Cultural Practices for Karnal Bunt Control
Extension, College of Agriculture & Life Sciences, The University of Arizona
Mike Ottman, Specialist, Plant Science
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 USA, Iran,
and South Africa. The disease has been reported in Nepal and Brazil in
Karnal bunt requires free water in the soil for teliospores to germinate.
The teliospores produce sporidia which are the spores that infect the
plants 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 florish. 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 of Northwest Mexico, the environmental conditions in March during
heading and flowering were: 79ºF average maximum temperature, 50ºF
average minimum temperature, 70 to 75% average relative humidity, and
0.24 to 0.50 inches of rain in two to three 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, the 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 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 because
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 will often
be destroyed during harvest. A "fishy odor" that may be detectable
from heavily infected grain is common to Karnal bunt as well as several
other bunt diseases and is caused by aromatic alkaloids present in the
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
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 reducing 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 reintroduction from
other fields is high especially in an area already infested with Karnal
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 to be found
in areas where lodging or water ponding have occurred.
Table: Effectiveness of various
cultural practices in reducing the incidence of Karnal bunt when teliospores
are present in the soil.
Environmental conditions from awn emergence through the
end of flowering are critical for disease development. Certain cultural
practices can reduce the occurrence and severity of the disease, but no
control measure is likely to eliminate the disease completely. Karnal
bunt is most likely to be found in areas where lodging or water ponding
have occurred. Zero tolerance for teliospores or bunted kernels can render
any control measure ineffective.
The author greatly appreciates the helpful suggestions
of Dr. G. Fuentes of CIMMYT. The original version of this pamphlet was
published by the Arizona Grain Research and Promotion Council in 1996.
The University of Arizona is an Equal Opportunity/Affirmative
Action Employer. Any products, services, or organizations that are mentioned,
shown, or indirectly implied in this publication do not imply endorsement
by the University of Arizona.
Document located http://cals.arizona.edu/pubs/crops/az1287/
Published July 2002
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