Loose smut on wheat: Causes and treatments

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There have been several reports in recent years of loose smut in wheat. It is not uncommon to find low levels of loose smut in wheat fields, and the symptoms will be obvious by this time of year.

It is easy to pick out plants with loose smut in a field. The spikelets of infected heads are completely black and sooty instead of the normal, healthy color. There is no grain. Instead, infected heads consist entirely of a mass of fungal spores.

Photo by Erick DeWolf, K-State Research and Extension.Loose smut in wheat. Loose smut is a seedborne disease that is caused by the fungus Ustilago tritici. The fungus that causes loose smut survives as dormant mycelia within the embryo of an infected wheat seed. When the seed germinates, the fungus becomes active again. The fungus develops within the growing point and moves into the developing grain tissue as the wheat plants grow.

When the head emerges, there are masses of black spores on the spikelets instead of flowering parts. By harvest only an erect bare rachis remains. The spores are released into the air and can be blown onto healthy wheat heads were infection takes place at flowering or the early stages of kernel development. If the infection is successful, the fungus begins to grow within the developing wheat seed embryo.

Newly infected grain appears healthy in every way, but when it germinates the following season, the plant that grows from the infected seed will produce nothing but a dark mass of spores instead of healthy grain. The yield loss on infected heads is total. On a field-wide basis, the amount of yield loss is proportional to the percentage of infected heads.

Cool (60-70 degrees), humid weather accompanied by light showers or heavy dews is most favorable for infection. Under favorable weather conditions, the wheat produced from a field with only one percent of the heads infected, can have seed with 10 percent or more infection of loose smut.

Once loose smut becomes evident in the field, it is far too late to control the disease. The best option at that point is seed treatment. If producers have a field that is infected with loose smut and plan to keep some of the grain back for seed, they should be sure to have the seed commercially treated with a systemic fungicide seed treatment such as Charter (triticonazole), a Dividend (difenoconazole) product, a Raxil (tebuconazole) product, or RTU-Vitavax (carboxin)-Thiram. These fungicides provide excellent control of loose smut, but good coverage of the seed is very important to ensure that the maximum benefit of the treatment is realized.

Another option is to sell all the wheat from the infected field as grain and buy certified seed to plant in the fall. Certified seed in Kansas is allowed to have as much as 10 heads in 1,000 (or 1 percent) that are infected with loose smut. There is no requirement that this seed be treated in order to qualify as certified seed by the Kansas Crop Improvement Association, but it would be a good idea to buy treated seed. The cost of having seed treated with a standard low-rate fungicide seed treatment for loose smut is relatively low. Costs are higher if the seed treatment also includes an insecticide, such as Cruiser or Gaucho.

There are no varieties are highly resistant to all races of loose smut.


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