Crop loss assessment and chemical control of rice [Oryza sativa] sheath blight disease
1989
Arunyanart, P. | Surin, A. | Dhitikiatpong, R. | Rajanahusdin, W. | Disthaporn, S. | Hevel-Rolf, B. (Department of Agriculture, Bangkhen, Bangkok (Thailand). Thai-German Plant Protection Programme)
Crop loss caused by sheath blight disease in rice was studied under conditions of natural infection in a farmer's field. Disease severity and incidence as well as grain yield were determined in 50 plots of 2 m2 per field. Absolute as well as relative lesion height were shown in two seasons to be closely related to yield reduction while disease incidence predicted crop loss with a non-sufficient statistical precision. The obtained damage function was utilized to calculate a tentative control threshold. According to the results of two seasons it will be profitable to spray when sheath blight severity has exceeded 12 cm lesion height or 18 relative lesion height. The uniformity of the relative position of the different leaves enables the farmer to sample relative sheath blight lesion height in a simple way, namely in terms of leaf position reached. In field studies on chemical control of sheath blight highest net returns were obtained with validamycin applied once at 60 days after sowing when lesion height was about 13 cm. The same treatment associated with a higher sheath severity before application resulted in a slightly lower economic benefit than a triple spraying of benomyl. In order to be able to control the disease efficiently with a single application, monitoring should be started in the late vegetative stage.
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