Evaluation of Bioagents for their Efficacy in Management of Root-knot Nematode Infesting Mulberry

The perennial plant mulberry (Morus alba L.)is highly adaptable to varied climatic conditions ranging from temperate to tropics and thrives well under different soils. Foliage is the major economic part of mulberry that ultimately decides the quality of raw silk since the silkworm feeds on mulberry leaf alone to derive its nutrients for growth and productivity. The quantity of quality leaf produced per unit area has a direct bearing on cocoon production and raw silk quality. Several factors contribute in obtaining a successful cocoon crop, among them mulberry leaf alone contributes to around 38.20 percent which is followed by microclimate in the rearing house (37.00%), silkworm rearing techniques (9.30%) and the breed (4.20%), which signifies the role of quality foliage in cocoon production. Apart from soil parameters, the biotic and abiotic stress factors greatly affect the quality of mulberry leaf. The nutritive values get degraded due to diverse biotic stresses viz., diseases (pathogens) and pests (insect/ non-insect) and the mulberry plants attract these pests and diseases due to its perennial, fast-growing and lush green characteristics (Miyashita, 1986). The biotic stressors affect both above and below ground parts of the mulberry plant, the symptoms of which can be easily traced through the foliage.

A markedly higher population of fungi and nematodes could be observed in the mulberry rhizosphere (Nandi et al., 2004). Around forty-two species of nematodes belonging to 24 genera were associated with mulberry from almost all countries, where sericulture is being practiced (Swamy and Govindu, 1965). Among them, the nematodes belonging to five genera viz., Meloidogyne, Rotylenchulus, Helicotylenchus, Hoplolaimus and Xiphinema were reported from India. The root-knot nematode (RKN), Meloidogyne incognita species alone is known to cause 20-50 percent mulberry leaf yield loss apart from deteriorated quality (Arunakumar et al., 2018).

The infestation of M. incognita (Kofoid and White) Chitwood on mulberry was first reported in India by Narayan et al. (1966). The infestation is more commonly noticed in sandy to loamy soils and under irrigated conditions. The RKNs are parasites of underground roots, which is difficult to recognize and hence the damage symptoms very often go unnoticed (Sengupta and Govindaiah, 1991). The affected plants show stunted growth, marginal necrosis and yellowing of leaves. The characteristic knots or galls appear on the roots and affect the utilization of water and nutrients resulting in poor plant growth (Govindaiah et al., 1991).

The RKNs have three life stages – egg, larva with four juvenile stages (J , J , J , J ) and adult of which, the J infects the host 1 2 3 4 2 plants (Biligrami and Dube, 1976). The female larva enters into roots, harbours in the sub-epidermal layer and starts feeding on the parenchymatous cells causing hypertrophy and hyperplasia that induce characteristic knots. The larvae undergo four moults in the roots and develop into mature oval or spherical egg-laying females. Each female lays 200-300 ellipsoidal eggs covered with a gelatinous substance. The eggs hatch and larvae are liberated into the soil under favourable conditions. The life cycle is completed in 30-40 days and 2-3 such cycles are noticed per annum. A temperature of 15-30 ̊ C and soil moisture of 40-60 percent are more favourable for the growth and development of RKNs (Padma et al., 2008). The size and number of galls induced by RKNs keep on increasing as the generations are repeated in the root tissues. Apart from damaging the parenchyma tissue, the cracks and holes in the galls invite secondary root infections. Thus, the root-knot infected plants show symptoms of both nutrient deficiency and other root diseases like root rot (Babu et al., 1996).

The studies reveal that the infestation of RKNs not only affects the growth and development of the crop but also affects the physiology ultimately resulting in inferior quality of foliage. Further, since mulberry leaf is the sole source of nutrition for silkworm, B. mori, the cocoon crop and quality of raw silk are severely affected causing huge economic loss to the farmer. Hence, effective management of RKNs is imminent for sustained productivity in sericulture. Though synthetic nematicides are available for the management of RKNs, environmental concerns, mammalian toxicity and longer safety periods limit their utility in sericulture. Alternately, the use of bioagents antagonistic to the nematodes serves the purpose of nematode management apart from improving soil health, which is an apt strategy to pace with the increasing environmental concerns. With this background, an experiment was planned to identify suitable bioagents for eco-friendly management of RKNs infesting mulberry.