HomeInternational Rice Research Notesvol. 22 no. 2 (1997)

Method for Detecting Rice Sheath Blight Pathogen in Soil Samples Using Mungbean

N P. Castilla | F A. Elazegui | W M. Lanip | S Savary

Discipline: Agriculture



This technique uses mungbean ( Vigna radiata (L.) Wilczek) as a bait plant to detect propagules of Rhizoctonia solani Kiihn, a soilborne fungus that causes rice sheath blight. The technique involves the collection of soil samples from the field, with each sample consisting of 300-350 g (Fig. 1). Each sample is placed in pots (11 cm diam × 16 cm height) with holes for draining, and each pot filled with soil is placed inside a clean plastic bag to avoid contamination among samples during transport. Soil samples are air-dried for 10 d at 30-33 °C. Extreme temperatures, above 37-40 °C, which cause destruction of soil organisms, must be avoided. After drying to a soil moisture content of about 17-21% (w/w), each sample is then pounded to a particle size of approximately 2-5 mm in diameter and dispensed in the original pots. Mungbean observations are done at 2- to 3-d intervals for 15 d, starting 4 d after sowing, of number of healthy plants, diseased plants, and ungerminated seeds in each pot. Seedlings infected with R. solani have dark brown lesions on the hypocotyl and/or cotyledons. A seedling is considered emerged when its cotyledon appears above the soil surface. Stems of infected seedlings that are more than 7 d old have dark brown lesions near the soil surface. Infected mungbean are soaked for 15 s in 5% calcium hypochlorite, and infected tissues are cut into small sections and placed on petri dishes with potato dextrose agar (PDA with 1 ml L-1 25% lactic acid. After 2 d, R. solani colonies are again plated on PDA. Three 8-mm leaf cuts obtained from third or fourth leaves of 40- to 50-d-old IR72 are placed on water agar (0.5 g agar L-1 distilled water). About 8 mm of a 4- to 5-d-old mycelial plug is placed on the center of each leaf section and incubated at 25-27 °C for 4 d to detect lesions.