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Interactive Effects of Ozone and Powdery Mildew on Pea Seedlings. H. Rusch, Former graduate research assistant, Department of Plant Pathology, Cornell University, Ithaca, NY 14853; J. A. Laurence, Plant pathologist, Boyce Thompson Institute for Plant Research at Cornell University, Ithaca, NY 14853-1801. Phytopathology 83:1258-1263. Accepted for publication 7 June 1993. Copyright 1993 The American Phytopathological Society. DOI: 10.1094/Phyto-83-1258.

We assessed the effects of ozone and powdery mildew infection on plant growth, ozone injury, colonization of leaves by the fungus, and pisatin content of pea (Pisum sativum) seedlings. Seedlings of cultivars Alaska and Bounty grown in the greenhouse were moved to controlled-environment growth chambers and exposed to ozone concentrations of 0.00, 0.06, and 0.12 ?l L1 for 6 h/day (1 ?l L1 = 1,960 ?g m3 at standard temperature and pressure). Fumigations were performed for 5 days before, after, or both before and after inoculation with conidia of Erysiphe polygoni f. sp. pisi, the causal agent of pea powdery mildew. Ozone at 0.12 ?l L1 suppressed growth in both cultivars only in the preinoculation fumigation treatment, indicating a protective effect of powdery mildew infection, which by itself did not affect dry weight. Bounty was more resistant to infection by E. p. pisi than Alaska and was also less sensitive to foliar ozone injury. Colonization of leaves by the fungus resulted in suppressed foliar ozone injury in both cultivars. Both preinoculation and postinoculation exposure to ozone at 0.12 ?l L1 significantly suppressed fungal leaf colonization. The apparent increase in resistance indicated a plant defense mechanism induced by the ozone exposure. Pisatin, a pea phytoalexin, was not detected in leaf tissue of noninoculated plants that were exposed to ozone, however, indicating that pisatin probably is not involved in this increased resistance.

Additional keywords: air pollution, pollutant-parasite interaction.