Do fungicides containing phosphites have a direct effect on Cercospora beticola Sacc., the causal agent of Cercospora leaf spot of sugarbeet?
Cercospora beticola Sacc., causal agent of Cercospora leaf spot (CLS), is one of the most serious foliar pathogens of sugarbeets (Beta vulgaris spp. vulgaris). Resistance to fungicide resistance action committee (FRAC) Group 1 and 11 fungicides and increasing insensitivity to Group 3 fungicides provide increasing challenges for CLS management in Ontario, Canada. The active ingredient mancozeb, has been an important fungicide in standard spray programs, but has been re-evaluated with a reduction in maximum applications from eight to five permitted. Evaluation of alternative fungicides in 2019, 2020, and 2021 field trials demonstrated that fungicides containing phosphites are a possible alternative to mancozeb, providing a significant reduction in disease through-out the season in all three field trials. To use these products effectively, it is beneficial to know if they directly suppress the pathogen or have an indirect effect such as increasing host resistance. Five concentrations of a commercial formulation of phosphites (Phostrol) (0.01, 0.1, 1, 10, and 100ul/ml agar) were tested for effect on mycelial growth of eight single spore isolates of Cercospora beticola. Single spore isolates were collected from sugarbeet leaves with CLS symptoms from the outer guard rows of research field trials in 2020, and 2021 in Ridgetown, Ontario, Canada. Phosphites were incorporated into the half V8 agar media, a mycelial plug placed in the center and the diameter of cultures was measured after five days. No concentration of phosphites decreased mycelial growth in comparison to the non-treated control (p-value less than 0.05). The 100ul phosphites per ml treatment (highest concentration tested, mean 18.4 mm) was statistically equivalent to that of the non-fungicide control (mean 17.6 mm) when analyzed using SAS 9.4 PROC GLIMMIX. This supports the hypothesis that phosphites stimulate host resistance to the pathogen and does not directly suppress the pathogen. Concurrent research on fungicide programs integrating phosphites is expected to provide growers with another option to combat CLS.