Interaction of Phoma terrestris and Soil Moisture Level on Yield of Two Onion Cultivars Differentially Susceptible to Pink Root. P. M. Coleman, Department of Fruit and Vegetable Science, New York State College of Agriculture and Life Sciences, Cornell University, Ithaca, NY 14853. L. A. Ellerbrock, and J. W. Lorbeer. Department of Fruit and Vegetable Science, and Department of Plant Pathology, New York State College of Agriculture and Life Sciences, Cornell University, Ithaca, NY 14853. Plant Dis. 76:1213-1216. Accepted for publication 16 June 1992. Copyright 1992 The American Phytopathological Society. DOI: 10.1094/PD-76-1213.

The effect of Phoma terrestris on yield of the onion (Allium cepa) cultivars Kodiak (tolerant) and Paragon (susceptible), using onions grown outdoors in flats, was investigated. Similar experiments in the greenhouse included a third factor, drought stress. When onions were grown in organic field soil, the addition of P. terrestris increased average levels of pink root at harvest from 40% (control) to 52% (amended) in the greenhouse. However, yield did not decrease. When onions were grown in greenhouse mix, the addition of P. terrestris increased average levels of pink root at harvest from 0% (control) to 71% (amended) in the flats and from 0% (control) to 62% (amended) in the greenhouse. The yield of onions in the greenhouse mix was decreased in the amended treatments in both cases, but in the flats, the yield of Kodiak was decreased more than that of Paragon. In amended treatments in all three experiments, Paragon had more pink roots at harvest than Kodiak. However, the yield of Paragon was higher than that of Kodiak in two of the three experiments, and their yields were equal in one experiment. Drought stress decreased the yield in both greenhouse experiments, but no interaction between drought stress and inoculum levels occurred. These studies indicate that onion yield can be decreased at high levels of P. terrestris.