Late blight of potato :

The section showing lemon shaped conidia of phytophthora infestans isolated by scrapping potato leaf from ICAR-NOFRI,Sikkim, as seen under foldscope

Symptoms

Late blight will first appear as water-soaked spots, usually at the tips or edges of lower leaves where water or dew tends to collect. Under moist, cool conditions, water-soaked spots rapidly enlarge and a broad yellow halo may be seen surrounding the lesion (Mohan et al., 1996). On the leaf underside, a spore-producing zone of white moldy growth approximately 0.1 – 0.2 inches wide may appear at the border of the lesion. Under continuously wet conditions, the disease progresses rapidly and warm, dry weather will slow or stop disease development. As conditions become moist and cool, disease development resumes. See pictures of late blight on stem and leaves.