BEING the eighth largest wheat producer of the world, the country every year suffers from acute shortage due to poor storage and marketing policy. Besides these, the crop is also exposed to many abiotic and biotic/living constraints during its lifecycle. Among biotic factors, insects, pests and diseases are the cause of reduction in yield and quality of the grain.

Wheat crop is susceptible to over 50 fungal, viral, parasitic nematode, bacterial and other diseases.

Karnal bunt or partial bunt is a fungal disease of wheat. It is called partial bunt when only part of the kernel is affected. Karnal bunt was first discovered in Karnal (India) for the first time in 2004 with the import of Australian wheat which was infested with the fungus. The disease is caused by smut fungus.

The infection occurs after heading when 20.sporidia, produced from spores germinating at the soil surface, are dispersed to wheat spike. Fungus threads (hyphae) from the spores penetrate the stomata and grow intercellularly to the base of the developing kernel.

The infection occurs in cold weather (59-72oF), cloudiness, overhead irrigation, or high humidity and rainfall favour development of the disease. In the absence of cold and rainy weather, chance of KB is reduced. High rates of nitrogen application and heavy manuring, may also increase the disease incidence. Cultivars sown between November 25 and December 15 are more prone to KB than those sown earlier between October 25 and November 24.

Propagation: Dissemination of spores before harvest is limited. However spores dispersed through wind after their release from fruiting bodies and dispersed during burning of wheat fields may contaminate seeds, soil and machinery. The spores can also disperse after ingestion by livestock and grasshoppers.

The spores may remain viable for 27-45 months in the soil and can germinate at 5-30 degrees C between pH 4 and 11, the optimum being pH 6-9.5. However, germination of spores is better in alternating light than in darkness, or near UV light. Germination in continuous light has also been reported.

Damage: Instead of all spikes of a plant, usually only a few irregularly distributed kernels are bunted. Further, infection of individual kernels varies from small points of infection to completely bunted kernels. Affected kernels are usually partially infected, and completely infected ones are rare. The embryo is largely undamaged except when infection is severe. The spikes of infected plants are generally reduced in length and in number of spikelets. In infected spikelets, the glumes may be flared to expose bunted kernels giving rotten fish like smell. Blackening of the grains at germinal end which become slightly swollen and gives slivery appearance is the main symptom of Karnal bunt in wheat.

Yield and grain quality losses: Partially bunted kernel produces normal seedlings than those with severe infection. Despite the fact that increases in disease severity result in proportional decreases in seed weight, yield losses are generally light up to 0.5 per cent. Even during years of most severe epidemics 89 per cent infected kernel with yield losses ranging from 20-40 per cent in highly susceptible varieties are reported.

However, KB may reduce flour quality in view of colour, odor, and palatability of whole meal. Usually 1-4 per cent infected kernels are unacceptable for human consumption. At five per cent infection, quality distinctly deteriorates; loss in flour recovery and chemical changes in composition of flour and gluten content cause poor dough strength. Washed and steeped grains in wheat lots with 7-10 per cent infected grains are considered acceptable for consumption.

Control measures: The most effective and economical method of controlling the disease is by growing resistance varieties, although the impact of weather during these periods is also recognised. The most important varieties affected by Karnal bunt in Pakistan are Iqbal-2000, A.S.-2002, Shafaq-2006; K.C.-050, a new wheat strain from Bari, Chakwal, was badly affected by the smut.

Seed Treatment: Since Karnal bunt is soil borne, air borne and seed borne disease. Hot water and solar energy treatments have been applied to KB-infected seeds. However, they have had limited application.

Soil fumigation: Soil fumigation to control spore germination with some success is reported. In wet soil, methyl bromide reduces spore germination by 98 per cent when buried at depths up to 10 cm. Metam-sodium (Vapam) and formaldehyde were effective only on the soil surface. In dry soil, Vapam reduced germination by 57-99 per cent when spores were buried at depths up to 10 cm.

Fungicide application: The combination of soil fumigation with Brassicol (pentachloro-nitrobenzene), seed treatment with Bavistin (Carbendazim), and foliar sprays with propiconazole provide a range of 50 to 80 per cent control; even combinations of foliar sprays with seed treatment or with soil fumigation also reduces KB incidence.

Industrial importance: From production point of view, Karnal bunt is a minor disease because it has little effect on grain yield. However, it has a major effect on grain marketability. Many nations do not import wheat from countries where KB is known to occur. Therefore, Karnal bunt is a major threat to wheat industry.

Although these diseases can be effectively controlled through non-chemical and chemical treatments, most farmers are unable to follow the recommended practices due to economic constraints, or health and environmental hazards. Breeding for resistance varieties has been a successful research activity and certainly is the best method to prevent the crop from such diseases in the long-term. However, variation in pathogen may sometimes create a threat to the crop production. It is needed to be vigilant regarding seed sources.