Look for the spreading of brown to black lesions on leaves surrounded by a thin halo of brighter pigmentations. Green fruits are more vulnerable to infection and could become marble-brown in appearance and the fruit becomes hard. In high humidity conditions, gray to white “fluff” will appear around the edges of darker lesions.
Late blight is a fungal disease, has the potential to destroy entire fields, and favours periods of heavy rainfall and high humidity.
Prevention spraying (pre-visible symptoms) is your main strategy. When conditions appear favorable for late blight or such conditions are expected in the upcoming days, spray applications of fungicides (based on one or more of the following generic active ingredients) in 5 – 7 days intervals: chlorothalonil, cymoxanil, dimethomorph, iprodione, azoxystrobin, propamocarb HCL, mandipropamid, copper hydroxide, and sulfate.
*Never perform two consecutive iterations with the same active ingredients.
It is extremely difficult to manage late blight organically. Organic growers should know the risks they are taking and do all they can in the cultural aspect. Though, some copper-based fungicides are considered organic and can be utilized.