Molya disease is widespread in wheat, barley, and oat growing areas of the world and is particularly important in Europe, Canada, Australia, and India. Molya disease has been first reported on wheat and barley in Rajasthan in 1957. It is now known to be widespread in the states of Rajasthan, Haryana, Punjab and Delhi.
The affected areas in Rajasthan usually suffer upto 50% crop-damage. The annual losses to the extent of Rs. 40 million in wheat and Rs. 30 million in barley are estimated in Rajasthan. The disease is also known as ‘cereal cyst nematode’ or ‘cereal cyst eelworm’ disease.
The host range of the pathogen (Heterodera avenae) is confined to the members of family Poaceae (= Graminae) such as wheat, barley, oats, rye, bajra, maize, and other various grasses. In other countries oats and barley are more susceptible than wheat, the condition is vice versa in India where wheat is more efficient host than barley.
Symptoms of Molya Disease:
The infected plants become dwarfed and pale appearing as they have been affected by nutritional deficiency. The leaves are discoloured to yellow and often become reddish from the tip. Tillering is markedly reduced and severely attacked plants fail to produce ear-heads. The roots are killed at apex hence do not grow further. New roots are produced near the killed apex in a characteristic resetting manner.
In badly affected plants the whole root system becomes dwarfed and matted. Since the development of long-roots penetrating into the deep soil layers is inhibited by the disease-effect, the affected plants are very susceptible to drought conditions. In advanced stages of infection, mild swellings appear near the root tips. This condition appears normally within 4-6 weeks after sowing, i.e., by the end of November or early December.
Glistening white bodies (females) are seen adhered to the roots by the middle of February. These bodies (cysts) become brown and may either remain attached to the roots or fall off in the soil within two weeks of before-harvest. In newly attacked fields, the disease appears in small patches of 2-3 feet diameter. These patches gradually increase in diameter every year until the whole field gets infested if the same crop is cultivated year after year in the infested field.
Causal Organism of Molya Disease:
The females of this nematode usually vary between 0.55 and 0.75 mm in length and about two-thirds as wide as long. They possess paired ovaries. Cysts are typically lemon-shaped, brown, measuring 470-1010 x 370-730 µm. They occur in soil and each cyst contains 225-250 eggs. On attaining to full length the embryo in the egg undergoes the first molt giving rise to second-stage larva.
These larvae completely develop within the egg inside the cyst. Fully developed second-stage larvae escape via the valve and other apertures in the cyst wall and migrate through the soil in search of a suitable host. The second-stage larvae are cylindrical tapering more in the posterior than in the anterior portion of the body and measure 490-615 mm in length including 50-79 mm long tail.
The second-stage larva penetrate the host usually just behind the growing root tip, grow rapidly, and three molts take place in the host. Sexes are differentiated at the third-stage larvae; the males develop a single testis, while the females form paired ovaries.
Shortly thereafter the third-stage, a fourth molt occurs and the fully developed fifth-stage adult male larva comes out of root, enters in soil, moves for some time and, finally, dies. In contrast, the fully developed female larva of fifth-stage becomes typical lemon-shaped cyst (adult female), which develops eggs that completely fill the body cavity. These cysts (adult females) can be seen adhering to roots and gradually become brown mature cysts after death of the female.
Molya Disease Cycle:
(i) Perennation:
The nematode survives in soil by means of its cysts. The latter get separated from the roots and fall into the soil. The eggs present inside the cyst do not hatch immediately. They require sometime to mature and the intervening period between two crop seasons serves as a maturation period for the eggs and hatching of the second-stage larvae.
(ii) Infection:
The fallen cysts are the main source of primary inoculum for the host crop in the beginning of a growing season. The second-stage larvae produced by egg-molting escape via the valve and other apertures in the cyst wall and come into the soil. They migrate through soil in search of suitable host.
They penetrate the latter usually just behind the growing root tip, enter inside, and establish within the host causing primary infection. Secondary infection does not take place as there is only one generation of the nematode in a year. Finally, the cysts adhered to the root surface of the host fallen in soil and serve as the source of primary inoculum for the next growing season.
Predisposing Factors:
The maximum emergence of second-stage larvae from cysts is reported to be at a constant temperature of 20-22°C under Indian conditions. Cysts present in well aerated soil hatch better. Light textured soils with good moisture, aeration, and drainage facilitate the migration of second- stage larvae in soil. Continuous cultivation of host crops in the same field favours inoculum build up year after year especially when the soil is not exposed to drying for more than four months.
Management of Molya Disease:
(i) Infested field should be kept fallow during the summer months and ploughed 2-3 times. This definitely reduces the primary inoculum because the cysts are very susceptible to desiccation and cannot tolerate hot sun and hot summer winds.
(ii) Since the nematode is highly host specific, long crop rotations (at least 4 years) are more effective hence recommended. If a non-host crop such as chickpea or mustard is sown for 2-3 consecutive years before host crop are planted, a profitable host crop can be raised in the field.
(iii) Green manuring of infested soil with chopped cabbage leaves heavily reduces the larvae in the soil. This practice increases the activity of nematode trapping fungi such as Arthrobotrys oligospora and Dactylaria thaumasia in the soil.
(iv) Early sowing of the crop (i.e., in early November) and soil application of Aldicarb at the rate of 2 kg a.i./ha results in significant control of the disease. Soil fumigation with D-D mixture, a volatile fumigant, normally kills 90% larvae. This treatment is effective for short time because the remaining larval population multiplies rapidly in presence of susceptible plants.
(v) No any wheat variety is available that could show resistance to the disease. However BP 263, C164, and BP 264 are the barley varieties that are highly resistant to molya disease in India.
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