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, R anger Russet, Red Pontiac, Russet Burbank, Russet Norkotah, Shepody, Snowde n, Superior, and Yukon Gold) were assessed for their response to inoculatio n with isolates of US-1 and to US-8 genotypes of Phytophthora infestans. Al l cultivars (with the exception of cv. Yukon Gold) developed significantly more surface necrosis due to US-8 (20% to 87% surface area necrosis) than U S-1 (13% to 46% necrosis). Penetration into the tuber tissue, measured as a verage depth of interior necrosis of stem, side, and eye regions, also reve aled 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 suscepti ble to infection by US-8.