Controlling Downy Mildew

A group of highly specialized holoparasites that affect many plant species, which can quickly become a problem.

Downy mildew, which was first described under the name Plasmopara in 1878, is a threatening plant pathogen for both agriculturists and horticulturists and can cause serious economic damage. Downy Mildew infests a large number of crops including maize, sorghum, pearl millet, onion, soybean, cucurbits, and grapes. The following article will give you an overview on how to identify, control and prevent Downy Mildew.

Favorable Conditions for Downy Mildew

Since fungi are closely related to algae, they absolutely need water to survive and spread. Which means that moderate temperatures (cooler than 65°F) and a relative high humidity, periods of fog, rain and frequent dews are favorable conditions for the infection and disease development of downy mildew. Some species can survive for years locally in plant debris, in the soil, on seeds or weeds. Other downy mildews can't survive in cold climates, but can be transported by the wind from warmer climates each spring.

Damage Symptoms

Downy mildew affects many plants and can be recognized as yellow to white spots on the tips of older leaves. On the underside, these areas are covered with white to grey, cotton-like fungi which are then responsible for the secondary spread of the disease. These "fluffy" masses are particularly easily noticed after rain or strong dew, however, they disappear again shortly after resumption of sunlight.

The disease causes leaf loss, which leads to a sharp drop in yield. In addition, plants that produce bulky fruits can suffer from sunburn of the fruit, due to the lack of roofing, which greatly impairs the quality and thus, the marketability of the products.

Mode of Infection

Only 4 days may elapse between the time of infection and the formation of new spores, but it usually takes 7-10 days. In all species, the sporangia germinate directly by forming a germ tube that penetrates the plant.

Preventive Measures

  • Select robust varieties
  • Choose well drained and well aerated sites
  • Do the planting at a greater distance to minimise leaf moisture
  • Irrigate your crops early in the day to reduce leaf moisture
  • Avoid overhead irrigation
  • Avoid excessive usage of nitrogen-based fertilizers
  • Weed control should be strictly adhered to
  • Use timely initiation of intercrop measures
  • Use of growth stimulants to strengthen plants
  • Use strictly sanitized agricultural machinery and tools

Biological Control

Seed treatment, as well as leaf spraying with Pseudomonas fluorescens, can be an effective measure for controlling downy mildew (4 to 5 g per kg of seed). Seeds treated with this beneficial bacteria increase seedling strength and inhibit sporulation of the downy mildew pathogen. Since the effectiveness is markedly higher, it is recommended to control downy mildew with P. fluorescens through seed treatment and leaf spraying.

Chemical Control

In general, there are several fungicides available for the treatment of downy mildew, including both protective and eradicus fungicides. Protective products, including Chlorothalonil, copper-based compounds and Mancozeb, can protect crops from infection early in the season. Once the disease occurs, eradicants such as Mandipropamide, Azoxystrobin, Fluopicolide + Fosetyl or Pyraclostrobin + Metiram are needed to penetrate the tissue. High spray pressures and high application rates are required to achieve sufficient spray coverage, and applications should be made before rain events to allow time for drying of treatments.

If you'd like to know which fungicide will be best for your crop in controlling downy mildew, please ask our experts in the Plantix community.