Severity of late blight tuber infection caused by US-1 and US-8 genotypes of Phytophthora infestans in 12 potato cultivars

Twelve potato cultivars (cvs. AC-Novachip, Atlantic, Chieftain, Kennebec, Ranger Russet, Red Pontiac, Russet Burbank, Russet Norkotah, Shepody, Snowden, Superior, and Yukon Gold) were assessed for their response to inoculation with isolates of US-1 and to US-8 genotypes of Phytophthora infestans. All cultivars (with the exception of cv. Yukon Gold) developed significantly more surface necrosis due to US-8 (20% to 87% surface area necrosis) than US-1 (13% to 46% necrosis). Penetration into the tuber tissue, measured as average depth of interior necrosis of stem, side, and eye regions, also revealed significant differences in the ability of the two genotypes to become established in internal tuber tissues. The cvs. AC-Novachip, Kennebec, Red Pontiac, Russet Burbank, and Russet Norkotah were penetrated more deeply by US-8 than US-1. The other cultivars were penetrated equally by isolates of both genotypes of the pathogen. These results indicate that introduced US-8 populations of P. infestans are more aggressive on tuber tissue than US-1 populations and that most commercially grown cultivars are highly susceptible to infection by US-8.