Increased Take-all of Wheat with Direct Drilling in the Pacific Northwest

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Abstract
Take-all, caused by Gaeumannomyces graminis var. tritici, occurred more frequently or more severely on consecutive wheat crops seeded directly (no-till) into the undisturbed stubble than on wheat seeded into plots that were prepared by moldboard or disk plowing. Increased take-all with direct drilling was found at each of 3 locations in Washington (where climate, soil type, and management all were different), in two seasons, and in both spring and winter wheat. Differences in soil temperature and soil moisture between tilled and no-tilled plots could not account for the greater take-all with direct drilling. When soil cores (15 cm diam, 17 cm deep) from a no-tilled wheat plot (naturally infested with G. graminis var. tritici) were either left undisturbed or given simulated tillage, planted to wheat, and incubated under the same conditions, take-all was greatest in the undisturbed cores. Differences in nutrition of the host plant likewise could not explain the effect; fertilizing plots with a mixture of N, P, K, S and trace minerals suppressed take-all more than fertilizing with only N and S, but suppression was similar in tilled and no-tilled plots. Several times more infested debris was recovered by sieving from no-tilled plots than from tilled plots just before sowing the next crop, and infested fragments from the no-tilled plots generally were larger. When plots were fumigated with methyl bromide and the take-all pathogen then introduced to provide the same amount of inoculum in all plots, disease severity was the same whether the plots were tilled or not. Disease with direct drilling apparently was increased because of more infested debris and because the inoculum source was ideally positioned for infection of the wheat crop.

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