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Effects of Temperature on the Development of Pseudothecia of Venturia inaequalis. David M. Gadoury, Graduate Extension Assistant, Department of Botany and Plant Pathology, University of New Hampshire, Durham 13824. W. E. MacHardy, Associate Professor, Department of Botany and Plant Pathology, University of New Hampshire, Durham 13824. Plant Dis. 66:464-468. Accepted for publication 30 July 1981. Copyright 1982 American Phytopathological Society. DOI: 10.1094/PD-66-464.

The ascigerous state of Venturia inaequalis was studied under laboratory and field conditions to determine the effects of temperature on the development of pseudothecia. Leaves were incubated at 4, 10, 15, and 20 C. Pseudothecia increased in diameter most rapidly at 10 C, but the number that formed per unit of leaf area was inversely proportional to temperature. Measurement of pseudothecial growth and productivity in field-stored leaves yielded similar results. Early leaf fall was associated with increased leaf decay and a resultant reduction in leaf weight and leaf area, but the date of leaf abscission per se did not affect pseudothecial development. Most pseudothecia were formed within 28 days of leaf fall. There was an inverse relationship between temperature and the number of asci that developed per pseudothecium. In pseudothecia from field-stored leaves, the mean number of asci per pseudothecium rose in early spring as asci matured and then declined as empty asci disintegrated. The thermal requirement of pseudothecia shifted during winter from 10 C for early diameter increase to 20 C for ascospore maturation. When the cumulative percentage of matured ascospores was plotted against time, the curves approximated cumulative normal distributions. There was a linear relationship between the probit of matured ascospores and degree day accumulation beginning with the date of the first discharge of ascospores.

Keyword(s): apple scab, disease management.