INCIDENCE AND SEVERITY OF LEAF AND FRUIT DISEASES OF PLUMS IN LATVIA

In the present study six plum orchards in Latvia were examined during 2014. One orchard was commercial with integrated pest management (IPM) practices, one was with organic management, two orchards were scientific collections and in two orchards plums were grown as a minor crop, using IPM practices. The shot-hole disease (Wilsonomyces carpophilus) and fruit rot were monitored in the field. Samples of twigs and leaves were taken for further examination if some other disease symptoms were observed. In total, 50 European plum (Prunus domestica) and six diploid plum cultivars were inspected. The fruit rot was assessed also in the laboratory to determine the latent infection with Monilinia spp. on immature fruits. Monilinia spp. isolates from all orchards were subjected to fungicide sensitivity tests. Incidence and severity of shot-hole disease was significantly different among various orchards when the same cultivar was compared, as well as between diploid and European plum cultivars. The average incidence of shot-hole disease was 41% in diploid plums and 80% in European plums, while the average severity was 9 and 15%, respectively. In the field, fruit rot caused only by Monilinia spp. was detected. The average incidence of brown rot on diploid plums was less than 1%, but on European plums it was 3.6%. The latent infection tests showed that plum fruits had higher incidence of brown rot than was observed in the field, up to 44% on particular cultivars. Additionally, from the fruits subjected to these tests, Botrytis cinerea, Diaporthe eres and Colletotrichum spp. were isolated. This means that in specific weather and management conditions the fruit rot incidence in the field could be several times higher. Examination of samples of twigs, leaves and fruits in the laboratory showed the presence of D. eres in samples from all orchards. In one of the scientific collections D. eres was isolated from twigs, leaves and fruits, and was more often found on the individuals located in the part of the orchard close to hedge. In other orchards this fungus was isolated only from fruits, and mainly from the latent infection tests. Monilinia spp. isolates showed high sensitivity to dithianon, penconazole, mancozeb and boscalid with pyraclostrobin, but lower sensitivity to cyprodinil. Several fungi that are known to be antagonistic to pathogenic fungi were isolated from all orchards: Epicoccum nigrum, Clonostachys rosea and Aureobasidium pullulans, mainly from the fruits of latency tests and leaves with disease symptoms.