Symptoms and Signs
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).
Comments on the Disease
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.
|Common name||Amount per cwt∆||REI‡||PHI‡|
|(Example trade name)||(hours)||(days)|
|Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first— the most effective and least likely to cause resistance are at the top of the table. When choosing a pesticide, consider information relating to the pesticide’s properties, efficacy, application timing, honey bees, and environmental impact. Always read the label of the product being used.|
|(RTU-Vitavax-Thiram)||5–6.8 oz||24||See comments|
|MODE-OF-ACTION GROUP NAME (NUMBER1): Carboxamide (7) and Multi-site contact (M3)|
|COMMENTS: This product is not labeled for use in California, but seed treated in and obtained from another state can be legally used in California even for a chemical not registered on small grains in California. Contact your retail seed dealer for information and availability. Do not use treated seed for feed, food, or oil purposes. Do not graze or feed livestock on treated areas for 6 weeks after planting.|
|B.||PCNB 24% EC||Label rates||NA||NA|
|MODE-OF-ACTION GROUP NAME (NUMBER1): Aromatic hydrocarbon (14)|
|COMMENTS: This product is not labeled for use in California, but seed treated in and obtained from another state can be legally used in California even for a chemical not registered on small grains in California. Contact your retail seed dealer for information and availability. Do not use treated seed for food, feed, or oil purposes.|
|(Dividend XL RTA)||1.0 fl oz||48||See comments|
|MODE-OF-ACTION GROUP NAME (NUMBER1): Demethylation inhibitor (3) and Phenylamide (4)|
|COMMENTS: For use on barley and wheat only. Do not use treated seed for feed, food, or oil purposes. Do not graze green forage for 55 days after planting. Do not plant any crop other than wheat within 30 days to fields in which treated seed was planted.|
|(Raxil-Thiram)||3.5–4.6 fl oz||24||See comments|
|MODE-OF-ACTION GROUP NAME (NUMBER1): Demethylation inhibitor (3) and Multi-site contact (M3)|
|COMMENTS: For use on barley, oats, triticale, and wheat. Do not use treated seed for feed, food, or oil purposes. Barley, oat, triticale, and wheat green forage may be grazed 31 days after seeding.|
|∆||Centum weight (cwt) is 100 pounds.|
|‡||Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI. The longer of two intervals is the minimum time that must elapse before harvest.|
|1||Group numbers are assigned by the Fungicide Resistance Action Committee (FRAC) according to different modes of action. Fungicides with a different group number are suitable to alternate in a resistance management program. In California, make no more than one application of fungicides with mode-of-action group numbers 1, 4, 9, 11, or 17 before rotating to a fungicide with a different mode-of-action group number; for fungicides with other group numbers, make no more than two consecutive applications before rotating to a fungicide with a different mode-of-action group number.|