Photo courtesy of Syngenta

In late spring and early summer, growers need to mind their mildews. Greenhouse Management recently spoke with Nancy Rechcigl, technical services manager at Syngenta, about these diseases and the role fungicides play in controlling them.

Greenhouse Management: May and June can be difficult months for mildews. What is the difference between powdery and downy mildew?

Nancy Rechcigl: Downy and powdery mildew pathogens are both obligate parasites but differ in their symptoms and fungal classification. Downy mildews are Oomycetes while powdery mildews are Ascomycetes. This distinction is important when selecting fungicides for control.

Powdery mildews form white colonies on the surface of leaves, stems and flowers. They will likely appear in the spring or early summer when temperatures are 62-72° F and the relative humidity is 70 percent or greater.

Plants infected by downy mildew will have blotchy, discolored areas on the leaves, often within the veins. Under the right environmental conditions, sporulation creates a fuzzy coating under the leaves below the discolored areas. Systemic infections can also cause distortion of emerging leaves and flower buds, defoliation and stunted plants.

Temperatures from 50-72° F are favorable for most downy mildew fungi, but some can grow in higher temperatures such as those infecting roses (up to 82° F) and salvia (up to 90° F). All downy mildews require relative humidity greater than 85 percent for six-plus hours when the leaves are wet.

GM: How should these diseases be controlled?

NR: When the environmental conditions are conducive to disease development, powdery mildews should be controlled by alternating fungicides from FRAC Groups M, 3, 7+11, 9+12. Horticultural oils and some microbials can also provide suppression. Downy mildews should be controlled by alternating fungicides from FRAC Groups M, 4, 21, 40, 43, 45, U-49. Fungicides in FRAC Group 7+11 can protect against downy mildew as well as other diseases. Preventative applications are generally less expensive than curative, and result in higher plant quality.

GM: Why is product rotation so important to preventing mildews?

NR: Fungicide resistance can occur quicker in pathogens producing a lot of spores. Downy mildews are known to develop resistance very easily when exposed to a single active ingredient over time. To avoid resistance, you must rotate at least two to three products with different modes of action for each target problem. Programs should include a combination of products with systemic activity and contact or translaminar activity. A well-structured program will maximize each product’s strengths and leverage modes of action when they are most beneficial.

GM: What solutions does Syngenta offer that fit into a well-rounded fungicide program?

NR: Our downy mildew program includes the following fungicides: Subdue Maxx® (4) early in production, rotating to Mural® (7+11), Micora® (40) and Segovis® (U49). Our powdery mildew program includes the following fungicides: Daconil Ultrex® or Daconil Weatherstik® (M5) early in production, rotating to Palladium® (9+12), then Mural (7+11). In outdoor production rotations, include Concert® II (M+3) for powdery mildew.

GM: What resources does Syngenta have to help growers stop these diseases?

NR: Syngenta offers free, downloadable programs to help control powdery and downy mildew, as well as other common diseases at

All photos are either the property of Syngenta or are used with permission.

©2019 Syngenta. Important: Always read and follow label instructions. Some products may not be registered for sale or use in all states or counties. Please check with your local Extension Service to ensure registration status. Concert®, Daconil Ultrex®, Daconil Weatherstik®, GreenCast®, Micora®, Mural®, Palladium®, Segovis® and Subdue Maxx® are trademarks of a Syngenta Group Company. Syngenta Customer Center: 1-866-SYNGENT(A) (796-4368).