Meloidogyne root knot nematodes are one of the most commercially important types of plant parasitic nematodes. They are a sedentary, obligatory root endoparasite that infects over 3000 plant species. They are high on the list of living pathogens affecting world food production and related sectors due to their economic importance on a global scale.
They are found in a wide range of geographical situations around the world but are more common in tropical and subtropical climate zones, and cause significant economic losses in a variety of agriculture and horticulture crops, including vegetables.
Meloidogyne obscura, Meloidogyne javanica, Kofoid and White Treub, Meloidogyne hapla and Meloidogyne arenaria Neal. The four main species reported to have a significant negative economic impact on vegetable crops are Chitwood. The first three species, which are prevalent in tropical and subtropical regions, are also found in temperate places, particularly if they are protected. M. hapla, the latter species, is found in temperate zones and at greater altitudes in the tropics.
Meloidogyne incognita is the most common species in vegetable agriculture in India, followed by Meloidogyne javanica and Meloidogyne arenaria. M. hapla has also been found to infest vegetables growing in temperate hilly locations. Furthermore, they not only decrease crop yields but also have an impact on vegetable quality.
Economic importance of Root-Knot Nematodes:
Vegetable crop loss from root-knot nematodes is estimated to be 10% annually around the globe. However, depending on the nematode type, location, and crop species, significantly higher % losses have been reported and the people of the soil. In addition to the direct damage they cause, root-knot nematodes act as a catalyst for the entry of soil-borne bacterial and fungal pathogens, increasing the problem by causing disease complexes and significant production losses in the range of 40 to 70% in vegetable crops cultivated throughout the country.
Furthermore, protected cultivation crops like carnations, roses, gerbera, and anthuriums are severely affected by root knots nematode species like M. incognita, M. javanica and reniform nematode (Rotylenchulus reniformis), which additionally infest crops like tomato, chilli, gherkins, okra, muskmelon and watermelon.
Races of Root-Knot Nematodes and their distribution
Four well-known species of root-knot nematodes, including M. incognita and M. javanica, M. arenaria and M. hapla have been discovered to contain races of these nematodes. Sasser was the first to note the variations in host ranges among the four principal Meloidogyne species.
Parasitism and symptom expression:
In the second stage (J2) infective juveniles wander freely in the soil in search of a suitable host, then inject hydrolytic enzymes released by oesophageal glands into plant cells through their stylet and migrate intracellularly until they reach the developing vascular cylinder. This migration continues for a short distance before J2 becomes sessile in the differentiation zone (cortical tissue. J2 incorporates its head in vascular tissue and the remainder of its body in the cortex, which is parallel to the longitudinal axis of the roots.
This allows it to start feeding on xylem and phloem cells and generates specialized multinucleate feeding cells or giant cells in the phloem or adjacent parenchyma. Multinucleated giant cells are feeding sites that form as a result of repeated end mitosis without cytokinesis. Adjacent cells multiply through hyperplasia, resulting in multiple knots or galls on the root system. These cells are highly specialized cellular modifications that provide nematode development and reproduction with a constant feeding source.
Second-stage juveniles feed optimally for 2-3 weeks and, under ideal conditions, moult into third and fourth juvenile stages in a short period (usually 4-6 days).
Management of Root-Knot Nematodes:
Under intensive vegetable cultivation, root-knot nematodes’ high reproductive potential, polyphagous nature and unique survival strategy made management more complex. It is nearly tough to eradicate root-knot nematodes once they have established themselves in the field.
Unless there is a regulatory requirement for total nematode control, eradicating nematodes is neither economically nor environmentally friendly. Management of root-knot nematodes in vegetable crops relies on cultural, physical, biological, chemical and genetic techniques.
Hafiz M. Rizwan Mazhar and Dr. Kamran Saleem
Plant Protection Division, Nuclear Institute for Agriculture and Biology, Faisalabad
