Role of Microsclerotia as Primary Inoculum of Microdochium panattonianum, Incitant of Lettuce Anthracnose. C. L. Patterson, Professor Emeritus, Department of Plant Pathology, University of California, Davis 95616. R. G. Grogan, Professor Emeritus, Department of Plant Pathology, University of California, Davis 95616. Plant Dis. 75:134-138. Accepted for publication 3 July 1990. Copyright 1991 The American Phytopathological Society. DOI: 10.1094/PD-75-0134.

Microdochium panattonianum, incitant of lettuce anthracnose, was soilborne. Lettuce planted in soil collected from fields 1–4 yr after an epidemic, or in soil artificially contaminated with residue infected with M. panattonianum, developed anthracnose. The pathogen was eliminated from soil, however, by fumigation with methyl bromide. Microsclerotia were observed in cells of lettuce tissues infected by M. panattonianum. Microsclerotia also were produced in vitro on a basal salts agar medium, germinated, infected, and incited anthracnose on inoculated lettuce; optimum conditions for germination and infection were 6 hr of continuous leaf wetness at 20–22 C. Anthracnose occurred on lower leaves of lettuce planted in soil contaminated with microsclerotia placed at 0-, 1-, or 2-cm depths. Thus, microsclerotia produced by M. panattonianum in infected lettuce residue were a primary source of soilborne inoculum that initiated anthracnose epidemics.