Bob Mulrooney, Extension Plant Pathologist; bobmul@udel.edu
Adapted for Delaware from an article written by Dr. Steve Rideout, VPI &SU
Causal Organism
Late blight of potato is caused by the airborne fungus (Oomycete) Phytophthora infestans. Late blight of potato is sporadic on the Eastern Shore of Virginia, but can be devastating if conditions favoring the disease persist. It has not been a production problem in Delaware for many years, although two fields last season had limited infections that were controlled. Disease is favored by moderate temperatures (60-80°F) with excessive rainfall or dews leading to high leaf moisture. Also of note, late blight of potato was the disease that caused the Irish Potato Famine in the 1840s which led to the immigration or death of over 3 million Irish. The late blight pathogen can also parasitize tomatoes.
Symptomology
Infected potato leaves show ‘greasy’ lesions that usually originate from the tip of the leaves (Figures 1). During periods of high moisture gray sporulation can be seen on infected leaves. If infection persists or becomes systemic, tubers may become discolored exhibiting black and greasy lesions (Figure 2). Infected tubers may also transmit the disease to subsequent crops if they are used as seed pieces. In severe infections, complete defoliation can occur if appropriate disease control measures are not employed.
Control
Use certified seed pieces to ensure that you are not transmitting late blight. Prior to disease appearance, growers should utilize a protectant fungicide (i.e. chlorothalonil or mancozeb) once sprays are either deemed necessary by the WISDOM prediction model or if the disease is present within the region. Once the disease is either present on Delmarva, surrounding areas or within your fields, systemic fungicides should be used for disease suppression. Systemic fungicides recommended for late blight control include: Curzate, Forum, Gavel, Omega, Presidio, Previcur Flex, Ranman, Revus Top, Revus, Super Tin, and Tanos. As always, follow pesticide labels for rates and usage.
