Disease Highlights

Humid Weather is Conducive to Powdery Mildew Diseases

The humid weather is triggering the development of many types of powdery mildew.  I’ve seen powdery mildew on many plants lately in gardens and in natural areas.  Almost any garden, landscape, or farm has plants that can be affected by powdery mildew.  Unlike most fungal plant pathogens, the powdery mildews do not require the surface of the host plant to be wet for infection to occur.  However, most powdery mildew fungi are favored by high humidity.  Many powdery mildews grow best in temperatures about 60-80 degrees.  Consider that while our daytime temperatures are much higher than that, our nighttime temperatures tend to be in the perfect range for powdery mildews.  Powdery mildew fungi produce massive numbers of spores that blow in the wind and cause disease to develop rapidly.

Before continuing the discussion of powdery mildew I would like to point out that powdery mildews are very different from downy mildews.  Downy mildews are not true fungi.  Rather, they are oomycetes (or “water molds”) and require wet conditions for infection.  Since they are not true fungi, some chemicals that control powdery mildew do not control downy mildew, and vice-versa.

Okay, on to powdery mildew.  Powdery mildew fungi are host specific.  Powdery mildew of oak, roses, cucurbits, bluegrass, etc are caused by different, but closely-related, species.   The powdery mildew fungi all grow as white or whitish-grey colonies on the surface of leaves.  Some powdery mildews also grow on flowers, young fruit, and other succulent tissue.  When conditions are conducive for disease the colonies can expand to cover the entire surface.  Young succulent shoots and leaves sometimes develop curling, twisting, or other distortion.  Severely infected tissue will dry out and turn brown.  Powdery mildew weakens the plant by reducing photosynthetic capacity and by causing early defoliation.  While not usually fatal, the disease causes severe blemishing.  Food crops can have reduced quantity and quality of yields.

Cultural methods can help to reduce disease pressure.  Since powdery mildews thrive in high humidity it is important to promote good air circulation with proper plant spacing, pruning, weeding, or trellising.  Avoid planting susceptible plants in shady conditions.  If possible, choose varieties with resistance to powdery mildew.

Chemical controls can help if applied at the right time.  If the powdery mildew is already at high levels chemicals will not do much good.  For homeowners some options include products with myclobutanil, chlorothalonil, triforine, or thiophanate-methyl.  Organic options include neem extract, potassium bicarbonate, horticultural oil, and sulfur products.  Again, it is important to catch the disease early before disease pressure builds up. 

Commercial growers have multiple options for chemical control.  However, it is important to note that the powdery mildew fungi are notorious for developing resistance to fungicides with narrow modes of action (some of the systemic chemicals).  As one example I’ll discuss cucurbit powdery mildew which is active this year in northeast Kansas.  Researchers have documented many cases nationwide where the pathogen that causes powdery mildew on cucurbits has developed resistance to several classes of fungicides (DMI and strobilurin).  Commercial growers with a history of powdery mildew problems should consider planting resistant varieties if possible.  Chemical applications should be done in a timely manner. 

As always, be sure to read and understand the label.  Products and labels can change and it is your responsibility to understand how to use the material.

Powdery mildew can be a challenging disease.  If you have a history of powdery mildew damage be sure to read up on the specific powdery mildew in your planting, and you can contact me at K-State Plant Pathology for more information. [Kennelly]

Soybean Rust Diagnosis

Thanks to a continuing grant from the Kansas Soybean Commission/Kansas Soybean Growers Association, the K-State Plant Disease Diagnostic Lab is once again able to provide FREE diagnosis of samples submitted for possible soybean rust infection.  There are two major leaf spotting diseases of soybeans in Kansas that could be confused with soybean rust, Septoria brown spot and bacterial blight.  By offering this service, we hope to be able to confirm when rust is NOT present.

To take advantage of the free diagnosis, suspect soybean leaves should be placed in a sealed plastic bag.  DO NOT      place any moistened towels or free water in the bags since this will cause the leaves to begin to rot while in the mail.  Also, do not place the leaves in paper bags or press them between newspapers because dried samples are also very difficult to work with.  Clearly mark the bags as being rust samples and include the normal diagnostic lab check sheet appropriately filled out.  Avoid mailing on Thursdays or Fridays when possible in order to keep samples from being left in post offices over the weekend.

Samples submitted for problems other than rust will be charged the normal fees. (Jardine)

Common Bunt of Wheat

We have received several calls and e-mails regarding a problem with some wheat in north central Kansas.  Two obvious symptoms present include large clouds of black
dust that occur while the wheat is being combined and a “fishy” smell to the wheat.  The problem has been reported in an area roughly from Washington to Beloit and
south to Lindsborg. The problem has been identified as common bunt. The following is an excerpt of some information provided by USDA wheat pathologist, Dr. Bob
Bowden. 

“There are several types of smut that can attack wheat: loose smut, flag smut, common bunt (stinking smut), dwarf bunt, and Karnal bunt. 

Loose smut replaces the grain and chaff with black powdery spores. You see it around flowering time. After that, the spores all fall off or blow away and all you have left 
is a bare rachis. It is not known to have a bad smell.

Flag smut causes grayish-black stripes on the leaves, and it is very rare in Kansas. It also is not known to have a bad smell

Common bunt, dwarf bunt, and Karnal bunt can all produce a fishy smell. Dwarf bunt and Karnal bunt are not known to occur in Kansas.
 

Common bunt causes the entire kernel to be replaced by bunt spores; however the outer pericarp usually remains intact. The result is a dark, puffy looking kernel called 
a bunt ball. These often survive the combining process and get into the harvested grain. Common bunt can easily be diagnosed by viewing the spores microscopically.

Question: How severe does common bunt have to be in a field to create an odor problem?  

Answer: Probably around 1% of the plants infected would start to cause a stink.

Question: What conditions may have existed to magnify the odor—humid weather, late harvest?

Answer: Common bunt is primarily seed-borne in Kansas. The most likely cause of the problem is continually planting bin-run seed without using a seed treatment. 
You need to either get new seed regularly or use a seed treatment fungicide. In very, very dry summers, the disease can also be soilborne. Either way, seed treatments 
should still work. 

Question: What seed treatments (currently available) are effective in the control of common bunt?   

Answer: Almost every commercial seed treatment should work on common bunt.  Vitavax, Enhance, Dividend, and Raxil, are all good to excellent.” (Jardine)

Disease Highlights

Honeylocust, ornamental pear, maple, cottonwood, willow/austree, rose, barberry - Environmental scorch (hot/windy weather)
juniper, euonymus - Drought stress (wide spread plant damage)
Ajuga  - Tobacco ringspot (KDA)
Apple - Cedar apple rust
Columbine - Powdery mildew
Cucurbits - Powdery mildew
Elm - Dutch elm disease
Hollyhock - rust
Hosta - Hosta virus X
Oak - Oak wilt
Pear - Some old fireblight, some environmental scorch, cedar-hawthorn rust leafspot
Tomato - Rhizoctonia root rot, tomato spotted wilt virus (store tomato), physiological leaf roll
Crops - Compaction 

Contributing Author(s):

Megan Kennelly           kennelly@ksu.edu 
Doug Jardine                jardine@ksu.edu

Web updated 9/01/06
Web comments to jpierz@ksu.edu