Bacterial Wilt or Brown Rot (bacterium – Pseudomonas solanacearum): Stems discolor (at first only on inside) and plants wilt and die. Tubers with a dark, vascular ring may decay. Avoid infested fields and plant seed from northern states. Discard tubers with dark eyes or with sticky ooze on surface.
Blackleg (bacterium – Erwinia carotovora subsp. atroseptica): The lower parts of affected stems develop an inky black , slimy , foul-smelling , soft rot. Leaves near tops of vines curl upward and become stunted. Aerial tubers usually form in leaf axils. Severely affected plants die. Tubers from affected plants show black stem-end discoloration and sometimes a severe black soft rot in storage. Plant certified, healthy seed in well-drained soil. Cut seed tubers with a disinfected knife. Treat seedpieces and cure properly before planting.
Early Blight (fungus – Alternaria solani): Mycelium and spores of Atlernaria survive in crop residue. Frequent rain and heavy dews are needed for heavy infection. First symptoms are oval to angular dark-brown to black target spots on the leaves (See Photo). Spots often have a yellow area around them. The lowest or oldest leaves are infected first. Leaves are often killed and yields are reduced. Tuber infection may occur with brown-black sunken spots, from to inch in diameter. The underlying tissue shows a brown, corky, dry rot more than inch deep. Control with 2 year rotation, good fertility and foliar fungicides [see table below].
|Early and Late Blight Fungicides:|
|Chlorothalonil||Chlorothalonil + sulfur|
|Metalaxyl + chlorothalonil||Metalaxyl + mancozeb (late blight only)|
Late Blight (fungus – Phytophthora infestans): Water-soaked spots enlarge rapidly and turn brown to black. Very humid conditions may produce white mold on the underside of the leaf. Under cool, wet conditions blight may attack petioles and stems, ruining a field in a few days. Blight spores can infect tubers at harvest or while in the ground. Late blight has been verified in the Winter Garden, but has not been reported on the High Plains. Some of the isolates were shown to be metalaxyl resistant. Use fungicide applications [see table above] and delay harvest about 2 weeks until vines are killed by chemical treatment.
Pink Rot of Tubers (fungi – Phytophthora parasitica and P. cryptogea): Diseased tubers are spongy and initially discolored around the point of stolon attachment. Later, they become discolored around the buds and lenticels. The internal tissues appear cream-colored when cut, but turn salmon pink after 15 – 20 minutes. They gradually become darker, turning black after about 1 hour. Diseased potatoes have been observed in the Texas High Plains primarily in furrow irrigated fields with clay soils. Control includes selecting sites with good soil drainage and avoiding excessive irrigation late in the growing season.
Ring Rot (bacterium – Corynebacterium sepedonicum): Resembles brown rot, but stem is not discolored. Use certified seed from areas where ring rot is not permitted.
Scab (fungus – Streptomyces scabies): Rough, corky areas that may be round or irregular occur on tubers. Injury does not extend far into the potato but the appearance is objectional. Scab is less prevalent in acid soils than in alkaline. Avoid alkaline soil amendments such as manure, lime, or ashes. Avoid severely infested fields. Apply heavy irrigation at the time of tuber or early root formation. Use a 4 to 6 year rotation under irrigation. A 3 to 4 year rotation is satisfactory under dry land conditions. Use certified seed of scab resistant varieties. Treat seedpieces (See in the chapter on “Seed Treatment“). Wire worm damage, commonly called “deep scab”, is not a disease. It differs in that the pits are deep and tissue has been removed by insect feeding. Sulfur may be used in home gardens to make the soil more acid.
Scurf and Stem Rot (fungus – Rhizoctonia sp.): Small, hard black bodies adhere to the surface of the tuber. Below ground part of stem turns brown. Aerial tubers are sometimes formed. Rotate crops. Use certified seed. Treat seed as for scab.
Curly Top, Several Mosaics, Leaf Roll, Spindle Tuber (viruses): Plants may be stunted and off-color. Foliage is mottled or leaflets tend to roll up. Tubers are often small, and in case of spindle tuber, elongated. Several viruses can infect potato without causing noticeable symptoms. However, yields can be reduced drastically. Use of seed certified to be true to variety and free of disease agents is the best way to control tuber-borne viruses. Control insects.
Spotted Wilt (virus – Tomato Spotted Wilt Virus): This virus is vectored by thrips insects. Early symptoms are dead spots on upper leaves and death of the upper plant parts. Early leaf symptoms may somewhat resemble early blight, but spotted wilt symptoms will usually be in the top of the plant rather than on lower leaves. Plant seed certified to be free of disease agents. Control broadleaf weeds in and around fields 4 to 5 weeks before planting.
Other Diseases: Potatoes are damaged severely by other diseases including Verticillium Wilt, Fusarium Wilt, Charcoal Rot, Southern Blight, Root Knot Nematodes, and Aster Yellows (also called Purple Top).