Light-induced inhibition of sporangial formation of Phytophthora infestans on potato leaves

Phytophthora infestans failed to produce sporangia on infected potato leaves under continuous light conditions. Blue light (λmax = 450 nm) was most inhibitory, while red light (λmax = 650 nm) was ineffective in inhibiting sporangial formation. Low intensity of blue light (3.7 μEinstein m−2 s−1) induced about 85% inhibition. The inhibitory effect of blue light upon sporulation was temperature dependent: it increased with rise in the temperature from 10 °C to 25 °C. 3,3,4-Dichlorophenyl-1,1-dimethyl urea (DCMU), which blocks the Hill reaction of photosynthesis, did not enable the fungus to sporulate on leaves under light conditions. Preceding dark treatments did not remove the inhibitory effect of blue light; this suggested that sporulation of the fungus is not a 'dark induction phenomenon.'