Evaluating Fungicide Sensitivity in Downy Mildew:

Nurseries supplying Brassica transplants face immense production challenges due to downy mildew, caused by the Oomycete pathogen Hyaloperonospora brassicae. Large-scale monoculture plantings at different growth stages, dense planting spacing, and overhead irrigation, consistently contribute to high downy mildew incidence and large quantities of downy mildew inoculum. Moreover, the cool climate and nighttime moisture of California’s Central Coast create optimal conditions for infection in outdoor nursery spaces and grower plantings. As a result, Nursery practitioners and growers depend heavily on fungicide applications for downy mildew management. Nonetheless, disease outbreaks continue to occur in nurseries despite rigorous chemical control (Figure 1).

Nursery seedlings such as broccoli and cauliflower may grow out of downy mildew infection after transplanting if field conditions are not conducive to disease development. However, under favorable conditions, the disease may continue to develop and reduce crop yield. In addition, infected seedlings may introduce inoculum into the field, leading to subsequent infections and production losses in leafy Brassica crops. For example, downy mildew signs including chlorotic and necrotic flecking, and gray-white sporulation renders infected leaves of baby leaf kale (Brassica oleracea)—an economically important salad green—unmarketable.

To facilitate the effective use of fungicides in downy mildew management, we conducted an in vitro fungicide sensitivity assay using eleven downy mildew isolates collected from broccoli grown in nurseries in Salinas and Nipomo during 2024 and 2025. Each isolate was tested for sensitivity to five fungicides, representing six FRAC groups at both high and low concentrations.

Fungicide information:

Overall, ten fungicide treatments were evaluated, comprising five fungicides applied at high and low concentrations (Table 1). These included two systemic fungicides with single-site modes of action (Mefenoxam and Fluoxapiprolin), one contact fungicide with a multisite mode of action (Mancozeb), and two translaminar fungicides with a single-site mode of action (Fluopicolide and Ametoctradin-Dimethomorph). These fungicide active ingredients were chosen based on prior use, efficacy, and/or resistance to downy mildews. The specific products used that contained Fluopicolide, Mefenoxam, and Ametoctradin-Dimethomorph active ingredients were labeled for downy mildew on most Brassica crops, Mancozeb active ingredient was labeled for downy mildew on broccoli and cabbage but not kale, and Fluoxapiprolin active ingredient was labeled only for grapevine downy mildew (Plasmopara viticola). Of these, Mancozeb was the only active ingredient consistently utilized in Brassica nursery programs where the isolates were collected.

* Because the product label for Zampro does not specify a low rate, a one-third reduced label rate was used as the low-rate treatment in this study.

Isolate maintenance and screening:

Cotyledons from the susceptible kale ‘Dwarf Blue Curled Scotch’ were excised from 2- to 3-week-old healthy plants, and fungicide treatments were prepared according to label rates in conical tubes containing 30 mL of deionized water and 30 µL of Tween 20 (Table 1). Each cotyledon was dipped into its respective fungicide solution using forceps, placed lower surface side up in a crisper box lined with moist paper towels, and incubated for 24 hours before being transferred to 1.5% water agar plates. 

Each downy mildew isolate was maintained on known susceptible kale varieties (Toscano OG, Dwarf Blue Curled Scotch, and Redbor F1; Locke-Paddon et al., 2025) for sporulation. The spores were collected in cold purified water to prepare a spore suspension. Two 10 µL drops of inoculum were pipetted onto each cotyledon and DI water was used as a control. Then the crisper boxes containing Petri plates with inoculated cotyledon leaves were inoculated in a humidity chamber for two days and then incubated on a growth rack for three days. 

Afterwards, cotyledons were scored for disease severity by examining the lower surface for gray-white downy mildew sporulation. Confirmation of disease development was determined by visible observation of the susceptible positive control. Disease severity was quantified according to the parameters outlined in Figure 2, adapted from Williams and Leung (1981).

Is reduced fungicide sensitivity driving downy mildew outbreaks in nurseries?

Across the eleven isolates tested, downy mildew was most effectively suppressed by Ametoctradin-Dimethomorph, Mancozeb, and Fluoxapiprolin at both concentrations (Figure 3). In contrast, higher disease severity was observed across all Fluopicolide and Mefenoxam treatments, with the greatest sporulation occurring under the low concentration of Mefenoxam.

While these patterns in disease severity can be explained by repeated fungicide exposure selecting for resistance, other factors, such as fungicide mode of action and phytomobility, may also contribute. Fluopicolide and Mefenoxam share the same mode of action and have been commonly used to manage Brassica downy mildew, and reduced sensitivity in plant pathogens to these fungicides has already been reported in literature. In contrast, Fluoxapiprolin is listed solely for controlling grapevine downy mildew as of 2024, suggesting that the tested isolates may have yet to be exposed to its mode of action and therefore remain highly sensitivity to it. Ametoctradin-Dimethomorph contains two active ingredients, providing a more robust option with dual modes of action, while Mancozeb is a multi-site fungicide, thereby lowering its risk for resistance.

Nonetheless, Mancozeb was the only active ingredient from the nursery program tested in this assay. Based on its performance, we infer that Mancozeb, when applied in the nursery appropriately, i.e. applied at a high label rate as a protectant with good spray coverage, can successfully inhibit downy mildew. Therefore, other factors besides fungicide resistance may be contributing to the ineffective control of downy mildew in the nursery.

One such factor could be improper fungicide use. Because Mancozeb is a contact fungicide, it is a protective measure preventing the initial infection from occurring. Conventionally, nursery spray programs follow a standard weekly schedule that is repeated as new transplants are grown, with the heaviest applications targeting downy mildew early in the production cycle. These sprays typically begin approximately ten days after emergence to protect the most vulnerable plant stage. However, if pathogen contact has already occurred or fungicide coverage is incomplete, disease control may be ineffective. This underscores the importance of proper fungicide application, including optimal fungicide selection, timing and coverage, as part of integrated pest management (IPM) strategies to improve disease control efficacy. The fungicide program should emphasize diversification, including the incorporation of fungicides with different modes of action and varying mobility within the plant, to improve overall efficacy. 

Given the conducive conditions in nursery production systems and the high susceptibility of plants to downy mildew, fungicides remain an important component of the IPM toolbox for disease management. Although fungicide resistance does not appear to be the cause of ineffective downy mildew control in nurseries, continued monitoring of fungicide sensitivity is essential to ensure long-term effectiveness.

References:

Locke-Paddon, E. M., Fernandez, M. A., Das, K., Brasier, K., Dowling, C., and Ding, S. 2025. Host resistance screening of baby kale against downy mildew isolates across the Central Coast of California. Plant Dis. First Look. https://doi.org/10.1094/PDIS-11-24-2290-RE.

Williams, P. H., and Leung, H. 1981. Methods of breeding for multiple disease resistant Chinese cabbage. Pages 393–403 in: Chinese Cabbage: Proceedings of the First International Symposium. N.S. Talekar and T.D. Griggs, eds. Asian Vegetable Research and Development Center, Shanhua, Taiwan.