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Factors Affecting Saprophytic Colonization of Wheat Straw by Fusarium roseum f. sp. cerealis ‘Culmorum’. R. James Cook, Research Plant Pathologist, Crops Research Division, ARS, USDA, Washington State University, Pullman 99163. Phytopathology 60:1672-1676. Accepted for publication 25 June 1970. DOI: 10.1094/Phyto-60-1672.

Saprophytic colonization of clean, bright, untreated wheat straw by Fusarium roseum f. sp. cerealis ‘Culmorum’ was severalfold greater than that of weathered straw buried in the same soil under the same conditions. Colonization of straw by Culmorum decreased as the period of exposure of the straw to weathering prior to burial increased. Weathered straws contained dematiaceous fungi, aspergilli, penicillia, and other airborne saprophytes. When weathered straws were autoclaved prior to burial, saprophytic colonization by Culmorum increased. Apparently, the establishment of saprophytic fungi in straw prior to burial prevents colonization by Culmorum after burial, and this accounts for the lack of saprophytic colonization of wheat straw by Culmorum in the Pacific Northwest. Effects of soil temperature and water could not explain the limited saprophytic colonization by Culmorum in the field. Colonization was maximal at 20-25 C, but occurred in significant amounts at 4 C. Colonization was maximal at water potentials of –50 to –60 bars (relatively dry soil), but occurred at –0.5 to –1 bar. Colonization was prevented only under extreme conditions (i.e., 1 C or soil nearly air-dry). Culmorum populations 10² and 10³ propagules/g of soil were the minimum levels to achieve colonization of straws in the laboratory. The limited saprophytic colonization of straw by Culmorum in the field cannot be attributed to insufficient inoculum of the fungus, since soils of several naturally infested fields contained Culmorum populations in excess of 10³ propagules/g of soil. Unweathered straws buried and incubated in these soils in the laboratory were 10-75% occupied by Culmorum within 1 week.