Pythium blight, a serious disease ofturfgrass, is caused by Pythium spp., including P. aphanideramtum, an oomycete fungus. Tank-mixing strategies, which deploy a mixture of two or more fungicides with different modes of action, have been used to suppress problems of fungicide resistance. However, few studies have examined how interactions within P. aphanidermatum populations may affect the proliferation of resistance. The isolates present in populations of P. aphanidermatum may vary in their fitness characteristics, and those characteristics may play an important role in competition among isolates. Different fungicide treatments impose characteristic selection pressures on Pythium populations, which may differentially affect the survival of resistant and sensitive isolates. In order to improve effectiveness of current strategies, the influence of interaction of fitness characteristics and fungicide treatments on proliferation of fungicide-resistant isolates needs to be elucidated.

A collection of 44 field isolates of P.aphanidermatum was used to investigate variation in fitness characteristics. Growth rate of mycelia, sporulation capacity, and aggressiveness on perennial ryegrass, along with sensitivity to mefenoxam, were quantified. Sixteen isolates showed complete resistance to mefenoxam. In 28 sensitive isolates, mefenoxam sensitivity was not correlated with the other three fitness characteristics. A wide variation in fitness characteristics was detected among the isolates.

In the second phase of the research, two test populations were constructed by mixing two mefenoxam-sensitive isolates possessing different fitness characteristics with a mefenoxamresistant isolate in appropriate ratios. Changes in the proportion of resistant isolates within each population under six fungicide treatments were monitored during five cycles of selection on perennial ryegrass under controlled conditions. The rate of proliferation of mefenoxam-resistant isolates was lower under application of a half-rate fungicide mixture of mefenoxam with propamocarb than under application ofmefenoxam fungicide alone. No significant difference in the change of proportions of resistant isolates was detected between a test population containing a relatively fast-growing mefenoxam-sensitive isolate and a test population containing a relatively slow-growing isolate in any fungicide treatment.

The results from this study indicated that half-rate tank mixing could delay the proliferation of mefenoxam-resistant isolates in a P. aphanideramtum population. An effect of fitness characteristics on rate of proliferation of mefenoxam resistance was not demonstrated.