Karnal Bunt of Wheat

Karnal bunt is first visible at the soft-dough stage as blackened areas surrounding the base of the grain; however, the disease is not usually noticed until the grain is threshed and partially smutted kernels are exposed. Unless the disease is severe, only a few florets per spike are affected and diseased spikes are not conspicuous because the glumes are not noticeably distorted. In severely infected spikes, however, the glumes may spread apart near maturity, exposing the infected seed. While diseased seeds usually retain a partial seed coat, the embryo and part of the endosperm are converted to masses of small black spores, which emit a fishy odor because they release trimethylamine.

Karnal bunt affects wheat, durum wheat, rye, and triticale. Although it only minimally affects grain yield, many countries have zero tolerance for the presence of its spores in seed. Consequently, this disease has regulatory significance.

In March 1996, the presence of Karnal bunt was confirmed in Arizona on certified durum wheat seed and later that year on seed that had been planted in New Mexico, Texas, and California. No known infections have been identified in California since that time, and quarantine regions are no longer established in the state.

The pathogen produces survival spores that can persist in soil and plant debris from season to season and can cause infection in future years. At harvest, these survival spores are easily dispersed throughout the field. When a new small grain crop is planted, the survival spores present in the soil germinate in response to moisture and subsequently produce numerous infectious spores at the soil surface, which infect the new crop. These new infectious spores are dispersed by either wind, splashing water, or insects. They have a short life span, even at high relative humidity, and generally survive for only short periods when airborne. Although these infectious spores are not long lived, they are continuously produced and can cause infection throughout the growing season.

Plants are most susceptible to infection when spikes emerge from the boot, but infection can occur throughout the flowering period. Newly produced spores infect the ovaries, directly penetrating the glumes and ovary wall. Diseased kernels may be partially or completely displaced by masses of spores that persist in soil in future years. These survival spores require a dormant period of up to 6 months before they can germinate, and they remain viable in the soil up to 45 months.

Cultural Control

Use of disease-free seed is essential. Durum wheat and triticale are less susceptible than bread wheat. In areas where the soil has become infested with the pathogen, rotate to crops other than wheat, rye, durum wheat, and triticale for up to 5 years. Mulching with polyethylene can be used to raise soil temperature and reduce spore germination. Planting dates can be adjusted so that heading does not occur during the cool and rainy conditions that are favorable for infection. Genetic resistance has been identified, but resistant varieties have not been evaluated as a management strategy in California.

Chemical Control

Although no seed treatment is 100% effective, several treatments that inhibit spore germination are available.