Packing a punch against powdery mildew

Powdery mildew usually does not result in plant mortality, but it can reduce a plant’s growth, and it greatly reduces the aesthetic appeal and marketability of infected plants.

Powdery mildew attacks a great number of bedding plants, perennials and woody ornamentals. Some of the most susceptible perennials include coreopsis, monarda, phlox, rosemary, scabiosa and veronica. Susceptible shrubs include azalea, crape myrtle, euonymus, hydrangea, lilac and roses. Although powdery mildew occurs frequently during production, it can be easy to prevent and control.

Recognize it
The most characteristic indication of a powdery mildew infection is the presence of whitish-grey talcum-like powdery patches (¼ to ½ inch in diameter) of fungal growth on the upper leaf surfaces. The first mildew colonies are often difficult to detect and usually are located on the lower leaves.

These patches, also referred to as colonies, initially go undetected until the environmental conditions become favorable for widespread infections. Early infections of powdery mildew often resemble spray residue and are often overlooked until the disease becomes more apparent.

The colonies of fungal growth often appear quickly, seemingly overnight. The patches often coalesce to completely cover the infected leaves, and in some cases, the entire plant becomes covered with fungal growth.

On many plants, powdery mildew can be more difficult to detect. In some instances, the leaves of infected plants develop a purplish discoloration without the formation of the white colonies. This can easily be mistaken as a nutrient disorder. In a few instances, leaves infected with powdery mildew develop brown scab-like spots with very little, if any, white powdery growth present.

Under severe infections, plant growth can be adversely affected. In some cases, the leaves may appear twisted or distorted and plant vigor is reduced. With many plants, the leaves turn yellow and wilt and die.

Fungus biology
There are several fungal pathogens (Erysiphe, Leveillula, Microsphaera, Podosphaera and Spaerotheca) that cause powdery mildew diseases. One characteristic that separates mildews from other fungal pathogens is that they are host-specific, which means the powdery mildew that infects one type of plant is usually different from the mildew that attacks another. They often attack plants within the same family, however. Some of the powdery mildew diseases are very host-specific and only attack one or two different species of plants, while others have a wide host range and can infect a wide range of plant species.

Another characteristic that differentiates powdery mildew diseases from many other pathogens is they can only grow and reproduce on live plant tissues.

Unlike many foliar diseases, powdery mildew does not require free water on leaf surfaces for infections to occur. Extended periods of high relative humidity (greater than 70 percent), particularly at night, trigger the formation of new spores and the onset of new outbreaks. The initial infections require high relative humidity and moderate temperatures (62°-72°F); disease infections occur more rapidly when the temperatures are warmer (70°-80ºF is ideal). Once the pathogen has infected the host plant, the relative humidity is no longer a factor, as powdery mildew can prosper regardless of the humidity.

Spores of this pathogen are spread by air movement such as wind or fans from heating and cooling systems. Once the spores land on plants, it takes three to seven days for an infection to develop and form visible colonies, provided favorable conditions are present. The spores release, germinate and cause infections without a film of water on the plant surface. The fungus grows into small colonies on the leaf surface.

Reduce the occurrence
The occurrence of powdery mildew can be reduced if high humidity levels or dramatic swings in humidity can be avoided. Providing ventilation two or three times during the night to dehumidify the air is particularly effective.

Growers can also take steps to reduce the humidity levels near the plant canopy where infections occur. First, maintain sufficient plant spacing. Second, increase the air circulation with horizontal air flow fans.

Scout the crop
It is beneficial for growers to identify and monitor any crops that are susceptible to powdery mildew diseases on a regular basis. Routine scouting should occur at least weekly, more if favorable conditions for this disease have occurred.

The first mildew colonies can be difficult to detect; look for small colonies on the lower and middle portions of the plants. Examine the upper and lower leaf surfaces. Powdery mildews can be distinguished from spray residues because chemical residues do not appear fluffy and often have a more droplet-like outline. Under conditions not favorable for spore production, plants infected with mildew do not have the distinguishing white colonies. Instead, the leaves have a purplish or red coloration which are often confused with nutritional disorders.

Control strategies
To obtain the most effective results, control strategies should be implemented as soon as colonies are detected. Preventive programs could be implemented, but they are not necessary, because powdery mildew can be eradicated when it is detected early. With preventive strategies, it is better to apply the control products during or just after periods that are optimal for the development of these fungi. When detected and treated early, mildew colonies can be eradicated with no reduction in plant performance or quality.

Several biorational compounds effectively control powdery mildew diseases. When using these products, it is important to achieve complete coverage. Multiple applications are often necessary. Some of the most effective biorational compounds contain the active ingredients bacillus subtilus, hydrogen peroxide, neem oil, paraffinic oil or potassium bicarbonate.

There are several fungicides that provide very good to excellent control of powdery mildew diseases. Fungicides containing the active ingredients azoxystrobin, copper sulphate pentahydrate, myclobutanil, petroleum oil (horticultural oil), piperalin, pyraclostrobin, pyraclostrobin + boscalid, triadimefon and triflumizole provide the best control of these diseases and are labeled for applications within enclosed structures. If chemical controls are necessary, be sure to rotate between chemical classes to reduce the likelihood of the mildew diseases from developing resistance to these products.

Using non-ionic spreaders with many of the above fungicides often helps these products penetrate through the established mycelium and improves the efficacy of the application. Refer to each product’s label for information regarding the use of spreaders.

With the host-specific nature of powdery mildews and several different species of mildews attacking ornamentals, there is not a single fungicide that works equally well on all mildew diseases.

The dead colonies will remain on the plant surface for several days after treatment. Infected plants should be checked after treatment to observe the efficacy of the treatment for signs of new growth or signs of continued mildew outbreaks. Multiple applications may be necessary to obtain control of powdery mildew. Growers should take note that it is difficult to eradicate existing, well-established mildew colonies.

Disclaimer: No discrimination is intended for products not mentioned in this article.

Paul Pilon is a horticultural consultant, owner of Perennial Solutions Consulting (www.perennialsolutions.com) and author of Perennial Solutions: A Grower’s Guide to Perennial Production. He can be reached at (616) 366-8588 or paul@perennial solutions.com.