Root Knot of Vegetables PDF

# [F] DISEASES CAUSED BY NEMATODES ## (1) ROOT KNOT OF VEGETABLES: * **Causal organism:** _Meloidogyne spp._ (Nematode). The common species causing root knot of vegetables are _M. arenaria_, _M. incognita_ and _M. javanica_. * **Host:** Potato, tomato, brinjal, bhindi, cucurbits, carrot, chillies,...

# [F] DISEASES CAUSED BY NEMATODES ## (1) ROOT KNOT OF VEGETABLES: * **Causal organism:** _Meloidogyne spp._ (Nematode). The common species causing root knot of vegetables are _M. arenaria_, _M. incognita_ and _M. javanica_. * **Host:** Potato, tomato, brinjal, bhindi, cucurbits, carrot, chillies, groundnut, radish, etc. * **Symptoms:** (Fig. 12.30 A) Infected plants show above-ground symptoms in the form of sickly and stunted growth. The leaves become yellowish-green to yellow, tend to droop and suddenly wilt. The underground roots and tubers (in case of potato) show gall formation due to enhanced cell-division. Usually the roots of infected plants bear spherical to elongated large-sized galls. * **Environmental relationship:** The root knot organisms are most destructive in sandy loam or silt loam soils. The most favorable temperature for infection, multiplication and gall formation ranges between 12°C to 35°C. High temperature above 40°C kills the larvae of these organisms. Survival and movement of pathogen larvae depends upon various physical, chemical and biotic soil environments. The movement of larvae is much slow in soils having excess moisture, clay content and lack of aeration. * **Host-parasite relationship:** (Fig. 12.30 B-D) The disease inciting organism belongs to a dominant group of plant parasitic nematodes. Larvae of these nematodes survive in the soil and remain motile for different durations. They are attracted by the root exudates towards the roots of susceptible host and held in mass around them. Since they are obligate parasites, they cannot survive longer in absence of hosts. The invading larvae first penetrate the host surface and migrate upto endodermis. The female nematodes insert their heads in the intercellular spaces near the endodermis. The male nematodes die. Growth of pith cells becomes arrested while cortical cells around the mouth of feeding females enlarge to become giant cells. Sometimes the walls of adjacent cortical cells may disintegrate. Adult female nematodes remain embedded in the host tissues. They lay eggs which are commonly found to be clinging to the surface of roots. The egg are ellipsoidal in shape. * **Disease cycle:** (Fig. 12.31) The pathogen is soil-borne: The eggs of nematode survive in plant debris. The first molt of larva differentiates within the egg which hatches under favourable conditions to release the second stage of infective larva. These infective nematodes possess piercing spears or stylet to penetrate the host surface. They apply their stylets on the weaker spots of host surface and exert pressure. An additional pressure is provided by surface tension of water film around the nematode body, normal reactions and frictions between soil particles and body of nematode. The pathogen is obligate parasite and derives its food from living host cells. The female adults (Vth stage of larvae) lay eggs which remain covered by host tissues. The eggs may also hatch on the surface of host tissues. Finally the larvae of soil infect roots of other hosts to repeat the disease cycle. * **Control:** 1. Application of soil fumigants, after destruction of root residues, in the field. ## FIGURE 12.30. Root knot of vegetables caused by _Meloidogyne spp._ (Nematode). * A. Symptoms; * B. Female adult; * C. Male adult; * D. Larva ## FIGURE 12.31. Disease cycle of root knot caused by _Meloidogyne sp._ (Nematode). * A. Eggs sac on the surface of old galls; * B-C. Single egg; * D. Ist stage larva; * E. IInd stage larva; * F. IInd stage larva free in soil; * G. IInd stage large attack rootlets; * H-KIInd stage larvae infecting and feeding on giant cells. The female starts laying eggs after 4th molt while adult male leaves the root; * M. Adult female starts laying eggs.