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Mildew - Powdery vs. Downy

Diagnosing diseases can be difficult, but the process is made even harder when two diseases sound very similar. There are numerous diseases that fall under the general description of powdery mildew. The actual injury to the plant varies greatly with the species and even the host attacked. Often the two mildews are confused and misdiagnosed. Let's try to separate the characteristics of the two.

A word about host susceptibility:
First, let's compare the facts that are common to the two diseases.

Most commonly grown greenhouse crops are susceptible to both powdery mildew and downy mildew. However, it is important to realize that there is high host specificity between the mildews and their individual hosts. The powdery and downy mildews are very specific to their individual hosts. If mildews are occurring on several different hosts simultaneously, it simply indicates that spores of the specific pathogens were present and the hosts were susceptible. The powdery mildew that infects and damages gerbera daisy is not the same powdery mildew that causes problems in begonia; the powdery mildew that infects zinnia is different than the powdery mildew that infects kalanchoe. This is true for hosts of downy and powdery mildew.
Both powdery and downy mildew pathogens are spread primarily by wind-blown, air-borne spores. Both of these mildew pathogens produce spores on the leaf surface (powdery on the upper leaf surface, downy on the upper and lower leaf surface) that can be readily blown and splashed from plant to plant.
Conditions that favor mildew are similar. Moderate temperatures, cloudy, cool, wet humid conditions when days are warm and nights are cool favor spore development. This usually happens in mid to late spring in our area with day temperatures in the 80s and high nighttime humidity that will induce mildew formation.

The most favorable conditions for powdery mildew are nighttime temperatures of 60ºF and relative humidity of 90 to 99%, with 80ºF temps and 40 to 79% relative humidity during the days.

Poor air circulation that results in air stagnation because such conditions favor the accumulation of pockets of high humidity around the leaf canopy is a major cause.

Powdery Mildew appears as an obvious dusty white to gray coating over the tops of leaf surfaces or other plant parts. It will begin as discrete circular, powdery white spots. As these spots expand they will coalesce, producing a continuous matt of mildew (similar to dirt or dust). The white powdery substance that occurs on infected leaves consists of powdery mildew spores and mycelium.

A white powdery "dust" quickly covers the leaf surface, and affected leaves gradually turn completely yellow, then turn brown and papery.

On some plants, powdery mildew may cause the leaves to twist, buckle, or otherwise distort. Eventually they die and fall off. In the case of fruit trees or vegetables the exposed fruit may sunburn

Here is our first distinct difference. Downy mildew has spots and powdery mildew does not. Host Plants: Powdery mildew fungi infect almost all ornamental plants.

Woody Plants consist of azaleas, buckeye, catalpa, cherry, dogwood, euonymus, honeysuckle, lilac, privet, roses, serviceberry, crape myrtles, magnolia, photinia (red tip and Japanese), sycamore, live oak, and maples.
Other plants are asters, impatiens, rhododendron, chrysanthemums, dahlias, delphiniums, kalanchoes, phlox, snapdragons and zinnias.
Houseplants: African violets and begonias.
Grasses: POA grass and Fescue.

Conditions that favor powdery mildew:
Powdery mildew requires humid conditions and thrives in warmer temperatures (around 80ºF). The fungus favors high relative humidity periods or close spacing of plants, densely growing plants, or shade.

During the winter on outdoor plants, the fungus survives on infected plant parts and in debris such as fallen leaves. The next spring spores within the resting structure cleistothecia (ascospores) are shot up into the air and carried by air currents to leaves of plants. Spores generally spread by wind or contact of infected plant material to un-infected plant material.

Conditions are common in late spring and early fall. Since the spores are sensitive to extreme heat and direct sunlight, plants in partial shade are especially vulnerable.

Powdery mildew is one of the oldest plant diseases known to man -- the ancient Greeks described it. Probably every gardener, unless they live in extreme cold regions, has witnessed its symptoms: a delicate webbing or thick felt that covers leaf surfaces and stems, giving the appearance that they've been dusted with flour.

Damage:
Leaves may turn yellow, die and fall off, reducing yields and weakening -- or killing -- the plant if infection is severe. On grapes and fruit trees, the disease is especially serious, attacking new growth -- buds, shoots, flowers and just-forming fruit -- as well as leaves and stems. Subsequent growth is dwarfed and distorted, often covered with the mildew; young fruit develop scars and corky skin.

Most garden plants are susceptible to powdery mildews, but pine trees and their relatives are totally immune. However, different species and genera of the fungus attack different plant species to varying degrees. Powdery mildew is especially serious on cucumber, squash, rose, phlox and lilac.

However, grasses, beans, peas, lettuces, grapes, small fruit trees and euonymus are also quite susceptible. The most common genera found around and in gardens are Erysiphe, Sphaerotheca and Podosphaera.

Control:
Certain plants have varieties that have been cultivated to resist the disease. If powdery mildew has been a problem in the past, choose a variety that has some resistance to the disease. Thorough management practices must be implemented in cases where plants are frequently plagued by powdery mildew. In most cases this fungal growth can be partially removed by pruning out and disposing of infected material.

Early detection of disease symptoms is vital to effectively controlling this disease. For outdoor plants, gather up fallen leaves in autumn and destroy them.

Begin fungicide sprays when the first white patches are noticed. Repeat as indicated on the product label during cool humid weather. It may be necessary to rotate among fungicide when disease becomes resistant.

Chemical treatment:
Here are some recommended products: Banner, Daconil, Bayleton, Triact, Terraguard, Systhane, Pipron, Rubigan, Banner Maxx (Novartis), Sentinel 40, WG (Novartis), Benomy, Mancozeb, Maneb, and Captan.

There is also a home remedy that is not usually on horticulture lists. Mix 1 tbsp liquid soap, 1 gallon of water, 1-½ tbsp baking soda, put in sprayer and spray leaf surfaces thoroughly. The soap helps the baking soda to stick to the leaves. Repeat for the next two days if needed. Rain or sprinklers can wash it off.

Downy Mildew can usually be found on the underside of the leaves and not just on the top of the leaves. Downy mildew begins with yellow leaf lesions that appear as indefinite yellow or brownish areas on the upper leaf surface that represents a spore producing fungus. The fungus grows within pods covering the seed with a white crust of spores. The texture of downy mildew is more cottony or fuzzy than powdery mildew.

With this is corresponding white, downy, or cottony fungal growth on the lower surface. Later, these areas become light to dark brown spots with yellow-green margins. Sometimes lesions are oily, somewhat angular, and are located between the veins. These yellow blotches usually occur on leaves in the lower plant canopy first; always examine leaves with these patterns of leaf yellowing for early detection of downy mildew.

The texture of downy Mildew is more cottony or fuzzy than powdery mildew. Downy spores grow in tree-like structure where powdery spores grow in chains.

Leaf blades in grass have white or yellow-green linear streaks running parallel to the leaf veins. Leaves turn yellow and there may be some browning of leaf tips. The disease is disfiguring and growth may be stunted. The disease is usually more severe in moist shaded areas.

Host Plants:
Downy mildew fungi infect almost all ornamental plants and many indoor plants. Greenhouse crops: snapdragon, salvia, alyssum, pansy, rose, rosemary, and ornamental cabbage. Perennials: aster, coreopsis, geranium, veronica and viola.

Conditions that favor downy mildew:
Requires humid conditions; thrives in cooler temperatures (around 65ºF)

A film of water is needed for spore germination and infection. Prolonged periods of leaf wetness promote this disease.

During cool wet weather, downy mildew can occur very rapidly and is difficult to control. This same host specificity exists with the downy mildews also; rose downy mildew will not go to pansy; downy mildew of snapdragon will not go to alyssum.

Downy mildews like it cool and moist.
Under ideal conditions of 65ºF temperatures. Below 40ºF the spores won't germinate, and they're killed by exposure to 80ºF temperatures for 24 hours.
Downy mildew is spread by splashing water.

Downy mildews like it cool and moist. Under humid, cool conditions, spores appear copiously on the lower surfaces of leaves, growing in tree-like formation on branched fruiting structures, unlike the powdery mildew spores, which are produced in chains.

In the presence of water from a recent rain or heavy fog, the spores will germinate within four hours; sporulation on leaf surfaces may occur in three days under ideal conditions of 65ºF temperatures. Dry winds and warm, clear days inhibit spore production. Roses, for example, are unaffected by downy mildew when humidity is less than 85ºF. Unlike powdery mildew spores, which are spread by the wind, downy mildew is spread by splashing water.

Damage:
More often, downy mildew is a problem for greenhouse or nursery growers. Thus it can become a problem for homeowners if they unknowingly purchase infected plants. Downy mildew infection results in a soft, fluffy gray, brown, white or purple fungal sporulation developing on the underside of leaves. On some plants, downy mildew infection may look similar to injury from foliar nematodes. In both cases, angular lesions are bounded by leaf veins.

Symptoms can also vary depending upon the host plant. Generally, leaves turn mottled and off-color with angular purple blotches. Pale green or yellow patches develop. Plant health and vigor are diminished.

Downy mildew fungi are parasites that can only grow on live plant tissue. Infection occurs inside the leaf. In ornamental crops, fungi in the genus Peronospora, and sometimes Plasmopara cause downy mildews. Different species of Peronospora affect closely related plants. The fungi that cause downy mildew are closely related to the Pythium and Phytophthora (rots) water molds.

Control:
Control recommendations are the same as for Powdery Mildew, with a few differences. Become familiar with the symptoms on plants that are susceptible to downy mildew. Check undersides of leaves for the gray, purple, brown or white blooms of spores. Undersides of leaves may have branching patterns that look like "deer antlers".

For outdoor plants, always water early in the day, so leaves can dry before nightfall. Use drip irrigation whenever feasible. Maintain proper plant spacing to reduce humidity levels within the plant canopy.

In some cases it may be beneficial to simply remove badly infected plants. Spores can be easily moved from plant to plant; subsequently, downy mildew may be carried over in infected plant debris in the soil and in weed hosts.

In the greenhouse, keep relative humidity below 85 percent to decrease fungal spores and disease development. Use fans to keep air moving in the greenhouse and to prevent cold spots from developing where condensation occurs.

Protect susceptible plants with preventative fungicide applications during cool, wet weather. Repeated applications may be needed.

Consider these treatments: Daconil 2787, Pace (rose only), Fore, Triact, Captan or Mancozeb, Maneb. The baking soda formula will not work on downy.

Please contact your county Extension Office for fungicide recommendations and instructions. ALWAYS READ AND FOLLOW PESTICIDE LABEL DIRECTIONS CAREFULLY.