Application of a New Laboratory Method for the Determination of the Survival of Helminthosporium Sativum Spores in Soil

A laboratory method is described for determining the survival of Helminthosporium sativum spores in soil. An oil–water emulsion containing the spores is mixed with potato dextrose agar supplemented with molasses; microscope slides are then dipped into the mixture, incubated, and examined for spore germination. Application of the method showed that viability of spores in a dry soil did not decline over a 9-month period whereas survival of spores in saturated soil dropped markedly in the same period. Intermediate moisture levels had intermediate effects on the spores. Studies on spores obtained directly from cultivated fields indicated little mortality from fall to spring. A summerfallow year between two wheat crops resulted in a substantial decline in percentage of viable spores.