Effect of post-application irrigation on fungicide movement and efficacy against Magnaporthiopsis poae

Abstract

Abstract Management of many crown and root diseases of turfgrasses includes the use of fungicides. The physicochemical properties of the fungicides used vary greatly, but most have low mobility and are not phloem mobile, which results in little active ingredient present in the basal and underground structures of turfgrass plants. Two studies were conducted in a laboratory setting to determine the effects of post-application irrigation amounts (0, 0.3, 0.6, 1.3, and 2.5\u202fcm) on the distribution of 14C myclobutanil and 14C tebuconazole in a soil profile of 90% sand and 10% peat moss by volume. In addition, growth chamber experiments were conducted to examine the effect of post-application irrigation amount on azoxystrobin efficacy against summer patch (Magnaporthiopsis poae) in ‘Penn A-4’ creeping bentgrass (Agrostis stolonifera L.). The creeping bentgrass was treated with azoxystrobin and immediately irrigated with 0, 0.25, 0.3, or 0.6\u202fcm of irrigation. Lastly, an in vitro fungicide sensitivity assay was conducted on three M. poae isolates to determine their sensitivities to two succinate dehydrogenase inhibitors (SDHIs), three demethylation inhibitors (DMIs), and four strobilurins (QoIs) to determine if fungicide concentrations in the soil profile reached levels high enough to suppress fungal growth. In both 14C experiments, more than 54% of the 14C was retained in the top 5\u202fcm of soil for all irrigation treatments. For the 14C myclobutanil experiment, lysimeters treated with 2.5\u202fcm of post-application irrigation resulted in 3.9% of 14C recovered in the 7.6–10.2\u202fcm sampling depth, which was higher than all other irrigation treatments. Post-application irrigation at 2.5\u202fcm in the 14C tebuconazole experiment yielded 6.3% of the 14C at the 7.6–10.2\u202fcm sampling depth and 2.3% at the 10.2–12.7\u202fcm sampling depth—recoveries at both depths were higher with 2.5\u202fcm of irrigation than all other irrigation treatments. No 14C was detected below 12.7\u202fcm for either experiment. Less disease was observed when azoxystrobin received post-application irrigation. Both 0.25 and 0.3\u202fcm of post-application irrigation increased turf quality compared to no irrigation; moreover, 0.25 and 0.6\u202fcm of post-application irrigation increased root length compared to no irrigation. In the in vitro fungicide sensitivity assay, isolates of M. poae were sensitive to all fungicides with only minor sensitivity to the SDHIs. In general, isolates were most sensitive to the QoIs with some variability in isolate sensitivities noted. Isolates of M. poae are sensitive to commonly used fungicides and efficacy is enhanced by post-application irrigation due to improved fungicide distribution into the soil profile.