How to Spot and Prevent Downy and Powdery Mildew

By Scott Ockey

Identifying diseases in your crops is vital to protecting them as they grow to prevent loss and increase yield. Although the onset of disease is almost inevitable, the earlier growers can work on control, the better the outcome will be for healthy crops.

How do you spot the signs and symptoms of powdery and downy mildew? Start with the basics of the diseases, think about the ideal conditions that cause the diseases, and determine the best methods for prevention and treatment.

What is Powdery Mildew?

The fungi Erysiphe cichoracearum and Sphaerotheca fuliginea  are examples of fungi that cause powdery mildew, and these fungi do not require the leaves to be wet to spread infection, but high humidity triggers it.

Appearance and placement of powdery mildew:

• Fungal growth often begins on the shaded undersurface of the crown leaves.
• On the upper surface of plant parts, a whitish, powdery, fungal growth will form, and it often appears as light grey or white spots, which begin to spread.
• Severely affected leaves may become dry first so that fungal growth is not evident, and then they become brittle and can wither and die.
  
  

Powdery mildew causes harm by robbing plants of water and nutrients, which causes leaves to yellow and become withered and distorted. Plants typically become weak, bloom less, and grow more slowly, impacting yield. If growth spreads, infected leaves typically become distorted, twisted, and wilted, fall prematurely, and infected buds may not open.

What is Downy Mildew?

Several fungus-like organisms of the phylum Oomycota cause downy mildew, which spreads under humid, cool conditions and germinates quickly from rain, dew, or heavy fog.

Appearance and placement of Downy mildew:

• During moist weather, the undersides of leaf surfaces become covered with a layer of fungus that can be white to purplish to almost black.
• Usually older leaves are affected first, then younger leaves as the disease spreads.
• The spots may turn brown or yellow, and then the entire leaf dies quickly.

Since downy mildew can spread rapidly (within 3 days under ideal conditions), crops can quickly lose leaves and affect the whole plant if you do not catch the disease early.

To prevent the spread of downy mildew, promote good air circulation and water early in the day to allow leaves to dry thoroughly, and use wide plant spacing to avoid the spread of disease.

Ideal Conditions for Powdery and Downy Mildew

Weather conditions can create adverse effects on the growth of crops, especially during rainy and humid seasons when conditions favor the growth of fungi, often causing powdery or downy mildew on growing crops.

In general, fungal diseases favor humid weather, shady areas, and tightly spaced planting to grow and thrive. That’s why growers should avoid these conditions as much as possible. When conditions are ideal for powdery and downy mildew growth, fungal spores are carried by air and germinate on the leaf surface, which then multiply and spread rapidly.

Be sure to leave adequate space between plants to prevent disease from spreading, ensure good air circulation and sunlight, help reduce relative humidity to control disease growth, and remove dead or diseased foliage.

Powdery and Downy Mildew Prevention and Solutions

When approaching disease management as a whole, prevention is the best medicine.

 Weather-driven foliar diseases should not be an ever-present problem each growing season. Biological fungicides are an excellent choice to include alone or within an IPM program to increase effectiveness in fighting disease or to combat resistance.

Steps for foliar disease prevention and control:

• Prep the soil with biofungicides per label directions
• Supplement with foliar sprays as needed upon signs of disease or weather forecasts show conditions favorable to disease infection and development.
• Perform root maintenance by ensuring proper fertilization, pH, watering, aeration, and other methods as needed
• Ensure plants are spaced apart appropriately to prevent the spread of foliar disease
  
  

Interested to learn more about how a preventive biofungicide can be used as a foliar spray or soil application to help control or suppress over 90 fungal and bacterial plant diseases such as powdery and downy mildew?

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