M A Y / J U N E 2 0 1 7 POTATO GROWER 23 plants where it produces airborne spores that can move to neighbor- ing plants. The late blight pathogen is favored by free moisture and cool to moderate temperatures. Night temperatures of 50-60o F and day temperatures of 60-70o F are most favorable for disease development. Free water from rain, dew and overhead irrigation sprinkler irri- gation all provide the water neces- sary for pathogen infection and development. Spores develop in 3- 5 days and require 12 hours of free moisture for infection to occur. Lesions on leaves and stems become visible as small flecks within a few days after infection. The lesions expand to water- soaked, gray-green areas on the leaf and sporulate if conditions are favorable. The spores are carried by wind and rain to healthy plants where the disease cycle begins again. A disease cycle can occur every 5-7 days resulting in rapid spread and movement of late blight. Tubers are infected by spores washed from lesions to the soil. Spores germinate and swim to tubers in free water and infect pri- marily the eyes. Tuber infections are characterized by patches of brown to purple discoloration on the potato skin. Cutting just below the skin reveals a dark, red- dish-brown, dry, corky rot. Management Effective control of this disease requires implementation an integrated disease m a n a g e m e n t approach. Late blight is a community dis- ease and effective m a n a g e m e n t requires community m a n a g e m e n t . Methods to help control the disease are listed. • Destroy all cull and volunteer pota- toes. • Plant late blight- free seed tubers. • Do not mix seed lots as cutting can transmit late blight. • Use a seed piece fungicide treatment labeled for control of late blight (current list of fungicides can be found in the NDSU Fungicide Guide PP-622). Recommended seed treatments include Revus, Reason and man- cozeb. • Avoid planting problem areas that may remain wet for extended periods or may be difficult to spray (the center of the pivot, along powerlines and tree lines). • Avoid excessive and/or night- time irrigation. • Eliminate sources of inoculum such as hairy nightshade weed species and volunteer potatoes. Figure 6. Actively growing late blight can cause white mildew-appearing along petioles. (Photo credit: Dr. Neil Gudmestad, NDSU) Figure 7. Brown, dry lesions following warm and dry weather. (Photo credit: Dr. Neil Gudmestad, NDSU)