DEVELOPMENT OF POTATO EARLY DYING IN RESPONSE TO INFECTION BY 2 PATHOTYPES OF VERTICILLIUM-DAHLIAE AND COINFECTION BY PRATYLENCHUS-PENETRANS

Verticillium dahliae isolates assigned to subgroups A and B of vegetative compatibility group 4 (VCG 4A and 4B) were tested for virulence to potato and ability to interact with the root-lesion nematode Pratylenchus penetrans in field microplot and greenhouse experiments. Microsclerotial inocula and the Verticillium-susceptible potato cultivar Superior were used. No differences in virulence were found between isolates from VCG 4A and 4B when plants were grown in soil infested with V. dahliae only. In contrast, plants grown in soil infested with V. dahliae isolates from VCG 4A plus P. penetrans exhibited higher disease severity and had lower tuber yields than plants grown in soil infested with V. dahliae isolates from VCG 4B plus P. penetrans. An increase in the total number of tubers formed per plant was found for plants infected with either VCG 4 subgroup of V. dahliae alone compared with control plants. This increase was due to a larger number of small (diameter <6.4 cm) and medium (diameter between 6.4 and 8.3 cm) sized tubers, while the number of large (diameter >8.3 cm) tubers was similar in all cases. Although total tuber weight from plants was similar regardless of treatment, total weight of large tubers was lower and total weight of medium and small tubers higher from plants infected by either VCG 4 subgroup. These data indicate that V. dahliae isolates from VCG 4A are able to interact synergistically with P. penetrans, while isolates from VCG 4B do not interact with P. penetrans or do so only weakly. Furthermore, although infection with V. dahliae may not necessarily result in lower total tuber weight than in controls, damage may still be significant in that total weight of marketable tubers is decreased.