ABST ACTIn a 5-year study of common rootrot of wheat in rotations at .Indian g"ta, S"it "t"h&;;, disease ratings were recorded three times -each crop r"rrori r"a isolations were made

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LEDINGMAM, R. J. 1970. Survival of Cochliobol~is sarivlrs conidia in pure culture and in natural soil at different relative humidities. Can. J. Bot. 48: 1893-1896. Conidia of Cochliobol~is sariv~is cultured on sterile wheat straw were nearly 100% germinable after 52 months when maintained at relative humidity of 50% or lower. At higher relative humidity, spore longevitv was nli~ch reduced. Increased temperature decreased spore longevity. Co~nparable results were obtained in natilral soil.

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X laboratory tnethod is described for deterriiirli~ig the survival of Iiel~~zirzLl~o-sporiunz sativu~n spores in soil. Xri oil-water emulsion co~itai~ling the spores is mixed with potato dextrose agar supplenle~lted with molasses; tnicro.;cope slides arq. then dipped into the misture, incubated, and esa~nirled for spore germ~nation. ,\pplication of the method showed that viability of spores in '1 dry soil did not decline over a 9-niorith period whereas survival of spore-; in saturated soil dropped ~narkedly in the s~ilne period. Inter~nedi~itc nloisture levels had intermediate eSfects on the spores. Stuciics on spores obtnined directly from cultivated fields indicated little mort,llity fro111 fall to spring. .I summerfallow year between two wheat crops r e s~~l t e d in ' 1 st~\)stallti,~l decline i l l percentage of viable spores.

Mouldboard ploughing of stubble land in preparation for a second consecutive wheat crop was compared with surface tillage for effect on common rootrot and yield in a study at the Experimental Farm, Swift Current, Sask., during the years 1951–58. Ploughing was found to be effective in reducing the incidence of rootrot in the seedling stage. As the season advanced increasing numbers of plants became infected and by maturity of the crop differences between ploughing and surface tillage frequently had disappeared. In general, yields were not significantly increased by ploughing.

This paper describes a flotation method by means of which spores of Helminthosporium sativum may be recovered from soil. Soil mixed with a small amount of mineral oil is shaken up in water. The emulsion which collects on the surface of the water contains most of the spores originally in the soil. Approximate spore numbers are determined by microscopic examination of aliquots of the emulsion. The technique is also well adapted for obtaining spores from soil for viability studies. It may be useful in comparing the effects of cultural and chemical treatments of soils on spore populations under field conditions.