M. K. Kwon, B. R. Kang, B. H. Cho and Y. C. Kim^*

*yckimyc@chonnam.ac.kr

Accepted for publication 13/01/03

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.

Fig. 1. Target leaf spot disease of cucumber caused by C. cassicola. (a) symptoms on artificially inoculated leaf, (b) symptoms of natural infections, (c) conidia of C. cassicola.

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 30ºC, plants were maintained at 25ºC. 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 ºC) increase disease severity. An isolated of C. cassicola isolate has been deposited in the Korean Agricultural Culture Collection (KACC40941).

This work was supported by grant R11-2001-092-01005-0 from the Korea Science and Engineering Foundation through the Agricultural Plant Stress Research Center at Chonnam National University.

Ellis MB, Holliday P, 1971. Corynespora cassicola. C.M.I. Descriptions of Pathogenic Fungi and Bacteria, No 303. Kew, UK: CAB International Mycological Institute.

Kang SW, Kwon JH, Chung BK, Cho JK, Lee YS, Kim HK, 1993. Identification and etiological of new disease, Corynespora leaf spot of cucumber caused by Corynespora melonis (Cook) Lindaw green house cultivation in Korea. RDA Journal of Agricultural Science 35, 332-336.

Wyszogrodzka AJ, 1987. Multiple-pathogen inoculation of cucumber (Cucumis sativus) seedlings. Plant Disease 71, 275-280.

The British Society for Plant Pathology