Downy Mildew Diagnosis & Treatment

Fungal mold like downy mildew is a common issue in gardens in humid conditions and regions. While a downy mildew infection may not be fatal for plants, it can still cause significant damage to the appearance of your plants but also to the yield of your crops.

Today, we’ll be walking through the facts you need to know about downy mildew and how you can control and manage it in the garden. Let’s get started!

What is Downy Mildew

Downy mildew is a common plant disease pseudoperonospora cubensis a water mold that is caused by oomycetes or fungus-like organisms that belong in the Peronospora or Plasmopara genus. These parasitic organisms are not true fungi. Instead, they are closely related to algae which means it likes cool temperatures and water to survive and spread.

They produce grayish, fuzzy-looking fungal spores and mycelium on the lower leaf surface. These downy masses typically appear after damp weather like rain or heavy dew and disappear when the weather becomes drier and there are warm temperatures.

It affects a wide range of different plants from edible to ornamental plants including:

• brassicas
• carrots
• grapevines
• lettuce
• onions
• parsnips
• peas
• poppies
• spinach
• roses
• cucurbits
• sweet basil

It should be noted that downy mildew has a narrow host range which means the species that affect one type of plant may not be the same one that affects another.

Despite having a similar name, downy mildew is not related to powdery mildew.

Symptoms of Disease

A crucial thing to remember about downy mildew disease is that its symptoms can vary depending on the plant host as well as the species of downy mildews that are infecting your garden. This makes it more difficult to recognize especially if you have several susceptible plants in your garden.

However, there are certain telltale signs you can watch out that include:

• Discolored leaf surfaces. Blotches may appear on the top side of leaves with colors ranging from pale green to purple, even yellow spots to brown depending on the host plant. Blotches on leaf veins may appear to have straight edges.
• Mold-like growth on the underside of the leaves. Grey or purple downy masses form under the leaves that correspond to the blotches appearing on the upper surface.
• Severely affected leaves may either shrivel and turn brown as in the case of tobacco plants or turn yellow and fall prematurely in plants like roses and impatiens.
• Infected plants look stunted and weak. In severe cases, host plants may die.

How Does Downy Mildew Occur and Spread

Downy mildew is spread by spore production on the underside of infected leaves. These can be splashed by rain and may also be carried for long distances through the wind.

Outbreaks are likely to occur in certain environmental conditions like wet weather when leaves are exposed to prolonged periods of wetness. This makes it a common occurrence in early spring or late fall when temperatures are cooler than 65˚F and relative humidity can approach 100%.

Downy mildew spores can survive for up to five years in the garden and in plant debris through crop residue and they can also overwinter. If a plant grows in contaminated soil, viable resting spores may infect them through the roots and can systemically infect the entire plant.

This can also happen through secondary infections when windblown zoospores from one leaf (usually from a systemically infected plant) land on the leaves of a nearby plant.

Control and Treatment of Downy Mildew

There are several non-chemical controls that can help you limit the effects and allow you to gain control of downy mildew in your garden. This includes:

• Picking off and disposing of affected leaves as soon as you spot symptoms. Be sure to place them away from your plants.
• Remove and destroy severely infected plants. Do not compost them.
• Providing good air currents and circulation in your garden. Avoid placing plants closely together and control weeds. With enough space in between your plants, water can easily evaporate.
• Avoid overhead watering in greenhouses to avoid periods of high humidity.
• Water your plants in the early morning to give upper leaf surface enough time to dry.
• Variety is key. Use crop rotation and avoid replanting the same host plants every growing season.

For most gardens, application of fungicides specifically for downy mildew control may be unnecessary. Broad-spectrum protectant fungicide such as chlorothalonil, mancozeb, and fixed copper are at least somewhat effective in protecting against downy mildew.

Use 100% cold-pressed Neem Oil for controlling downy mildew. Mix 2 and 1/2 tablespoons in a gallon of water and a squirt of dishwashing liquid – to help keep the oil and water mixed. Use lukewarm water to help mix the oil as well.

Spray on affected plants or as a preventative measure every 7-14 days. Apply generously making sure to cover both the undersides of the leaves and tops of leaves. While you spray be sure to shake the mixture up every 30 seconds to a minute to keep the oil and water mixed.

What problems do you have with pest or disease development in your garden? Comment below and I’ll be sure to answer.

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