In this article we will discuss about the covered smut of barley caused by basidiomycetes.
Host: Hordeurn vulgare
Pathogen: UstHago hordei
Introduction to the Covered Smut of Barley:
The disease is of common occurrence in the barley growing tracts of India. It causes considerable damage in the Punjab, Uttar Pradesh, Madhya Pradesh and certain parts of Bihar.
Symptoms of Covered Smut Disease:
The first symptom of the disease makes its visible appearance at the flowering stage. From the flowering time onwards the ears of the infected barley plants in the fields appear blackened and shrivelled as soon as they emerge out of the boot leaves.
They can be seen from a distance. The smutted ears emerge from the boots at the same time as the healthy ones. They usually remain shorter and are retained within the boot leaves for a longer time.
Sometimes they fail to come out. Every ear in a diseased plant and every ovary of the smutted ear contains a compact spore mass called a smut sorus. The grains are thus replaced by the smut balls or sori. The spores in the sorus do not fall apart due to the deposition of a fatty substance.
They are held together by an investment provided by the walls of their respective ovaries and the basal parts of glumes of the spikelet’s. The smut spores in each sorus being firmly enclosed within a white, shining, silvery membrane are not blown by the wind. Flower infection is thus precluded.
The smut spores do not serve as means of propagation of the disease during the growing season. They are liberated by the rupture of the surrounding membrane as the ears get crushed under heavy pressure during threshing. The spores thus released adhere to the rough skinned healthy grains (Fig. 22.15 A).
Causal Organism:
The causal organism of this disease is Ustilago hordei (Pers.) Lagerheim.
Mycelium of Covered Smut Disease:
The mycelium is internal and dikaryotic. The hyphae are septate and branched. They ramify in the intercellular spaces between the cells in the host tissue. The mycelium grows in the stem as well as in the leaves but generally it is confined to the stem.
By the flowering time it reaches the ear where it grows more vigorously. The fungal hyphae enter the ovaries of the flowers. There they branch repeatedly and become additionally septate. The short, binucleate segments thus formed round off to form the binucleated smut spores.
The latter remain compacted together into a ball-like mass in each ovary. They are liberated at the threshing time by the breaking of the walls of the ovary. The spores thus set free remain attached to the surface of the healthy grains.
The spores are brown in colour but lighter on one side and globose to ellipsoidal in shape. In a mass they appear black. Each measures 9 p in diameter. The spore wall is smooth.
Disease Cycle:
The disease is externally seed borne and perennates through the smut spores which adhere to the healthy grains during threshing-time (A).
(a) Germination of Teliospores:
The binucleate smut or teliospores present in the soil or sticking to the grains (B) germinate in the presence of warmth and moisture when the contaminated barley grains are sown in the field.
Prior to germination, the two nuclei of dikaryon in the spore fuse to form a single fusion nucleus or the synkaryon. The diploid teliospore or probasidium as it is now called, germinates to produce a germ tube, the promycelium or the epibasidium.
In the epibasidium, the synkaryon undergoes meiosis to form a row of four haploid nuclei. Segregation of plus and minus strains takes place at the time of division. Two of the nuclei are of plus and two of minus strain.
Septa are laid between the nuclei. The epibasidium now consists of four haploid segments or cells (G). Each segment of the epibasidium produces one uninucleate sporidium or basidiospore. Two of these basidiospores are of plus and two of minus strain.
They are ovate to oblong in form. The basidiospores of covered smut of barley are capable of multiplying by budding in the presence of nutrition like yeast. The secondary spores produced by budding germinate like the basidiospores and are capable of infecting fresh barley plants.
(b) Infection:
The basidiospores or secondary spores germinate on the surface of the grain. Each produces a slender haploid germ tube. The haploid germ tubes by themselves are incapable of infecting the barley seedlings.
The germ tubes from two different basidiospores of opposite, strains growing in the vicinity fuse (C). As a result one of them becomes dikaryotic. The dikaryotic germ tube or the infection thread infects the barley seedling at a very early stage.
The covered smut of barley therefore provides an example of seedling infection. The dikaryotic hypha penetrates the young coleoptile and enters the growing tip of the embryo. There it grows upwards keeping pace with the growing point (D).
It forms a dikaryotic mycelium. The mycelium grows upwards in the leaves and the stem. It grows more vigorously in the stem than in the leaves. By the flowering time the mycelial hyphae reach the inflorescence region (E1) and enter the ovaries to produce the smut spores (E2). Infection fails to take place if the primary shoot of the host has come above the soil surface.
Effect of Covered Smut Disease:
The host is not killed outright. Its vitality is no doubt lowered. Thus the infected plants show stunted growth. In severe cases of infection the yield may be lowered by 50 per cent of the crop.
Control Measures of Covered Smut Disease:
Since the disease is surface borne external application of fungicides to the grain is quite effective.
1. Chemical Grain Dressing:
It may be applied in the form of liquid dressing or in the form of dust.
(a) Liquid Grain Dressing:
Dressing the barley grains with a solution of one fluid oz. of formalin to 2 pounds of water gives very good results. The practice of liquid gram dressing is, however, not so popular these days.
Firstly, because the procedure requires cumbersome apparatus and secondly, the grains treated with liquid dressings do not store well. They have to be sown promptly after the treatment.
(b) Dry Grain Dressing:
Chemical compounds in the form of lime dust such as Agrosan, Ceresan Sulphur dust. Formalin dust and Spergon etc., are very popular as grain dressings. Care should be taken that each grain is lightly coated with the fungicide.
Since the dusts are persistent the dusted grains can be stored. Moreover, the dry grain dressings with disinfectants protect the seedling not only from the surface borne parasite but also from the disease organism in the soil.
Agrosan GN at the rate of 4 oz. per 37 kilograms of seed gives excellent control. Grewal and Dharam Vir (1964) reported that Ceresan and Agrosan 5 W completely control the disease.
Sulphur dust which is capable of passing through a sieve with 300 meshes per linear inch (300 mesh) is most effective to control the disease.
2. Growing resistant varieties:
Acting on the principle, “Prevention is better than cure” the most effective method is to grow disease resistant varieties of barley. Mathur, et al. (1960) reported that K12, K18, K19, C50, C84, and CN249 are moderately resistant.
3. Rogueing of plants with smutted ears is another preventive measure. They should be uprooted and burnt with great care so that the smut spores may not fall on to the ground and be a potential source of infection.