Development of an effective screening method for partial resistance to Alternaria brassicicola (dark leaf spot) in Brassica rapa
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In order to develop a method to measure resistance to Alternaria brassicicola (cause of dark leaf spot disease) in Brassica rapa, the effects of inoculum concentration, leaf stage, leaf age and incubation temperature of inoculation on infection were studied under controlled conditions using several B. rapa genotypes. Three inoculation methods (cotyledon, detached leaf and seedling inoculation) were evaluated for this purpose. The detached leaf inoculation test was the most suitable for screening B. rapa genotypes because clear symptoms were observed on the leaves in less than 24 h, and there was a significant positive correlation between the results from the detached leaf inoculation test and the seedling inoculation test, an established method considered to yield reliable results. In addition, it was very easy to screen plants for resistance on a large scale and to maintain standard physical conditions using detached leaves. For successful infection, inoculum concentration should be adjusted to 5 × 104 conidia ml−1, and incubation temperature should be between 20 °C and 25 °C. The 3rd/4th true leaves from 30 day-old plants were optimal for inoculation. In a screening test using 52 cultivars of B. rapa, the detached leaf test effectively discriminated between various levels of partial resistance among cultivars. As a result, we identified two cultivars, viz Saori and Edononatsu, as highly resistant and five cultivars, viz Tokinashi Taisai, Yajima Kabu, Purara, Norin-F1-Bekana and Tateiwa Kabu, as having borderline resistance.
Keywordsinoculum concentration incubation temperature inoculation technique leaf position leaf age rapeseed
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The authors are grateful to the seed companies of Japan, Gene Bank of the National Institute of Agrobiological Resources, Japan, and Bangladesh Agricultural Research Institute, Bangladesh, for providing seeds of Brassica rapa. The authors sincerely thank Dr. Y. Sato of Toyama Prefectural University, Toyama, Japan and, Dr. T. Shirakawa of the National Institute of Vegetable and Tea, Tsukuba, Japan, and Gazi M. Mohsin from East West seed (Bangladesh) Ltd., Bangladesh, for their valuable suggestions. The first author is supported by a scholarship from the Ministry of Education, Culture, Sports, Science and Technology, Government of Japan (Monbukagakusho-MEXT).
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