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Title: Controlling Powdery Scab: The Breeding Approach

Author
item Brown, Charles
item JOHNSON, D - WASHINGTON STATE UNIV.
item CUMMINGS, TOM - WASHINGTON STATE UNIV.
item BATCHELOR, DALLAS - WEATHER OR NOT, PASCO, WA
item MILLER, JEFF - UNIVERSITY OF IDAHO
item OLSEN, CHRIS - OTHELLO, WASHINGTON

Submitted to: Proceedings Washington State Potato Conference
Publication Type: Proceedings
Publication Acceptance Date: 3/30/2006
Publication Date: 9/15/2006
Citation: Brown,C.R., D.Johnson, T.Cummings, D.Batchelor, J.Miller, C.Olsen. 2006. Controlling Powdery Scab: The Breeding Approach. Proceedings 45th Annual Potato Conference of Washington State Feb 5-7, Moses Lake, WA. pp.5-11

Interpretive Summary: When a disease appears and starts to reduce the quality and yield of crop plant, scientists can modify the environment by applying chemicals to the crop and they can search for resistance. Often resistance is found in distant crop species and in advanced breeding lines or existing varieties. Powdery scab is a disease of potato that has gone from a rarely occurring problem to a geographically widespread and major production factor. It is caused by a fungus that inhabits the soil. The fungus is mobile and can swim in the soil water. It is more damaging in fields which retain water due to soil composition and/or receive perhaps too much water from irrigation. No chemical treatments that have been tried are effective in controlling the damage at an acceptable level. However, we have found resistance in wild and cultivated potato to the root damage caused by the fungus. When these genetic materials are grown in affected fields they show very little galling and the root systems are large and healthy. This is important because the main effect on the varieties grown in the Columbia Basin is to lower overall yield and especially to reduce the size of the tubers. Often the contracts offered to growers for processing potatoes specify incentives for tuber size. The failure to achieve a sufficient percentage of the larger sizes can reduce the contract payments to the point that the grower does not make a profit. Development of new varieties will result in a healthier business profile for potato growers in powdery scab affected areas.

Technical Abstract: Powdery scab is a serious disease of potato caused by the fungus Spongospora subterranea. S. subterranea is a member of the Plasmodiophorales, also known as the parasitic slime molds. The primary diagnostic sign of the disease are cystosori, spongelike aggregates of cysts or resting spores. The motile life stage appears as zoospores with two flagella, which explains the correlation of abundant soil water to level of damage. The fungus is also the vector for Potato Mop-Top Virus, a quarantine virus in the United States. Powdery scab can damage the skin of the tuber to the point that it is totally unmarketable for fresh market or processing. However, russet skin varieties are usually only lightly blemished on the skin. In contrast, the root systems can be heavily galled resulting in a smaller and less efficient root. Germplasm of potato has been screened for three years in infested fields in Washington State and Idaho. The focus has been to determine if significant resistance to root-galling exists in advanced or primitive germplasm. The results have indicated that there is significant root-galling resistance that has been stably expressed in two widely distant test locations, over two to three years. The resistant clones include several that are also root-knot nematode (Meloidogyne chitwoodi) resistant. The common genetic thread that links all resistant materials is that there are two backcrosses to the variety Summit Russet in the pedigree. Summit Russet was not suspected of being powdery scab resistant, but was found to have a high level of resistance to root galling in follow-up tests. There are no known chemicals or fumigation treatment to control powdery scab. It is easily introduced to fields on scabby seed tubers. The development of root-galling resistant cultivars will increase the profitability of potato production in areas where the pathogen has established.