Characteristics of Sclerotium cepivorum Berk. sclerotia from a single infested field in Mexico

Sclerotium cepivorum Berk. sclerotia from different positions in a single f ield in Mexico were tested for numbers and viability, germination potential and colonisation by microorganisms. Sclerotia were collected in March 1994 from infection sites within a field located in the Bajio region of Mexico. The samples were derived from several infection sites, each infection site comprised two depth sites (0-10 cm and 11-20 cm). Sclerotia were removed f rom infested soil and Petri dishes were filled with moist soil and covered with nylon gauze. Firm sclerotia were transferred to the gauze and the lid replaced. The dishes were transferred to plastic boxes containing DADS, inc ubated at 15 degrees C. Germination was recorded after 5, 10 and 15 days. Up to 572 firm sclerotia were produced (numbers of firm sclerotia/30 gram d ry soil) in the field in samples from 0-10 cm depth and 72-100% survived af ter 8 months burial in soil. The viability of apparently healthy sclerotia was variable and never exceeded 86% when assessed after 4 months; the viabi lity of this sample had declined to 19% by 12 months. The viability of a se cond sample was 47% after 4 months, rose to 100% by 8 months and then decli ned to 67% after 12 months. Colonisation by microorganisms, particularly fu ngi and bacteria was peaked at 64% after 4 months, but decreased to 3% afte r 12 months. Sclerotial germination varied and appeared to follow 6 different patterns o ver the 12 months. In some samples germination was much less after 12 month s than at 4 or 8 months. However, the high rates of loss of field-produced sclerotia after 8 months burial in soil, together with the novel patterns o f germination of surviving sclerotia, suggest that other types of behaviour may occur.