Downy mildew
Also known as Peronosporaceae, Peronospora, Plasmopara, Pseudoperonospora
In short
Downy mildews are caused by fungus-like water moulds (oomycetes), not true fungi, and are unrelated to powdery mildew. They need long periods of wet leaves, so they are worst in wet weather and humid greenhouses. Spores form on the underside of leaves, often at night.
How it looks
Discoloured blotches on the upper leaf surface in pale green, yellow, purple or brown; on many hosts the blotches are angular or blocky because they are bounded by leaf veins, and they expand quickly and turn brown. Directly beneath, the lower surface carries a soft, fuzzy mould-like growth that is white, grey or purple-brown depending on the species.
Signs
Yellow, then brown, angular patches on leaves; leaves wither and drop in severe cases, and growth and yield decline. Symptoms can spread rapidly through a group of plants during cool nights with long dew periods.
Life cycle
Downy mildews are obligate parasites that must live on a living host. Many produce their spores at night on leaf undersides; the spores are splashed by rain or irrigation and sometimes carried far by wind, and infection requires long periods of leaf wetness. Most species also form tough resting spores in infected tissue that pass into the soil as it rots and can survive for up to about five years.
What favours it
Extended leaf wetness, high humidity, cool nights with long dew periods, overhead watering late in the day and crowded plants with poor air flow.
Prevention
- Water at the base, early in the day, so leaves are dry before night.
- Space plants and control weeds so air can circulate; ventilate greenhouses and grow tents.
- Remove affected leaves as soon as you see them.
- Rotate annual crops such as basil or cucurbits so resting spores in soil do not reinfect them.
- Grow resistant cultivars where available (for example of lettuce and onion), although resistance can break down when new races of the mildew appear.
Treatment
Start with the first step and move down only if the problem persists.
- Remove and bin infected leaves or whole plants; do not compost heavily infected material.
- Reduce humidity and keep foliage dry.
- Where fungicides are permitted, oomycete-specific products and protectants are used commercially; home gardeners in some countries (such as the UK) have none approved. Follow the product label.
Pros and cons
Cons
- Can spread very fast in wet, humid weather.
- Few or no controls are available to home growers in some countries, so prevention matters most.
Look-alikes
- Powdery mildew
- Powdery mildew is a dry white dust, often on the upper surface, and does not need wet leaves; downy mildew has coloured blotches above and fuzzy growth below.
- Nutrient deficiency
- Deficiency yellowing follows a pattern across old or new leaves and has no fuzzy growth underneath.
- Fungal leaf spot
- Leaf spots are usually round with darker margins and no downy growth on the underside.
Affected plants
Grapes, cucurbits, basil, hops, spinach, soybeans, sunflowers, lettuce, onions, peas, brassicas, and ornamentals such as impatiens, pansies and columbine; each downy mildew species has a limited host range.
Related
Sources
Written in our own words from these sources. Check a product label before using any treatment.