Does Coinoculation with Different Verticillium dahliae Genotypes Affect the Host or Fungus?

Inferences about Verticillium dahliae are often deduced from experiments where hosts are inoculated with one isolate. The assumption that the outcomes from these experiments scale with V. dahliae diversity is untested. The objectives of this research were to test the hypotheses that (i) coinoculation with combinations of isolates affects plant biomass, disease expression, and fungal colonization; and (ii) hosts select for the specific isolates. Potato, mustard, and barley plants were coinoculated with seven combinations of three isolates. Genotypes recovered from infected plants were genotyped with microsatellite markers. Disease expression and fungal colonization but not plant biomass of potato was affected by coinoculation (F = 7.07, P < 0.0001; F = 2.36, P = 0.0427) and depended on the isolates with which plants were inoculated. One genotype was disproportionately selected for by all hosts. Putative heterokaryons were recovered from mustard plants coinoculated with isolates of different vegetative compatibility groups (VCG). These results support the assumption that mixed infections have marginal impacts on plant biomass but challenge the assumption that they do not affect disease expression and fungal colonization. Finally, this research provides evidence that plants select for specific V. dahliae genotypes and isolates from different VCGs can anastomose in planta.