Downy mildews

Powdery mildews are common diseases that mainly affect the leaves of a range of different plants. Each species of downy mildew usually has a narrow range of plants it will attack, which is made up of one species or a few closely-related plants. So, onion downy mildew won’t attack peas, rose downy mildew won’t attack busy Lizzies, for example.

Commonly affected plants include the following.

Ornamental plants

Aquilegia
Digitalis (foxglove)
Geum
Impatiens (busy Lizzies)
Nicotiana (tobacco plants)
Papaver (poppy)
Viola (pansies)
Roses

Edible plants

Brassicas
Grape vines
Lettuce
Onion
Parsnip
Pea
Rhubarb
Spinach

They produce a range of different symptoms, depending on the plants affected, which can make identification of the disease difficult. These include:

Discoloured blotches on the upper leaf surface. These can be yellow, pale green, purple or brown.

Corresponding white, grey or purple mouldy growth on the underside of the leaf.

Severely affected leaves may either turn yellow and fall prematurely in summer, or shrivel up, turn brown and die.

Badly affected plants lack vigour, grow poorly and may produce stunted or distorted growth or even die.

Sometimes, plant parts other than the leaves can be affected. These include flower buds, fruit, pea pods and cauliflower curds.

• Herbaceous perennials
• Annuals
• Vegetables
• Roses

Downy mildews are caused by several different fungus-like organisms. They spread throughout the plant and from plant to plant by spores, which are splashed from the soil onto the leaves by rain or when watering, and can be carried long distances by winds.

Wet or very damp weather and leaves, particularly at night, are needed for infection to occur, and severe outbreaks are only likely during a wet summer. A humid atmosphere is needed in greenhouses, and the disease can be present all year round indoors.

Downy mildews overwinter as resting spores either on fallen leaves or in the soil, releasing spores to infect plants the following spring, or as infections within the plant. The resting spores can survive for up to five years and will infect new plants through the roots to cause an infection that will spread throughout the plant.

Try to keep plants growing as strongly as possible to help them fend off disease attacks. Avoid the use of high nitrogen fertilisers, which produce soft growth, using high potash ones instead, which help to harden the growth. However, when large outbreaks of downy mildews occur, this may not prevent or control them.

Carefully pick off and dispose of affected leaves when the disease first strikes to reduce or slow down its spread. This is highly unlikely to actually eradicate the disease. Try not to disturb the spores, or you will be spreading the disease further.

Prune out badly infected growth, or carefully dig up badly affected plants, and dispose of it by either burning it or put it in the council’s green waste bin. Don’t compost it.

Similarly, carefully pick up or rake up affected fallen leaves – especially in autumn – to try and reduce carry-over through the winter.

Use a four-year crop rotation for vegetables, and avoid replanting beds and borders with the same susceptible plant for at least a couple of years, to prevent infection from the soil-borne resting spores.

Unfortunately, there are no fungicides available to home gardeners to control downy mildews.

Avoid watering plants over their leaves, watering the soil or compost instead. And water early in the morning or during the day, rather than in the evening, so that wet leaves dry out quickly. Watering in the evening means that the leaves remain wet overnight, and many downy mildews produce their spores at night.

In greenhouses, water plants carefully to reduce excessive water increasing humidity, and open doors and vents whenever possible to help reduce humidity further.

Don’t plant too densely, to help ensure there is good air circulation around the plants.

Some plant varieties are listed as being downy mildew resistant. These include lettuce, onion and spinach. Bear in mind that over time such resistance can break down.

• Secateurs
• Leaf rakes