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Title: Powdery Scab

item Brown, Charles - Chuck

Submitted to: Potato Country USA
Publication Type: Popular Publication
Publication Acceptance Date: 5/15/2007
Publication Date: 7/10/2007
Citation: Nitzan, N., Johnson, D., Batchelor, D., Brown, C.R. 2007. Powdery Scab. Potato Progress. July/August 2007, pp 6-7.

Interpretive Summary: Many diseases of potato reside in the soil and invade the plant through the roots. One of these is powdery scab. It gets its name because is causes blemishes on the tuber that look like scabs. If rub the scab a powder is released, thus the name. Powdery scab is caused by a fungus which lives in the soil. The fungus can also be on the skin of tuber seed pieces. The fungus prefers cool temperatures and moist conditions. It attacks the roots and tubers, but tubers of russeted varieties are usually not blemished to the extent that they will be unmarketable. The root damage is, however, a serious matter. The roots do not develop normally, but rather have pruned almost singed appearance. Field with powdery scab have lower total yield and are lacking in the larger size tubers that are desired in mixture with smaller tubers to make varying size French fries. Growers trying to achieve size incentives specified in their contracts will lost the payment and suffer a profit squeeze. There are no known management protocols. None of the agricultural chemicals have an affect on powdery scab. Hence the search for resistance is the best course of actions. We have found resistance to root galling in some varieties, “Summit Russet,” and “Tarago,” an Australian variety. In addition several breeding lines show resistance. Incorporating resistance will increase the profitability of the potato crop and obviate the need to apply chemicals to control powdery scab.

Technical Abstract: Powdery scab (PS) is incited by a member of the Plamodiophorales, or parasitic slime molds, Spongospora subterranea. It invades the roots forming multinucleate plasmodia, release zoospores, and forms galls on the roots and lesions on the tuber skin. It is an emerging disease in the Pacific Northwest, and has become a major production factor. The roots are infected early on with plasmodia. Galls form on the roots and the tuber skins will have lesion if they are white or red. Russeted skin resists lesion formation, although small lesions do occur. The root growth is curtailed in highly susceptible genotypes with a small overall size and general reduction of functionality. This leads to a reduction in yield and an inability of the plant to produce large-sized tubers. Many fungicides have been tested, including fumigants. Nothing reduces PS to economically acceptable threshold. Host resistance has been found in several varieties and breeding lines. Since russet skin counteracts skin lesions, the primary goal is to reduce compromise of the root system as judged by galling or overall root vigor. A root system impaired by PS results in a plant less able to withstand heat and drought stress. A root system that cannot keep up as the plant grows will limit yield. Genotypes resistant to root damage were tested in three successive years in Washington and Idaho fields infested with PS. A number of breeding lines showed consistent resistance over the course of testing. Varieties “Summit Russet” and “Tarago” show resistance. Incorporating resistance into new varieties will contribute to the sustainability and profitability of the potato industry in the Northwest.