Influence ofGliocladium virenson Germination and Infectivity of Sclerotia ofSclerotium rolfsii

Abstract

A system was developed for: 1) testing the ability of sclerotia of Sclerotium rolfsii to infect snap bean plants or hypocotyls after their incubation in soil amended with Gliocladium virens (strain G1-3); 2) relating infectivity to colonization of sclerotia by the antagonist; and 3) evaluating germination of antagonist-infected sclerotia on agar. Sclerotia were recovered from soil and plated (with or without surface treatment with 1% sodium hypochlorite) on a medium selective for Sclerotium to test for germination and on modified TME medium to determine the extent of sclerotial colonization by G. virens. Sclerotia recovered from soil were placed on 9-day-old snap bean plant hypocotyls cut into 9-cm sections. Hypocotyls were incubated at 26 C for 3 days and examined for lesions developing from germinating sclerotia. Infectivity of sclerotia also was assayed by placing two sclerotia 2 cm below the soil surface in contact with hypocotyls of snap bean plants growing in nonsterile field soil in the greenhouse. Positive correlation was found between the percentage of colonization of sclerotia by G. virens in soil and reduction of infectivity and germination when strain Sr-1 of S. rolfsii (small sclerotia, about 1.0 mm in diameter) was used. Rapid loss of viability and infectivity of sclerotia of strain Sr-1 (within 3 days in soil) was observed even at a low concentration of strain G1-3 (3 .times. 103 colony-forming units [cfu] per gram of soil). In contrast, sclerotia of strain Sr-109 (sclerotia 1.5-2.0 mm in diameter) recovered from soil amended with 3 .times. 103 cfu of G1-3 per gram of soil were about 60% infective at 32 days. Although 100% of sclerotia of strain Sr-3 of S. rolfsii (sclerotia > 2.5 mm in diameter) from soil amended with G1-3 were colonized by the antagonist at all times, their germinability was not reduced after 24 days and only slightly reduced after 32 days. Although all sclerotia of Sr-3 were germinable, only 73 and 42% were infective after 24 days at 6 .times. 103 and 9 .times. 103 cfu of G1-3 per gram of soil, respectively. Sclerotia of the three strains of S. rolfsii tested from control soil were 100% viable and from 80-100% infective, depending on the strain.