Karnal Bunt of Wheat

Karnal bunt is first visible at the soft-dough stage in the form of 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 have been converted to masses of small black spores, which emit a fishy odor (due to the presence of trimethylamine).

Karnal bunt only minimally affects grain yield, but many countries have a zero tolerance for the presence of its spores in seed; consequently this disease has regulatory significance.

Karnal bunt is indigenous to the Punjab area of the Indian subcontinent, where it was first reported in 1930. It first appeared in Mexico in 1972. In March of 1996, its presence was confirmed in the U.S. in Arizona on certified durum wheat seed, and later that year on seed that had been planted in New Mexico, Texas, and California. Since that time, however, it has not been identified in California.

The disease spreads from spores that are present on infected seed and in soil contaminated from the previous crop. The delicate outer layer that surrounds each sac of teliospores is easily broken during harvest, dispersing the spores to contaminate the soil. The teliospores germinate in response to moisture and produce numerous sporidia at the soil surface. These spores are forcibly ejected from the sporidia and dispersed by either wind, splashing water, or insects. Sporidia have a short life span, even at high relative humidity, and generally survive for only short periods when airborne.

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

Use of disease-free seed is essential. Resistant cultivars are being developed, but at present, no cultivars are immune. Durum wheat and triticale, however, are less susceptible than bread wheat. In areas where the soil has become infested, rotate to crops other than wheat, durum wheat, and triticale for up to 5 years. Mulching with polyethylene can be used to raise soil temperature and reduce teliospore germination. Planting dates can also be adjusted so that heading does not occur under weather conditions conducive to infection.

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