Packing a punch against powdery mildew

Most growers have experienced a powdery mildew disease on certain crops at one time or another. 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.

Although powdery mildew occurs frequently during commercial production of ornamentals, it can be very easy to prevent and control. Learn to manage this group of diseases to minimize crop damage and losses.

Recognition
The most characteristic indication of a powdery mildew infection is the presence of whitish-gray, talcum-like powdery patches (0.25- to 0.5-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 diseases 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. However, they often attack plants within the same family. 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 all are obligate parasites. This means they can only grow and reproduce on live plant tissues. The fungus will die without the presence of live plants.

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°F to 72°F); disease infections occur more rapidly when the temperatures are warmer (70°F to 80°F 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, obtaining nutrients from the plant by penetrating into leaf cells with specialized structures called haustoria.

Reducing the occurrence
The occurrence of powdery mildew can be reduced if high humidity levels or dramatic swings in humidity can be avoided. When growing plants inside enclosed structures, it is usually beneficial to purge or ventilate the greenhouses regularly to keep the humidity levels below the optimal levels for disease development. 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. The first step they can implement is to maintain sufficient plant spacing; allowing adequate amount of space between the plants reduces the humidity levels at the plant canopy. The second method of reducing the humidity at plant level is to increase the air circulation using horizontal air flow fans.
 

Scouting
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 on at least a weekly basis, 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 since 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.

Detecting powdery mildew early and implementing control strategies upon detection will help growers greatly reduce the severity of the infections. It is very difficult to eradicate existing, well-established mildew colonies; early detection will allow control strategies to be most effective.

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, since 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’s 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. One fungicide may provide excellent control on one mildew species but provide little, if any, control on another. Although the powdery colonies look similar between species, they each have different fungicide sensitivities.

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 very difficult to eradicate existing, well-established mildew colonies.

A new control option
There is a new total-release product from BASF containing the active ingredient triticonazole that has provided excellent control of powdery mildew, according to research conducted by Dr. Ann Chase (Chase Agricultural Consulting LLC). Using total release delivery can be an improvement in foliar disease control, as the foliage does not remain wet following applications.

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

Editor’s Note: For more information about identifying and managing powdery mildew, see this month’s Plant Health column on page 55.

Paul Pilon is an author, horticultural consultant and owner of Perennial Solutions Consulting (perennialsolutions.com) He can be reached at (616) 366-8588 or paul@perennialsolutions.com.

Photos courtesy of Paul Pilon