Occurrence of target leaf spot disease caused by Corynespora cassicola on cucumber in Korea

This is the first report of target leaf spot of Cucumis sativus grown in open fields, caused by Corynespora cassicola, in Chonnam province of Korea. A similar disease caused by Corynespora melonis was observed in 1991 in greenhouse-cultivated cucumber in Jinju area of Korea (Kang et al., 1993).

A survey of commercial fields conducted between 2000 and 2001 in Chonnam province, revealed severe damage on Japanese cucumber variety, Baeksung 3, but not on Korean varieties. However, by 2002 these varieties were severely affected by target leaf spot in fields in Chonnam and Gyeongnam provinces. Leaf symptoms began as small brown spots with yellow halos, enlarging and becoming irregularly shaped before defoliation occurred. The morphology of the fungus consistently obtained from these symptoms matched the description of Corynespora cassicola (Ellis & Holliday, 1971). Conidiophores were erect and simple, pale to mid-brown, 205-725 µm long and 5-8 μm wide. Conidia were formed singly or in chains, obclavate to cylindrical, straight or curved, subhyaline to pale olivaceous-brown, 22-300 µm long and 5-10 µm wide at the base, with 7 - 11 pseudosepta. The 610 bp PCR-products derived using primers to amplify ITS of the 18S rRNA regions from the new Korean isolates and a reference strain ATCC64204 of Corynespora cassicola (Wyszogrodzka et al., 1987) were identical.

A conidial suspension (10⁴ conidia per ml) of Corynespora cassicola was prepared from 3 week-old cultures grown on potato dextrose agar and sprayed on cucumber plants with completely developed third leaves. After incubation in a dew chamber for 24 hours at 30°C, plants were maintained at 25°C. Small brown spots appeared on leaves within three days. Leaves of water-sprayed control plants remained healthy. Corynespora cassicola was re-isolated from the leaf lesions and was identical morphologically to the original isolate of C. cassicola sprayed on the test plants. Greenhouse studies indicated that a long dew period and high temperatures (25-30 °C) increase disease severity. An isolated of C. cassicola isolate has been deposited in the Korean Agricultural Culture Collection (KACC40941).