Laboratory studies were conducted to evaluate the risk of Phytophthora capsici developing resistance to two morphlines,dimethomorph and flumorph. Metalaxyl, the well-known high risk of resistance fungicides, was used as referencefungicide. Resistant mutants for the three fungicides were isolated by treating mycelium with ultraviolet radiation.Metalaxyl-resistant mutants were obtained with high frequency and exhibited high level of resistance with factors morethan 100 folds, while mutation frequency for dimethomorph-resistance was relatively low and the resistance factorsranged from 3.0 to 13.9 folds. Most dimethomorph-resistant mutants decreased in hyphal growth rate and the spoulationability, which have a large impact upon the epidemic development of dimethomorph-resistant populations. Theseresults suggested that the risk of resistant pathogen population was much lower for dimethomorph than for metalaxyl.Both the frequency of developing resistance and level of resistance (resistance factors = 1.8-14.6) to dimethomorphwere similar to those of its structure analogue flumorh. Moreover, the cross-resistance were found between them,which suggested the risks of developing resistance to dimethomorph and flumorph in the pathogen were very closelyrelated. As P. capsici can potentially develop resistance to dimethomorph and flumorph, and oomycetes usually havethe high risk to develop resistance to fungicides, appropriate management against resistance development should betaken.
