Of 14 fungicides with different modes of action, cyproconazole and tolclofos‐methyl were generally inhibitory both in vitro and in vivo against all tested isolates of five Rhizoctonia species belonging to the teleomorphs Thanatephorus cucumeris, Waitea circinata or Ceratobasidium cereale. Triadimenol and carboxin provided considerable variation in activity against different species and isolates, whereas prochloraz was ineffective against all isolates. Imazalil and fenarimol showed moderate control, whereas flusilazole, propiconazole, fenpropimorph and benomyl showed strong activity against R. zeae and R. oryzae, but were much less effective against R. sasakii, R. cerealis and R. solani. Benodanil and iprodione controlled all isolates of R. cerealis and R. solani, but were not very effective against R. zeae and R. oryzae. Pencycuron showed strong activity against R. sasakii and most R. solani isolates, moderate activity against R. zeae, and was ineffective against R. oryzae and R. cerealis.

Of eleven tomato cultivars ‘Baby’ was the most susceptible to Phytophthora infestans. The sporangium concentration of the fungus and the methods of fungicide application had a significant effect on the efficacy of oxadixyl, mancozeb and cymoxanil when tested singly or in mixture against sensitive and phenylamide resistant strains of P. infestans on tomato. Mancozeb was the least effective fungicide, its activity decreased significantly with increasing numbers of sporangia used for inoculation (increasing disease pressure) and its efficacy was dependent on the method of application. Oxadixyl and cymoxanil showed much stronger antifungal activities (except oxadixyl against resistant strains) which did not depend on the number of sporangia or method of fungicide application. The mixture of oxadixyl, mancozeb and cymoxanil was equally active and was independent of application type, sporangium concentration and level of sensitivity of the fungus. Mixtures of the single components showed synergistic interactions up to levels of 20, depending on the activity of the individual fungicide and the different disease intensities. Three‐way mixtures containing oxadixyl, mancozeb, and cymoxanil represent therefore promising possibilities in practice for strong disease pressure, even when resistance problems are expected.