Association between the potato cyst‐nematode, Heterodera rostochiensis Woll., and Verticillium dahliae Kleb. in the early‐dying disease of potatoes

SUMMARY Investigation of a possible association between Verticillium dahliae and H. rostochiensis (pathotype E, British notation) was based on field observations and an examination of disease development in single‐stemmed potato plants grown in pots. An association was found in the distribution of the nematode and the fungus in the field, and the disease was far more severe with a combined infection than with either pathogen alone. Studies on leaf‐area development and yield reduction suggest there is synergism between fungus and nematode, the reductions produced by the combined infection exceeding the sum of those produced by either pathogen alone. Fungal mycelium and the extent of host colonization by V. dahliae were greatly increased by the presence of the nematode. The possible benefits to V. dahliae in the fungus‐nematode complex are discussed.