Addressing Downy Mildew and Powdery Mildew in the Home Garden

Although downy mildew and powdery mildew appear to be fungal infections, there are distinct differences in the biology, symptoms, life cycle, and plant damage of the two diseases. It is important to note these differences in order to prevent or treat them in the home garden.

Description

Downy mildew is an obligate parasite of vascular plants. Although downy mildew is similar to fungi and absorbs nutrients from infected plants, it is not a fungus. It is now classified in the Peronosporaceae family and of the phylum Oomycote. It is a disease that thrives in cool, humid regions and environments.

While downy mildew is an oomycete pathogen (water mold disease), powdery mildew is a fungal disease caused by a variety of species. It consists of a large number of microscopic spores (conidia) borne in chains. As the fungi spread on the plant, it develops structures that insert into plant cells to extract nutrients.

Symptoms

Downy mildew first displays as a colony of sporangia that looks like mats of fluffy growth on the underside of plant leaves. It generally has a bluish tint, but the color varies from white to gray to purple. Small yellow dots will then appear on the upper surface of the leaf opposite the site of the sporangia growth. Eventually, the leaf will wilt and wither. It can infect the plant's leaves, stems, flowers, and fruit. Infected leaves and branches might distort and die. In some plants, such as roses and grapes, downy mildew initially presents as an oily spot on the leaves.

Powdery mildew looks like flour dusted on the surface of leaves. It starts as circular powdery white spots on leaves, stems, fruits, or flowers. Layers of mildew made of many spores form across the top of leaves. It will spread to cover most of the leaf and usually appears on the upper leaf. Young foliage is the most susceptible. At maturity or in the fall, round, black specks can form in the mildew. These are sexual fruity bodies known as cleistothecia.

Damage

Downy mildew is potentially more damaging than powdery mildew. Fully infected plants lead to damaged fruit or new growth problems. It is considered an important pathogen since left untreated, it can do severe damage to crops and is capable of killing all plants within a week.

While not as potentially damaging as downy mildew, powdery mildew can still be harmful to the plants in the garden. Powdery mildew can slow the growth of the plant. If the infection is severe, leaves can turn yellow and wither. The mildewed parts of the plant might become distorted and stunted, including flowers which will also be fewer in number. Fruit yield and quality are sometimes compromised.

Life Cycle

The differences in the life cycle between downy mildew and powdery mildew are important to note in order to understand the best ways to prevent infection.

Downy Mildew

Cool temperatures (58-78°F) and relative humidity (>85%) near the plant or leaf surface favor infection. A sporangium germinates by forming a germ tube that penetrates the plant and lives off the plant’s water supply. In some species, sporangia germinate indirectly by releasing zoospores. Post-infection, some spores begin to appear as soon as four days, but 7 to 10 days is more typical. The spores penetrate the plant with mycelia. The mat of sporangia releases more spores which are windblown to other areas of the plant or other plants in the area. It disperses into the soil via rotting plant debris. An area can be infected for up to five years. In southern climates, some species overwinter in soil and plant debris and reappear when infected matter touches plant leaves and new plant growth in the spring. However, they do not survive in the cold winters in Pennsylvania and are blown in from southern climates in the spring.

Powdery Mildew

The growth of powdery mildew is favored by a warm (60-80°F), dry climate. High relative humidity encourages spore formation, and low relative humidity encourages spore dispersal. Cool days and humid nights are ideal conditions for establishing and spreading the disease. This disease does not require free water for germination and infection but does require high relative humidity around the plant to spread. It does not spread in cooler, rainy conditions and is slowed down by high temperatures (>90°F). Powdery mildew is most severe in crowded, shady, and poorly ventilated areas. It affects plants more in shady areas than in sunny areas and is less common in summer. Once germinated, new spores are produced every 3 to 14 days. In the fall, cleistothecia crack open to release spore sacs that contain ascospores that blow onto nearby plants and infect them. New outbreaks sometimes come from dormant spores in old vegetive material or weeds. Overwintering occurs as mycelia form mats on crops or weeds. It can survive the winter attached to plants and plant debris.

Affected Plants

The various species of downy mold largely specialize in a particular species of plants. A wide variety of trees, shrubs, field crops, vegetables, brush fruit, and garden plants can be affected. The following is a list of species that are of particular concern to the home gardener:

• Basil downy mildew (Peronospora belbahrii)
• Cucurbit downy mildew (Pseudoperonospora cubensis) – squash, pumpkin, melons, cucumber
• Downy mildew (Hyaloperonospora parasitic) affecting Brassicaceae – broccoli, brussel sprouts, cabbage, cauliflower, kale, radish, and turnips
• Downy mildew (Peronospora sparsa) affecting roses, blackberries, raspberries, and other Rubus spp.
• Impatiens downy mildew (Plasmopara obducens)

Powdery mildew affects hundreds of species of trees, shrubs, vines, flowers, vegetables, fruits, field crops, and grasses. Like downy mildew, specific species of powdery mildew affect specific species of plants, whether they be cucurbits, nightshades, legumes, or roses. Here is a list of some common garden plants that can be affected by powdery mildew:

• Azalea
• Blueberry
• Cucumber
• Crabapple
• Crepe myrtle
• Dahlia
• Delphinium
• Dogwood
• English ivy
• Euonymus
• Lilac
• Oak
• Pecan
• Phlox
• Photinia
• Pyracantha
• Rhododendron
• Rose
• Spirea
• Snapdragon
• Squash
• Wisteria
• Zinnia
• Management

Once infected, it can be difficult to eradicate either downy or powdery mildew, so the focus of the home gardener should be on prevention rather than treatment. Many preventative actions are common for both diseases. As always, it is best to start with cultural solutions to prevent infection.

When selecting plants, choose disease-resistant cultivars. Regularly scout susceptible plants and zones in the garden that are susceptible based on the life cycle of the diseases and the at-risk plants for early detection. Remove plant suckers as new growth is most susceptible. Ensure good air circulation by staking plants and/or selective pruning, keeping the relative humidity around the plants low. Avoid overcrowding, overwatering, and poorly drained soil. Eliminate weeds for the same reason. Replace overhead watering with drip or soaker irrigation. If overhead watering cannot be avoided, do it in the early morning so that the plant has time to dry during the day.

If the symptoms of either disease are detected, begin by eliminating infected leaves, branches, flowers, or fruits. For a severely infected case, it might be necessary to remove the entire plant to protect the rest of the garden. Dispose of the infected material by burning or throwing it away in a closed container. Do not compost diseased plant material, as most home composters do not reach temperatures high enough to kill the spores. Sterilize garden tools between use on infected versus uninfected plants to prevent the transfer of spores. Remove plant debris, especially in the fall, to prevent overwintering, and dispose of it in the same manner as the infected plant material.

There are fungicidal treatments available for both downy and powdery mildew, but they do have limitations and other effects on the environment. For organic treatments, neem oil and copper or phosphorus fungicides can be used. Both have shown good results but are not guaranteed to eliminate the disease. Therefore, they are best used as a preventative measure with repeated applications during the season when the conditions are ripe for germination and spread. Copper-based treatments have a negative effect on some plants, so further research before use is required. Neem oil kills all insects, including bees, that come in contact with it. It should be used judiciously and applied in the evening when there is less insect activity. Some spores have developed fungicidal resistance, so if applying this type of treatment on a large scale, it is best to periodically vary the fungicide.

Always carefully follow all label directions when using a fungicide. Contact the Penn State Extension office in your area for specific recommendations.