Powdery Mildew Questions and Answers

Bob Mulrooney, Extension Plant Pathologist; bobmul@udel.edu and Gordon Johnson, Extension Ag Agent, Kent Co.; gcjohn@udel.edu

Powdery mildew has been a major disease in wheat in DE. However, we have seen relatively little in the last 2 years. Why?
The reason is the lack of favorable weather for infection and subsequent disease development. The weather has been drier and the relative humidity in the spring has been lower due to windier weather.

In what types of growing seasons should we expect more powdery mildew?
The seasons when powdery mildew is troublesome are those that are favorable for luxuriant fall growth followed by cool, wet springs.

What impact can powdery mildew have on yield if left untreated?
Powdery mildew can have a tremendous negative impact on yield if the disease is able to infect a high percentage of the leaf area on the three topmost leaves.

How do powdery mildew epidemics start and progress in a wheat crop?
Powdery mildew oversummers on old infected leaves from the previous crop. The fungus produces sexual spore producing bodies in the old infected leaves that protect these spores until fall when they forcibly eject the spores into the air and they are carried to newly planted wheat. These primary infections occur in the fall primarily and the fungus overwinters on the infected leaves. In the spring the fungus resumes growth and begins producing large numbers of asexual spores called conidia that spread the disease within the field and to nearby fields. The amount of disease that we see depends on the weather, the nutritional status of the crop and its resistance to powdery mildew. Powdery mildew is favored by temperatures between 60 and 75 °F and periods of high relative humidity.

When is powdery mildew control necessary?
Powdery mildew control becomes necessary if there is ample infection present on the lower leaves, 5-10 % of the fully expanded upper leaves are infected and the weather conditions are favorable for continued infection.

Do fall infections impact spring levels?
In most cases, yes. Fall infections can lead to increased levels in the spring. But if weather is not favorable and the variety is moderately resistant to resistant no further infection may occur.

How should a farmer scout for powdery mildew?
Growers are encouraged to begin weekly scouting for powdery mildew at Feekes stage 5 (tillers strongly erect) just before jointing begins(Feekes GS6). Examine plants from 5- 10 randomly selected areas within the field.

What are the thresholds for applying controls?
As mentioned before only apply a foliar fungicide when 5-10 % of the fully expanded upper leaves are infected and the weather conditions are favorable for continued infection.

What are the timings for controls?
The timing for application of fungicides is usually from flag leaf emergence through head emergence (GS 10.5), but can be applied as early as jointing if necessary.

Many varieties have some resistance to powdery mildew. Describe the rating systems for powdery mildew in wheat varieties.
Usually Cooperative Extension sources of information will use a scale for rating powdery mildew and other diseases. The scale usually is VS= very susceptible, S= susceptible, MS= moderately susceptible, MR= moderately resistant and R= resistant. Usually it’s the varieties that fall into the moderately susceptible rating and lower that require fungicide treatment when weather conditions are favorable for disease.

Where can farmers find information on the powdery mildew resistance levels in their varieties?
Extension publications and Extension and company websites are good sources of information on disease reactions of wheat varieties. You can find the ratings at this site on page 3-22 (PDF file) http://www.ext.vt.edu/pubs/pmg/fc3.pdf

Sometimes varieties that are listed as having powdery mildew resistance still get the disease at threshold levels. Explain.
This can occur especially in the spring either just at jointing or later if excessive amounts of nitrogen have been applied. The succulent growth is much more susceptible to infection, overcoming the genetic resistance in the plants. The other event that can happen is a change in the fungus population. Genetic changes in the fungus can occur, sometimes rather quickly (one season), that overcome the resistance genes in the wheat variety. This allows the fungus to infect a previously resistant variety. This happens when you have a fungus that reproduces sexually like powdery mildew and has the potential to produce mutations that overcome the resistance genes. The most recent case of this was loss of resistance in ‘Roane’. The resistance really isn’t “lost” the fungus overcomes the resistance genes bred into the variety rendering it susceptible.

Management strategies for powdery mildew include planting resistant varieties, using certain seed treatments, and foliar applied fungicides. How effective are each of these?
Planting varieties with ratings of resistant to moderately resistant should prevent the need for other controls. It is the best and least expensive control. However, if moderately susceptible or susceptible varieties are grown fungicide seed treatments containing Baytan or the high rate of Dividend will reduce powdery mildew levels by preventing fall infections. Foliar fungicide applications are very effective if the right fungicides are applied at the right time on susceptible varieties.

If foliar fungicides are needed, what products are available for control? What are the critical timings? How late can they be applied? When should they not be applied?
At the present time propiconazole (Tilt, PropiMax), trifloxystrobin + propiconazole (Stratego), azoxystrobin + propiconazole (Quilt), azoxystrobin (Quadris), pyraclostrobin (Headline) are the best products for powdery mildew control. Tilt, PropiMax, Quilt and Stratego would all be rated as very good to good for control of powdery mildew. All these fungicides can be applied up to head emergence except for PropiMax which can only be applied up to GS 8 (ligule of the flag leaf has emerged). The critical timing for application is from flag leaf emergence through heading.

Timing for powdery mildew control is most commonly earlier than for other wheat head and leaf diseases. Explain.
Timing is earlier because this disease is favored by cool conditions that we typically see in early spring depending on the season.

Any final thoughts on powdery mildew in 2008?
One of the advances in breeding for resistance to powdery mildew is the addition of genes that provide slow mildewing and the increase in varieties with mature plant resistance. Slow mildewing results in some powdery mildew being present throughout the plant canopy but infection levels increase very slowly resulting in very little to no yield loss. Adult plant resistance is increased resistance in the uppermost fully expanded leaves often just the flag leaf (sometimes it’s even triggered by temperature) but what you see is little development on the upper fully expanded leaves. This is important to know so that fungicides are not applied needlessly when light infections are present.

I would end the discussion by saying that disease control is especially important given the increased value of wheat, but remember that fungicide applications protect the yield that is present and at least as fungicides are applied on Delmarva, they do not increase yield. Fungicides protect what inputs have already been applied. The economics of fungicide application are most favorable for fields with good to excellent yield potential (>60 bu/A) and when powdery mildew is a threat to reaching the full yield potential of the crop.

Questions by Gordon Johnson, Extension Agriculture Agent, UD and responses by Bob Mulrooney, Extension Plant Pathologist, UD.

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