In this article we will discuss about the wilt of arhar caused by deuteromycetes.
Host: Cajanus Cajan
Pathogen: Fusarium oxysporum f.udum
Introduction to the Wilt of Arhar:
The wilt disease of arhar is a most severe threat to the arhar producing states in the country such as U.P., M.P., Bihar and in parts of Bombay.
Symptoms of Wilt Disease:
The susceptible plants are attacked when young about five to six weeks old. The symptoms are variable. Typically the first symptom is the premature yellowing of the leaves. The next symptom is the wilting or withering of the leaves of the diseased plants. The plant is in a blighted condition.
The leaves appear to be affected progressively from the bottom towards the top. Sudden wilting is rare. Finally the entire plant completely dries up. In severe cases of the infection more than half the plants in the field may fall victim to this disease.
The wilting is brought about by the plugging of the vascular tissue of the stem and roots of the host plant by dense masses of mycelial hyphae. The free flow of water to the leaves is thus interfered with.
This results in drooping and wilting of the leaves of the host plants. In addition the fungal hyphae produce toxic substances which kill the plant cells concerned in the ascent of sap. In cases where plugging of the vascular tissue of the host takes place on one side only, the wilting is partial.
Causal Organism:
The causal organism is Fusarium oxysporum f. udum Butler. The mycelium is septate and branched. It is colourless. The hyphae are both intercellular and intracellular.
Disease Cycle. (Fig. 22.22):
Asexual reproduction (Fig. 22.20) takes place by the formation of three kinds of spores, microconidia, macroconidia and chlamydospores.
(i) The microconidia (Fig. 22.20) are generally tiny structures produced in large numbers from the tips of conidiophores which are indistinguishable from the vegetative hyphae. The spores may be rounded, curved or elliptical and are formed within the tissues of the host.
Usually the microconidia are unicellular, sometimes with one or two septa. In cultures they are held together in a ball or false head.
(ii) Macroconidia (Fig. 22.20) are long, sickle-shaped or crescent-shaped spores. They are septate (with 3-4 septa) and have pointed tapering ends. The macroconidia are produced at the tips of short conidiophores which arise from the upper surface of cushion- shaped stromatic mass of hyphae.
The whole fruit body is called the sporodochium (Fig. 22.21). The macroconidia are shed as soon as they reach maturity.
(iii) The chlamydospores (Fig. 22.20) are thick-walled, rounded or oval cells formed in the hyphae. They occur singly or in chains and may be terminal or intercalary. The chlamydospores become separated from the parent hypha after maturing. They function as resting spores.
Infection of Wilt Disease:
Wilt of arhar is a soil borne disease. The microconidia, macroconidia and chlamydospores remain viable in the soil. At the sowing time the spores germinate and initiate new infections.
Infection takes place through the young roots or rootlets possibly through the root hairs Infection of the aerial parts of the plant through air-borne spores never takes place.
The fungus mycelium also exists in the soil as an active facultative saprophyte so long the dead host roots and other parts are present in the soil. It gains entry into the host through the rootlets and finally passes into the larger roots.
Control Measures of Wilt Disease:
The best method to control this disease is to produce wilt resistant varieties N.P. 15 and N.P. 38 are fairly resistant to the disease. Crop rotation, field sanitation and hot weather cultivation are some of the methods to eliminate soil borne infection.
The other recommendation is to protect the plants from the attacks of the wilt organism by the use of antibiotics and chemicals. Nothing positive has been done in this direction so far. Vasudeva. et al. (1952, 1954) reported that Bacillus subtilis produces an antibiotic called bulbuformin which inhibits the growth of the pathogen.