VIEW ARTICLE

Ecology and Epidemiology

Development and Inoculum Potential of Peronospora tabacina in the Fall Season. Joseph Rotem, Department of Plant Pathology (on leave from Volcani Center, Bet Dagan 50-250, Israel) and Department of Ecology and Climatology, Connecticut Agricultural Experiment Station, P.O. Box 1106, New Haven 06504; Donald E. Aylor, Department of Plant Pathology (on leave from Volcani Center, Bet Dagan 50-250, Israel) and Department of Ecology and Climatology, Connecticut Agricultural Experiment Station, P.O. Box 1106, New Haven 06504. Phytopathology 74:309-313. Accepted for publication 6 October 1983. Copyright 1984 The American Phytopathological Society. DOI: 10.1094/Phyto-74-309.

Development of a Peronospora tabacina epidemic on tobacco at season' s end in Connecticut was studied from mid-September until mid-November. Plant growth, disease incidence, and production and dispersal of inoculum as affected by weather changing from suboptimal to marginal to submarginal were measured. Under these conditions, the epidemic developed in proportion to inoculum pressure. Although the number of sporangia per lesion decreased as the season advanced, most sporangia in the field appeared during late September and early October, when lesions were most numerous. Shortly after, the amount of inoculum and the number of lesions decreased owing to continuing destruction of leaves by frosts. Inoculum disappeared in mid-November, and no sporangia were left for overwintering. The following phenomena were quantified during the epidemic: Most lesions on older leaves were necrotic, and as the season progressed, most lesions on all leaves became necrotic. Necrotic lesions yielded fewer sporangia than chlorotic lesions with or without necrotic margins. Also as the season progressed, all lesions, but especially necrotic ones, became infested by Alternaria sp. Although abundant after nights with free moisture on leaves, sporangia were also present after dry nights. Mechanical spore traps and potted plants usually trapped sporangia in proportion to the number measured directly on leaves of sporulating plants, but when sporulation was low, mechanical traps did not collect spores. Sporangia caught by potted plants during the afternoon and night accounted for about one-third of the daily infections. The frequency of nighttime infections appeared to be impaired by low temperatures late in the season.