Downy Mildew

Causal Agent: Fungal relatives called Oomycetes, the pathogens are in the genera Peronospora or Plasmopara. These fungal-like organisms, troublesome in during cool, moist weather, attack tender, above-ground plant parts, dispersing rapidly in films of water.

Hosts: Ornamentals are common hosts for downy mildew fungi. Common ornamental hosts include aster, coreopsis, roses, rudbeckia, snapdragon and violets. However, most downy mildew fungi have a restricted host range.

For example, the species of downy mildew that affects roses (Peronospora sparsa) does not cause disease on any plant but roses. Other downy mildews such as Plasmopara halstedii can infect many members of the daisy family including rudbeckia, but it does not affect plants outside this family.

Symptoms: Symptoms of downy mildew most often appear on the upper leaf surface as a patchy purple or brown discoloration bound by leaf veins.  The disease may be mild to severe depending on the host species or cultivar.

Affected leaves may defoliate prematurely. To understand why the disease is called “downy mildew,” turn the leaf over.  Evident on the lower leaf surface are the characteristic downy tufts of white to purple/grey fungal growth.  This growth consists of spores (called sporangia or conidia) that are easily dislodged and disseminated by wind and splashing water.

Disease Cycle and Development: Understanding the disease cycle is critical to managing downy mildew and minimizing reliance on chemicals. The fungus inoculum is dormant in or on plant parts as mycelium or oospores (thick-walled, gumball-like structures that form the resting stage of the pathogen). Temperature and humidity play key roles in the pathogen’s development.  During cool (10-22^oC), wet conditions with high relative humidity (85 percent or higher), downy mildew outbreaks develop.

Prolonged periods of leaf wetness promote spore germination and the disease’s spread, so keeping plants dry minimizes the spread of this disease. Increasing air circulation around the plant by thinning and pruning it, reduces humidity and minimizes infection.

The disease cycle, from the initial infection to the production of additional spores and secondary infection, is usually seven to ten days, but can be as short as four days under warm and humid conditions – again, this varies depending on the species of downy mildew. In the absence of susceptible plant tissue, downy mildew pathogens in cold climates survive in plant debris, soil, or weeds.

Disease management: Preventive controls for downy mildew are critical.  To manage this disease, first “manage the moisture”:  space plants to ensure good air circulation and rapid drying of foliage after irrigation.

Avoid overhead irrigation when the weather is generally cool.  In greenhouses, reduce relative humidity to less than 85% by properly spacing plants, and use air flow fans to improve air circulation throughout the house.  Fill the house with warmer, drier air by venting and heating if possible. Downy mildew epidemics generally cease when it’s hot and dry. However, keeping plants dry may prove impossible, or nearly so. When that’s the case, proper fungicide use comes into play to prevent additional infection from occurring and cure already infected tissues.

Warning: To reduce the possibility of the development of fungal resistance to some of the fungicides, avoid the sole use of any fungicide for extended periods of time when other reliable products are available.

Spray Program