethylation inhibitor) fungicides reduced disease development, but failed to provide satisfactory control. DMIs were less effective than the strobilurin, kresoxim-methyl and polyoxin B in controlling S. pannosa on fruits and leaves of field-grown Japanese plum trees. However, spraying of sulphur significantly enhanced the inhibitory effect against powdery mildew and provided the highest level of disease control. In the light of experience with S. pannosa in nectarine, it is suggested that intensive use may have led to reduced efficacy of the DMI fungicides in controlling S. pannosa in nectarine and plum orchards. An alternative disease management programme, which integrates early and timely applications of effective fungicides, and tank-mixtures of DMI fungicides with sulphur, may provide an improved level of powdery mildew control and could be valuable to avoid the spread of powdery mildew and to minimize the development of fungicide resistance.