BP Singh

144 A. Majumdarobserved under a compound microscope, whenever required. Incase of stored seeds, saprophytes such as species of Alternaria,Aspergillus, Mucor, Penicillium and Rhizopus may create hindrancesin observations. Using this method in our laboratory we coulddetect a large number of seed-borne pathogens including thosewhich are not reported from India.Sometimes the growth of seedlings in the blotter testinterferes during recording of observations under stereobinocularmicroscope. To overcome this problem the germination of seedsto be tested by the blotter method may be retarded or stopped byusing a 0.1-0.2% water solution of the sodium salt of2A-dichlorophenoxyacetic acid (the herbicide 2A-D) in place ofpure water for dipping the blotters (Neergaard, 1969). Thisprocedure is widely used in testing the seeds of crucifers for Plzomalinganz, the black rot pathogen.Another way of retarding the germination of seed is by usingdeep freezing method (Limonard, 1966, 1968). In this case seedsafter plating are kept at 20°C for 24 hours, then shifted to -20"Cfor the nest 24 hours and finally incubated at 20"C for theremaining 5 days. Exposure to -20°C kills the germinating seeds,thus, making available enough food material for better growth ofcertain fungi. The method has been used for the detection ofDrechslera, Fusarium and Septaria infection in cereals and grassesbut can also be used for many other seed borne fungal pathogens.Rolled paper towel method can be applied for fungi whichproduce visible symptoms on seedlings.(b) Agar plate method: The principle of recording in the agarplate test is macroscopic examination of fungal colonies. Themethod is qUite reliable and quick but one has to be wellacquainted with colony characters of different fungi on agar media.Seeds are generally pre-treated with mild disinfectant suchas sodium hypochlorite, before plating on agar to prevent overgrowth on the seeds by saprophytic fungi. A soak for 10 min. ina 1% solution of chlorine in water is the usual pretr~atment butin certain seeds it has been observed that the pretreatment from30 seconds to 40 min. has little effect on the test results (Hewett,1979). Other pretreatment chemicals used are Ca (OCl)2' H 20z andHgCl2_' Except with the mercury pretreatment washing of seedsis not necessary. .