High levels of insensitivity to phosphonate fungicides in Pseudoperonospora humuli

Authors: Gent, David - Dave; BLOCK, MARY - Oregon State University; CLAASSEN, BRIANA - Oregon State University

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/18/2019
Publication Date: 3/20/2020
Citation: Gent, D.H., Block, M., Claassen, B. 2020. High levels of insensitivity to phosphonate fungicides in Pseudoperonospora humuli. Plant Disease. 104(5):1400-1406. https://doi.org/10.1094/PDIS-10-19-2067-RE.
DOI: https://doi.org/10.1094/PDIS-10-19-2067-RE

Interpretive Summary: Downy mildew is one of the most important diseases of hop worldwide. Management of the disease often relies on cultural and chemical control measures because of the lack of host resistance in most varieties. One of the most commonly used fungicides for managing the disease is phosphonate. Development of insensitivity to phosphonate compounds is believe to be low risk and has been documented only rarely in plant diseases. In the case of hop downy mildew, reduced sensitivity to the phosphonate compound fosetyl-Al was described in the early 2000s but the disease could still be managed by increasing rates of fosetyl-Al. In this research, we identified strains of the hop downy mildew organism that possess even higher levels of insensitivity to fosetyl-Al and other phosphonate fungicides. The sensitivity of the downy mildew organism from two farms reporting disease control failures was 1.6 times the maximum allowable rate of the phosphonate fungicide utilized. We found insensitivity to the maximum allowable rate of a phosphonate fungicide was widespread within and among hop farms in Oregon, being detected in 96% of samples analyzed. We also demonstrate that simply increasing rates of these products is insufficient to control highly insensitive strains of downy mildew. This finding has implications for management of the disease not only in Oregon, but also other production regions should insensitive strains be introduced on infected planting material.

Technical Abstract: The Fungicide Resistance Action Committee has deemed phosphonate to be at low risk of resistance development and reduced sensitivity to phosphonate has been reported only occasionally in plant pathogens. Reduced sensitivity to the fungicide fosetyl-Al was documented in the hop downy mildew pathogen, Pseudoperonospora humuli, in the early 2000s but disease could still be managed if the fungicide rate was doubled from 2.24 to 4.48 kg/ha. In this research, we document the occurrence of isolates of P. humuli that possess even higher levels of insensitivity to fosetyl-Al and other phosphonate fungicides. The median EC50 for isolates collected from the two farms reporting disease control failures was 2.7% (v/v) phosphonate (range 1.6 to 164.2), which is 1.6 times (range 0.9 to 96.0) the maximum labeled rate of the phosphonate fungicide utilized. In contrast, the median EC50 for isolates obtained from experimental plots that have received only a single application of a phosphonate fungicide was 0.6% v/v phosphonate (range 0.11 to 2.3), or 0.3 times the maximum allowable rate. Sensitivity of isolates to a phosphorous acid fungicide, fosetyl-Al, and a plant nutrient product containing phosphorous acid were linearly related. Insensitivity to the maximum allowable rate of a phosphorous acid fungicide was widespread within and among hop farms in Oregon. Among 54 isolates assayed for phosphonate insensitivity, 96% had EC50 values that exceeded the maximum allow rate of the fungicide used in the assays. Field studies conducted in two years further demonstrated that a phosphorous fungicide and fosetyl-Al failed to provide commercially acceptable suppression of downy mildew when applied at the maximum allowable rates and even double these rates, whereas fungicides with different modes of action provided 91% or greater disease control. The totality of this research indicates that P. humuli has been selected to tolerate fosetyl-Al and other phosphonate fungicides at rates four-times greater than those used nearly 40 years earlier to obtain satisfactory suppression of downy mildew.