Effects of Interrupted Light on Plant Disease

Abstract

RESULTS of experiments with scab disease on MM 109 apple rootstock shoots showed a marked lack of reproducibility, even when done in a glasshouse with some control of light and humidity as well as temperature (the mean visible light intensity was controlled to 1,300 ± 200 cal per cm² per month¹, ambient temperatures to 20 ± 2° C during the day and 15 ± 2° C at night with corresponding relative humidities of 70% ± 5% minimum and 90% ± 5% maximum). The disease system seemed to be very susceptible to residual fluctuations in the environment. Although this situation could be partly attributed to mechanical failures, examination of results and sunlight radiation records collected over a period of years indicated that erratic and short reductions in sunlight intensity, due to small cumulus clouds passing during the end of the first week after infection, could account for a considerable part of the irregularity. This study, though stimulated by previous observations on apple scab disease, was done with Venturia inaequalis and Padosphaera leucotricha on MM 109 apple rootstock shoots, and strawberry latent ring-spot virus on cucumber plants, in order to investigate the influence of interrupted light on plant disease generally.