WHEAT PRODUCTION IN AN ARID ENVIRONMENT .2. ROLE OF SOIL PATHOGENS

In the Negev Desert agro-system, wheat (Triticum aestivum L.) yields under fallow management practices have been shown to be 84% greater than those from continuous wheat. The higher yields for the fallow management were achieved without improved soil water storage, and soil nitrogen was not a limiting factor in either system. Yields were similarly increased after several years in continuous wheat, by chemical control of soil pathogens. The objective of this study was to identify the dominant soil pathogens in the different management systems and to assess their influence on grain production. A 19-month fallow period or a chemical soil treatment significantly reduced the population of two dominant nematodes, Pratylenchus mediterranea and Heterodera avenae. The nematicide aldicarb, which was used to differentiate between fungicidal and nematicidal effects of the broad-spectrum biocide metham-sodium, increased wheat yield significantly in one out of four years at Gilat, and in two out of four years at Magen. These results emphasize that, although the parasitic nematodes reduced yields in some years, no clear and consistent correlation between nematode population and grain-yield could be shown. The root-rot fungus Cochliobolus sativus was the dominant fungal pathogen in the soil, but frequency of recovery of the fungus from sub-crown internodes was not related to yield differences between management systems. Under irrigation, the pathogen Gaeumannomyces graminis var. tritici (the cause of take-all) became dominant, and greatly reduced grain-yields. When the disease was controlled either chemically or biologically, yields returned to the level prior to disease onset. No symptom of take-all was found in the same years and treatments under dryland conditions. It is concluded that the high yields achieved with the fallow wheat management are a result of improved soil sanitation, which justifies an effort in developing pest and disease-tolerant/resistant cereal varieties for continuous wheat cropping systems.