Growth and differentiation of sporangia and sporangiophores of Psudoperonospora cubensis on cucumber cotyledons under various combinations of light and temperature

The inhibitory effect of light upon sporangial and sporangiophore formation of Pseudoperonospora cubensis on cucumber cotyledons was studied. The following major findings may be pointed out. (a) Light, especially of the blue region of the spectrum, inhibited sporangial production but did not prevent emergence of sporangiophores through stomata. Sporangiophores emerging from leavesexposed to high light level were abnormal, but could further produces sporangia upon transfer to darkness. (b) The inhibitory effect of light upon sporangial formation was temperature dependent. The average rates of inhibition, as compared to dark control, were 18, 84 and 90% at about 15, 20 and 24 °C, respectively. (c) At 10 °C, light did not inhibit, but rather stimulated sporangial production. (d) Preceding dark treatments (PD) of 2 to 4 h greatly diminished the inhibitory effect of light-more so at 20 to 30 °C than at 10 °C, but inhibition was fully restored upon extending the PDT to 6 h. (e) Indirect irradiation, achieved by partial covering of a leaf with an aluminum planchette, reduced sporangial yield of the pathogen on the unirradiated tissue. The possible involvement of a chemical and/or physical antisporulant in the inhibitory effect of light is discussed.