SCREENING TECHNIQUES FOR DOWNY MILDEW RESISTANCE IN GHERKIN CUCUMBERS

Abstract

Downy mildew (Pseudoperonospora cubensis) causes significant losses in cucurbitaceous
crops worldwide. P. cubensis isolates were obtained from infected cucumber leaves in Lampang province,
Thailand during 2010-2012 in order to determine the downy mildew virulence. The isolates were tested for the
presence of highly compatible reactions on specific hosts and categorized as the pathotype 3. This study
determined a screening technique that could improve downy mildew resistance of gherkin cucumber lines
(Cucumis sativus L.) grown under greenhouse and field conditions including inbred lines, gherkin varieties and
hybrid parents of cucumbers. The pedigree method was applied to four generations of gherkin cucumber lines
(F2-F5). The results showed an average mildew resistance score for 8 elite gherkin lines from the F5 generation
after 40 days of transplanting. The score of the 8 elite lines was less than an average score for other gherkin
and commercial cucumber varieties and was equivalent to an average score for downy mildew resistant
varieties. On average, these 8 elite lines from the F5 generation had maximum yields as measured by a mean
output of 26.9 tons per hectare (more than that of other gherkin cucumber varieties), mean fruits per plant of
32.6 fruits, and a mean weight of each fruit of 19.4 grams. It was concluded that a disease resistance breeding
program using this screening technique and its application was successful in controlling and improving gherkin
cucumber lines resistance to the downy mildew pathogen, and which led to increased productivity.