Competition for Nitrogen between the Take-all Fungus and the Roots of Crop Plants

Abstract

SURVIVAL of the take-all fungus (Ophiobolus graminis) in buried infected cereal stubble is prolonged by an adequate supply of soluble nitrogen from the surrounding soil1,2. If the soil is left in clean fallow after harvesting a cereal crop affected by the take-all disease, then the ammonia and nitrate nitrogen set free by the activities of soil microorganisms will become available to O. graminis in the buried infected cereal stubble, and the survival of the fungus is likely to be prolonged. But in soil under crop, much of the nitrogen thus set free will probably be taken up by the plant roots before it can reach O. graminis. With this possibility in mind, I have recently compared the survival of O. graminis in lots of 250 lengths each 1½ in. long of infected wheat straw buried in fallow soil, and in soil under trefoil, mustard, and oats, respectively. Eight weeks after burial of the infected straws in the soil, the percentage of straws containing viable mycelium of O. graminis (as revealed by a wheat seedling test1) was 68 under fallow, 18 under trefoil, 17 under mustard and 4 under oats.