Defence reactions in two potato cultivars following infection with two races of Phytophthora infestans

The kinetics of accumulation of chitinases, glucanases and phytoalexins wer e studied in two potato cultivars, differing in their degrees of vertical a nd horizontal resistance, when infected with two races of Phytophthora infe stans. Tuber disks of cvs Kennebec (susceptible, low horizontal resistance) and Huinkul (tolerant, high horizontal resistance) were infected with eith er race "0" (avirulent) or "C" (complex, race 1.3.5.7.11) of Phytophthora i nfestans. Extracts of tuber tissue (0-7 days) from cv. Kennebec infected wi th race "0" showed a strong increase in phytoalexin production and in chiti nase and glucanase activities when compared with those infected with race " C". These results indicate that race "C" is able to block defensive reactio ns. No significant differences were observed in cv. Huinkul infected with b oth races. The contribution of these reactions to horizontal resistance is unknown, and our results would not support a conclusive role for them in th e interaction.