Causal Agent

Downy mildew is caused by fungi known as Oomycetes, specifically those belonging to the genera Peronospora or Plasmopara. These fungal-like organisms are a problem during cool and moist weather. They mainly attack the delicate parts of plants above the ground and spread quickly in water films.

Hosts

Downy mildew fungi commonly infect ornamental plants. Some examples of common ornamental hosts include aster, coreopsis, roses, rudbeckia, snapdragon, and violets. However, most downy mildew fungi have a limited range of plants they can infect.

For instance, the type of downy mildew that affects roses (Peronospora sparsa) only causes disease in roses and not in any other plants. Another downy mildew called Plasmopara halstedii can infect various members of the daisy family, including rudbeckia, but it does not affect plants outside this family.

Symptoms

The symptoms of downy mildew usually appear as patchy purple or brown discoloration on the upper surface of leaves, bordered by leaf veins. The severity of the disease varies depending on the host plant or cultivar.

Infected leaves may drop prematurely. To understand why the disease is called “downy mildew,” simply flip the leaf over. On the lower leaf surface, you will notice distinct downy tufts of white to purple/grey fungal growth. These tufts consist of spores called sporangia or conidia, which can be easily carried away by wind or splashing water.

Disease Cycle and Development

Understanding the disease cycle is crucial for managing downy mildew effectively without relying heavily on chemicals. The fungus remains dormant as mycelium or oospores (resting structures resembling gumballs) on or inside plant parts. The pathogen’s development depends on temperature and humidity. Downy mildew outbreaks occur during cool temperatures (around 10-22°C) and wet conditions with high humidity (85% or higher).

Extended periods of leaf wetness facilitate spore germination and further spread of the disease. Therefore, it is important to keep the plants dry to minimize the disease’s spread. Enhancing air circulation around the plant by thinning and pruning helps reduce humidity and lowers the risk of infection.

The disease cycle, from initial infection to spore production and secondary infection, usually takes seven to ten days. However, under warm and humid conditions, it can be as short as four days. The duration varies depending on the specific downy mildew species. In cold climates, where susceptible plants are absent, downy mildew pathogens survive in plant debris, soil, or weeds.

Disease Management

Implementing preventive measures is crucial for managing downy mildew effectively. The first step is to “manage the moisture.” Properly space plants to ensure good air circulation and quick drying of foliage after watering. Avoid overhead irrigation during cool weather. In greenhouses, reduce relative humidity to less than 85% by spacing plants adequately and using fans to improve air circulation.

Venting and heating the greenhouse can help fill it with warmer, drier air. Downy mildew outbreaks typically stop in hot and dry conditions. However, it may be challenging or nearly impossible to keep plants completely dry. In such cases, the proper use of fungicides becomes necessary to prevent further infection and treat already infected tissues.

Warning: To minimize the risk of fungal resistance to certain fungicides, avoid using a single fungicide continuously for extended periods when there are other reliable products available.

Spray Program

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