Evaluation of a short-day adapted tetraploid potato population with horizontal resistance to Phytophthora infestans under long-day conditions in Northern Maine

Phytophthora infestans (Mont.) de Bary, the cause of late blight, has rapidly overcome major (R) gene resistance in potatoes. A population of short-day adapted tetraploid potatoes with horizontal resistance to late blight was developed at the International Potato Center in Lima, Peru. True seed from this population was obtained from the NRSP-6 Project at Sturgeon Bay, WI for the purpose of evaluating its potential to contribute to the breeding effort for late blight resistance in the United States. In 1996, 552 single hills were planted on Chapman Farm, Presque Isle, ME; only 448 tuberized. In 1997, these clones were planted on Chapman Farm for increase; 129 were saved, 53 failed to tuberize, and 266 were discarded because they were sprouted at harvest. In 1998 and 1999, 69 clones were tested for their reaction to late blight in replicated plots on Aroostook Farm, Presque Isle, ME. Percent infected foliage was estimated three times toward the end of the season and used to compute area under the disease progress curve. Broad-sense heritability for horizontal resistance to late blight was estimated as 0.78 with a 95% confidence interval of 0.64 to 0.86. Using detached leaflet assays, these clones were inoculated with US-8 strains of P. infestans, and the diameter of the lesion was measured 7, 8, 9, 10 and 11 days after inoculation. The correlation between field resistance and the detached leaflet assay was very low (0.18<r<0.24). Some clones from this population are highly resistant to the US-8 strain of P. infestans and represent another source of breeding material for developing late blight resistant varieties in the future. However, very late maturity, short dormancy, poor tuberization and low fertility levels may limit the usefulness of this germplasm in northern USA latitudes.