Loose smut of wheat

Loose smut of wheat

Pathogen:- Ustilago nuda

Systematic position of pathogen:- Class- Basidiomycetes; order-Ustilaginales; family- Ustilaginaceae.

Distribution :- It is fairly common disease in most of the wheat growing tracts of India, without often becoming epidemic or doing much damage. Sometimes the disease causes severe damage, destroying about30 per cent ears of some infected fields. Its distribution is throughout the world. This disease is of common occurrence in western districts of utter Pradesh.

The pathogen:- the loose smut of wheat is caused by Ustilago nuda. The innumerable smut spores develop on the mycelium found in the intercellular spaces of the host. The smut spores are very minute, pale olive-brown, lighter on one side, spherical or occasionally oval, and measure 5-9 micron in diameter. The outer wall of it called exospores is thick and echinulate, whereas the inner wall exospores, is thin and smooth. On maturity, the two opposite-strained nuclei of each smut spore are fused together and a diploid nucleus is formed. The smut spores are carried by wind, insects and other agencies to the stigma of the flowers where they germinate producing basidia or promycelia. The exosporium ruptures and the tubular basidium comes out. The diploid nucleus of smut spore divides reduction ally forming 4 haploid nuclei. The basidium becomes segment and the four uninucleate forming two of + and two of – strains. From each segment single infection thread develops. The two opposite-strained infection threads unite and adikaryotic cell is produced. The dikaryotic hypha develops from the dikaryotic cell. This process is called dikaryotization. The dikaryotic hypha enters the style and reaches the embryo where it develops into thick irregular and branched mycelium. The mycelium remains dormant in the embryo. The seeds with dormant mycelia in their embryos look quite healthy in outer appearance.

                The dormant mycelium established in the embryo of the seed becomes actives at the time of the germination of the infected seed and grows along with the apex of coleoptiles. The optimum temperature for the growth of the mycelium ranges from 20-25 degree Celsius .The fungus is systemic and the mycelium travels throughout in the stem. On the emergence of the ears, the mycelium reaches in the floral parts and become activated. The thousands of smut spores are producing in smut sori on the floral axis. All the floral parts become infected except awns. The haustoria are not formed. The food is absorbed by diffusion method.

Nature and recurrence of disease :- the disease is seed borns. The dormant mycelia perennate inside the embryo of infected seeds and whenever such seeds are sown the seedling become infected from the very beginning. The disease happens to be internally seed borns and systemic.

Control measure

Resistant varieties:- this is the simplest and most important method of control. Every year many resistant varieties are evolved at research centers and distributed to the farmer for sowing. The important resistant varieties are: NP 710, NP 718, NP 770, Bansipali 808, Bansi 224, NP 792, NP 729, MP160, MP168, MP169, etc. These resistant varieties are recommended for cultivation in utter Pradesh, Punjab, Bihar and Rajasthan.

Rogueing :- the infected ears should be burnt outside the field, so that, the smut spores may be destroyed. This method can easily be practiced, because of the earlier emergence of the infected ears from the boot leaves.

Seed  selection:- since the disease is deed-borne, the seeds should be selected from healthy crops.

Hot water treatment:- to activate the dormant mycelia the seeds are soaked  for four or five hours in the water at normal temperature ranging from 26-30 degree Celsius . thereafter the seeds are being transferred in hot water at 54 degree celsiuys for ten minutes. The activated mycelium dies at this temperature. These grains may be used as seeds. The temperature of water should not exceed in any way because the embryo of the seed dies at 56 degree Celsius. This process seems to be risky and not practical.

Solar radiation:- this is the modification of hot water treatment. The seeds are soaked in water for four hours, and then the seeds are dried up on the floors in the open sun in very thin layer from 12:00 noon to 4 P.M, in the month of may and june. The dormant mycelium become activated on soaked the seeds in the water for 4 hours, and dies when  the seeds are spread in thin layers in the hot sun and the seeds become free of pathogen.

Chemical treatment:- it has been suggested that after soaking the seeds in water for six hours, they should be kept for 40-50 hours in 2 per cent spergon or chlorose at 50⁰ F to 72⁰  F by this method the pathogen is destroyed and the seed become fit for sowing.