Understanding Powdery Mildew

Posted on: Feb 5, 2021

Powdery Mildew is a fungal disease that has a high impact on production ornamentals. It grows in highly humid and warm environments, and it is easily identified by its white, powdery spots on the leaves and stems. Three of our experts from across the country share more about this fungal disease that has a substantial impact on production ornamentals.

Dan Dillman has been in the industry since 1980 in various capacities. He has a degree from Penn State in Horticulture and Plant Pathology, and he has worked for Helena Agri-Enterprises since 1997. Dan covers Western Pennsylvania and Eastern Ohio. 

Also contributing to the conversation will be Victor Gonzales, who has been in the industry since the mid-90’s. Victor earned a degree in Agriculture from Miami Dade College with 30+ credit hours in Nursery and Landscape Management.  His career with Helena began in 2011, and he covers the Southern Florida Nursery and Greenhouse markets. 

Our last participant is Ches Lemmert, who covers the North/Eastern Texas Grower market, as well as Oklahoma and Arkansas.  Ches brings 21 years of industry experience with him and has worked at Helena for a total of 18 years.  Ches holds a degree from Texas A&M-Commerce in Broadfield Agriculture with a focus on horticulture.

Why is Powdery Mildew control necessary?

Ches:If Powdery Mildew isn’t controlled, an unsightly white powder forms on the top of the leaves.  This disease, as with all diseases, stresses the plant, so that it is unable to thrive and grow to its fullest potential. It will eventually turn the leaves yellow and dry them out, so that they will fall off the plant.  Typically, mildew prefers 60-80 degree temperature, less sunlight or more shade and high humidity. However, unlike other fungus, spores are released in dry conditions. So this fungus disease needs both high humidity to germinate spores on a plant and low humidity to spread the disease. Without a quick response, its proliferation can become more difficult and more costly in potential loss of plant material and chemical control measures.


Are there cultural steps that could be taken to prevent Powdery Mildew, or is it primarily environmental?

Dan:Cultural practices, like keeping a clean growing environment, can help significantly with control. Spores can be on both live and dead tissue. It is recommended to not overcrowd plants, which allows good air circulation and more sunlight to keep leave surfaces dry and also keep good ventilation. Something important for all fungus control, too much nitrogen will encourage succulent plant cells, which are more susceptible to mildew and other diseases.

In your geographies, what have growers had the most success with for Powdery Mildew control? 

Ches: My experience has been in the Texas and Oklahoma market. Powdery mildew has been pretty easy to control with chemicals, and with all the preventative sprays in my area for other diseases, it isn’t as much of a problem as other diseases.  But for contact products, I suggest propiconazole, myclobutanil and potassium bicarbonate.  For preventative, longer-term control, Pageant®, Orkestra® and Heritage® are often used.

Dan:In the Western Pennsylvania and Eastern Ohio areas that we service, control for powdery mildew is broad spectrum. Anti-desiccants, like Transfilm®, will put a coating on the leaf surface, which will not allow spores to germinate.  Overhead irrigation can promote leaf disease by keeping leaves too wet, but a disinfectant can be used when watering overhead such as Oxidate®, SaniDate®, X3 or ZeroTol®. These products used occasionally will prevent leaf disease.  Softer chemical options, like Neem oils, MilStop® (potassium bicarbonate), EcoSwing®, light oils and sulfur, can all be effective. The quickest and most effective control is using a contact fungicide, such as chlorathnoil, mancozeb and captan. With a systemic fungicide, almost any DMI, strobilurin or newer combination product, add a good adjuvant such as Joint Venture® or Cohere® according to label instructions. With all leaf fungus, complete coverage is really important. If you have an outbreak of powdery mildew, chances are you will have other fungus issues creeping in, and the combo of contact and systemic with an adjuvant will prevent other problems.

Victor:  In southern Florida, I recommend Group 3 (propiconazole, myclobutanil, triticonazole), Group 11 (azoxystrobin, pyraclostrobin, trifloxystrobin), Group M1 coppers, Group M3 manzate and Group M5 chlorothalonil.

Is it a single application or a rotational program? 

Victor:  Read the label and choose the rate. As is the case with most pathogens, a preventative approach is better than a curative one. Do not forget to add an adjuvant to ensure proper coverage of your preferred control method according to all labels involved.

Lastly, are there any known resistance instances in Powdery Mildew control?

Victor:  A lot of fungicides have resistant strains of pathogens. Powdery Mildew is no exception. It is always wise to switch up your modes of action to avoid any possible opportunities for resistance to occur.

For more solutions to Powdery Mildew problems, and to find the right products for you, contact your local Helena representative.

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