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Influence of Inoculum Composition on the Black Point Disease of Durum Wheat. J. E. Huguelet, Associate Professor, Department of Plant Pathology, North Dakota State University, Fargo 58102; R. L. Kiesling, Professor, Department of Plant Pathology, North Dakota State University, Fargo 58102. Phytopathology 63:1220-1225. Accepted for publication 10 March 1973. DOI: 10.1094/Phyto-63-1220.

Helminthosporium sativum and Alternaria alternata were repeatedly isolated from black-pointed durum wheat and were found to be pathogenic by greenhouse inoculations and using 5,000 - 10,000 conidia/ml as the inoculum and a 48-hr period of high humidity following inoculation. Both organisms, either alone or in combination, caused decreased kernel weight but the decrease was more pronounced when the ratio of the pathogens in combination favored H. sativum. On plants inoculated first with one organism and then 24 hr later with the other, the fungus initially introduced rapidly colonized the wheat heads and restricted the isolation frequency of the second organism. The severity of black point was greatest on the middle to upper part of the spike and on the outer florets of each spikelet. The growth rate of each organism was reduced more than 40% when grown in the presence of the other organism, presumably due to competition for nutrients and not to the production of a diffusible inhibitor.