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Light Inhibition of Thallus Growth of Armillaria mellea. James E. Doty, Research Division, Los Angeles State and County Arboretum, Arcadia, California 91006; P. C. Cheo, Research Division, Los Angeles State and County Arboretum, Arcadia, California 91006. Phytopathology 64:763-764. DOI: 10.1094/Phyto-64-763.

Potato-dextrose agar (PDA), when pre-exposed to a light source for 21 days, does not support growth of Armillaria mellea. On fresh PDA, 2 h exposure to light every 24 h during culture, inhibits thallus growth 50%. Different isolates of A. mellea on PDA vary in their sensitivity to light inhibition. Light was less inhibitory to less vigorous isolates and was most inhibitory to isolates normally producing abundant rhizomorphs. Replenishing cultures in light and dark incubators with light and dark-treated media resulted in little inhibition of thallus development under dark incubation, but significant inhibition under light incubation. These results are interpreted as indicating light may directly inhibit thallus development.