ARS | Publication request: Genotypic Differences in Advanced Breeding Lines for Resistances to Black Dot and Powdery Scab

Submitted to: Proceedings Washington State Potato Conference
Publication Type: Proceedings
Publication Acceptance Date: April 15, 2009
Publication Date: July 17, 2009
Citation: Brown, C.R., Nitzan, N., Johnson, D., Miller, J., Cummings, T., Batchelor, D., Olsen, C., Quick, R.A., Hutson, W.D., Castaneda, R.B. Genotypic Differences in Advanced Breeding Lines for Resistances to Black Dot and Powdery Scab. Proceedings of the 48th Annual Washington State Potato Conference. pp. 5-12.

Interpretive Summary: Black dot is a disease of potato caused by a fungus. It is present as a contaminant on seed potatoes, is a long-time resident of soil and has become a constraint on potato yields in many parts of the US. It is expensive to control chemically requiring numerous applications. The disease involves the rotting of roots and stem parts toward the end of the growth period. It is highly influenced by the environment and can appear and disappear for unknown reasons. Powdery scab is a disease caused by a different fungus. It can blemish the skin of the tuber and cause root rot. It also is ubiquitous as a contaminant on seed tubers and lasts many years in the soil even when potato is not grown. Thus black dot and powdery scab have in common that they are already present in many fields and can easily be introduced because so much of the seed supply is contaminated. Powdery scab is not controllable by any chemical means. Both of theses pathogens are best controlled by host resistance in the potato crop. It is difficult to identify resistance, but the parameters that seem to seem to be the best indicators are the amount of galling on the roots caused by powdery scab and the degree to which the black dot fungus grows up the stem as the plants senesce. We have found germplasm of potato that is resistant to both fungi. So far there are no commercially viable breeding lines, but it is an objective of the breeding program to produce commercializable breeding lines for the industry. It is likely that double resistance to the fungi will reduce yield losses of up to 25 %. This will enhance the profitability of potato production and make it more competitive globally.

Technical Abstract: Black dot is a disease of potato caused by a fungus, Colletotrichum coccodes. It is present as sclerotia contaminating seed potatoes and since the slerotia are viable many years it also almost a permanent soil and has become a constraint on potato yields in many parts of the US. It is expensive to control chemically requiring numerous applications. The disease involves the rotting of roots and stem parts toward the end of the growth period. It is highly influenced by the environment and can appear and disappear for unknown reasons. Powdery scab is a disease caused by a different fungus, Spongospora subterranea, a parasitic slime mold. It can blemish the skin of the tuber and cause root rot. It also is ubiquitous as a contaminant on seed tubers and Powdery scab is not controllable by any means known. Both of theses pathogens are best controlled by host resistance in the potato crop. We have investigated many parameters to score resistance in potato breeding lines. The best indicators so far are the degree of galling on the roots caused by powdery scab and the presence of black dot sclerotia on the stem. In the latter case the degree of advancement apically in the interior and on the surface of the stem correlates with resistance. Furthermore it appears that the ability to produce a large system in the presence of both pathogens is a good indicator of resistance or tolerance. Since both organisms are often present together in production fields it is hard to separate the individual contributions to yield reduction. Yield reduction occurs primarily as a failure to produce larger tubers. This means that the payment from the contractor will be lowered according to specific incentives for tuber size. Germplasm with resistance to both organisms has been found. A common feature is the presence of the variety Summit Russet in the ancestry.