![]() Blight infections on tubers usually become visible within the first month of storage, but symptoms may continue to appear throughout the storage season.Ĭultural control Regular scouting for the disease is critical for all types of management strategies. Infected tuber skin at first is discolored brown to purple later there is a brownish dry or wet rot. Lesions, with the occurrence and duration of moisture, may be found in the lower canopy while none are visible in the drier, upper canopy. In cool, wet weather, blight may advance rapidly and ruin an entire field in a few days. Under humid conditions, a ring or surface of white mold may appear on the leaf underside at the lesion's edge. Symptoms Water-soaked spots appear on leaves and stems, enlarge rapidly, and turn brown or black. The disease is less often seen in Idaho and seldom seen in central Oregon or Klamath Falls. Since 1991, yearly outbreaks have occurred in the Columbia Basin of Oregon and Washington. The disease is often found in the Willamette Valley of Oregon and in northwestern Washington. The disease has been found throughout the PNW. Scout areas where water pools in the field, in low spots or wheel tracks, near the center of a center pivot irrigation system, and where irrigation overlaps occur. Fields should be scouted early and continuously, and the use of fungicides is required. A relatively new variety, Premier Russet, is moderately resistant. Varieties Norkotah, Russet Ranger, and Shepody are especially susceptible to late blight tuber rot. The variety White Rose is similar to Russet Burbank in susceptibility. Varieties Norchip, Hilite, Russet Norkotah, Superior, Shepody, and Red LaSoda are more susceptible than Russet Burbank. Nearly all potato varieties are susceptible to late blight. The existence of compatible mating types also brings the risk of new strains that could be more difficult to control. The occurrence of a second mating type allows for the formation of a different survival structure, the oospore, providing an additional means for the fungus to overwinter, this time in the soil. Most isolates of the pathogen are resistant to Ridomil, so its use is not recommended. In the 1990s, isolates resistant to Ridomil (metalaxyl) were found, as was a second mating type that was discovered in the Pacific Northwest. ![]() This can be a very serious problem in storage. Indirect damage occurs when secondary infection, by soft rot bacteria, rots the infected tuber and then spread to other tubers below in the pile. If potatoes with late blight are stored, the blight can directly or indirectly damage tubers. Sporangia and/or zoospores also can infect tubers throughout their development or at harvest or in storage. Many cycles can occur in the growing season. With favorable conditions, the pathogen can complete one cycle in as little as 4 days. At the lesion's margin, new sporangia form through the stomata to continue the disease cycle. ![]() The leaf or stem is colonized quickly, and large portions become necrotic as the microbe advances. Under cool, wet conditions, sporangia can produce swimming spores (zoospores). Sporangia can germinate on a wet leaf or stem and penetrate either directly through the cuticle or a stomate. Rain or irrigation spread sporangia to healthy tissue. Spores (sporangia) form on infected culls, volunteers, or crop plants. Cause The fungus-like microorganism, Phytophthora infestans, survives in tubers in cull piles or on tubers left in harvested fields, on seed potatoes, or is blown in as spores with rainstorms.
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