RECENTLY PUBLISHED: Fungicide resistance in strawberry powdery mildew in California

Strawberry powdery mildew caused by Podosphaera aphanis, is an important disease in California strawberries (Fig. 1). Strawberry Center master’s student Michael Palmer recently published his work on fungicide resistance in the journal Plant Disease ( Because P. aphanis is an “obligate parasite” all of the experiments were done using live plant tissue (Fig. 2). This makes the research much harder and more time-consuming. Still, Palmer was able to determine fungicide sensitivity of 19 isolates of the pathogen to seven active ingredients (Table 1).

Figure 1. Severe powdery mildew on strawberry leaf under greenhouse conditions.

Table 1. Fungicides used for resistance screening and their respective Fungicide Resistance Action Committee (FRAC) code.

Active ingredientTrade nameFRAC code
fluopyram + trifloxystrobinLuna Sensation7 + 11
Figure 2. Graduate student Michael Palmer inspects resistance bioassay for growth of P. aphanis on detached strawberry leaflets.

The six fungicide treatments and their average disease incidence on treated leaves for the 19 isolates are listed below and shown in Fig. 3.
• Penthiopyrad (51.4%)
• Quinoxyfen (41.5%)
• Myclobutanil (39.8%)
• Trifloxystrobin (19.8%)
• Cyflufenamid (19.3%)
• Fluopyram + trifloxystrobin (3.5%)

Figure 3. Average disease incidence (%) of each fungicide treatment for 19 isolates of Podosphaera aphanis according to the lab fungicide assay. Treatments that do not share a letter are significantly different according to Tukey honestly significant difference separation of means. Error bars represent standard errors of the mean.

This work is the first report of resistance in P. aphanis to any fungicide in California and the first report of resistance to penthiopyrad and quinoxyfen worldwide. Two isolates collected from organic production systems were sensitive to all fungicides.


Fungicide efficacy against powdery mildew in California is compromised by fungicide resistance. This information should be considered when selecting which fungicides to use. Host resistance should be incorporated where possible to reduce selection pressure from fungicides. Integrating fungicides and host resistance should result in high levels of disease control and a longer effective life for fungicides.


Palmer, M. G. and Holmes, G. J. 2021. Fungicide sensitivity in strawberry powdery mildew caused by Podosphaera aphanis in California. Plant Disease 105:2601-2605.