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The interacting effects of light and temperature on spore formation of P. tabacina on tobacco leaves were investigated. The following points indicated that an enzymic build-up of an antisporulant during a wet light period and its enzymic decay over a dry dark period may explain the inhibitory effect of light upon sporulation, and its reversal by darkness. (1) Fluorescent blue light of relatively low photon flux density (3 �7 /lE m - 2 S -1) inhibited sporulation by 99 % at 20oe. (2) Light level and temperature during the sporulation period determined spore yield of the pathogen: at high temperatures (in the range 8-24�C) sporulation was inhibited at low light level, whilst no inhibition occurred at much higher light levels at low temperatures. (3) Preceding dry dark treatments given at 200e considerably diminished the inhibitory effect of light, but not if given at 100e. (4) The diffusion of an inhibitory compound from irradiated to unirradiated areas of detached leaves was demonstrated. (5) The continuing photosynthetic activity of the host in the light at 20oe, and the lack of sucrose following dark periods at lOoe, were not associated with the inhibitory effect of light. The similarity between the role of light in the present system and the role of light in activation and decay of phenylalanine ammonia-lyase is discussed.

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Conidia of Peronospora tabacina, the incitant of tobacco blue mold, attached to infected leaves or suspended in aqueous suspensions remained infective for more than 3 mo at-20 C. Conidia on sporulating lesions survived after six repeated cycles of freezing (-20 C) and thawing (25 C), whereas conidia suspended in water survived after two such cycles. Conidia on sporulating lesions stored at 5 C (100% RH) survived more than 34 but less than 57 days. The possible epidemiologic implications of this feature of the pathogen are discussed.

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