Karnal bunt is a disease of wheat, durum, and triticale caused by the fungal pathogen Tilletia indica Mitra. Karnal bunt was first reported in India in 1931, and later in Pakistan, Mexico, the U.S., Iran, and South Africa. The disease has been reported in Nepal and Brazil in isolated areas.
Karnal bunt requires free water in the soil for teliospores to germinate. The teliospores produce sporidia spores which infect the plant’s florets. Cool, cloudy, and very humid conditions or rainfall between awn emergence and the end of flowering, is required for sporidia production, infection, and for the disease to flourish.
The incidence of Karnal bunt is usually very low and rarely seen if the environmental requirements are not met.
In two years of high incidence of Karnal bunt in the Yaqui Valley in northwest Mexico, the environmental conditions in March during heading and flowering were: 79 degrees F. average maximum temperature, 50 degrees F. average minimum temperature, 70 degrees to 75 degrees F. average relative humidity, and 0.24 to 0.50 inches of rain in 2-3 rainfall events.
The disease cycle starts with deposition of Karnal bunt teliospores in the soil. Teliospores may remain dormant but viable for several years. The source of teliospores could have been seed, wind, animals, contaminated equipment, or other sources.
Teliospores located at the soil surface germinate in response to moist conditions and produce sporidia. The plants are susceptible to infection for a 2 to 3 from awn emergence to the end of flowering when sporidia infect the florets and fungal hyphae enter the ovary.
Subsequent disease development in the embryo end of the kernel results in the formation of new teliospores which are deposited back in the soil at harvest, adding further to soil inoculum.
Karnal bunt is not easily detected in the field since few florets are typically infected and the area of the kernel affected might be small and facing inwards.
A mass of black teliospores is found at the embryo end of the kernel and, at higher levels of infection, along the crease or in the entire kernel. Generally, only a portion of the kernel is occupied by teliospores (partial bunt).
Fully bunted kernels are often destroyed during harvest. A ‘fishy odor’ that may be detectable from heavily infected grain is common to Karnal bunt plus several other bunt diseases and is caused by aromatic alkaloids present in the spores.
The effectiveness of any control measure for Karnal bunt is questionable since the disease incidence is usually very low or sporadic. Also, as long as zero tolerance for spores or bunted kernels exists, control measures will not solve the problem of Karnal bunt since complete control is unlikely.
Nevertheless, control measures can be classified into four broad categories:
Genetic: Development of resistant varieties is most effective long term strategy to minimize disease development.
Chemical: Some foliar fungicides are highly effective against Karnal bunt but none are currently registered for use. Seed treatment is not effective on the current crop since the fungus does not infect seedlings systematically. Furthermore, seed treatments have not been proven to reduce the viability of teliospores.
Crop environment modification: Lowering seeding rate and nitrogen fertilizer amounts and altering irrigation timing are only slightly effective unless taken to extremes where yield potential is jeopardized.
Changing the crop environment by delayed planting, however, can be highly effective if rainfall and optimum environmental conditions are avoided between awn emergence and the end of flowering.
Unfortunately, delayed planting has the potential for reduced yields. In Northwest Mexico, seeding two rows on 30-inch beds has shown to reduce disease incidence compared to drill seeding. Cultural techniques can suppress disease development but not eliminate the disease.
Soil teliospore reduction: Reduction of teliospore load in the soil by rotating to non-host crops or disinfecting the soil is highly effective, but the chances for teliospore re-introduction from other fields are high especially in an area already infested with Karnal bunt spores.
Environmental conditions between awn emergence and the end of flowering is the overriding factor in disease development.
Cultural practices may be partially effective in controlling Karnal bunt but cannot eliminate the disease completely.
Karnal bunt is most likely found in areas where lodging or water ponding have occurred.