Root-knot nematodes: understanding them to prevent them from spreading

Widespread throughout Italy, nematodes can cause direct and indirect damage to many cultivated plants, trees and herbs, particularly in the presence of certain conditions, such as mild temperatures, loose soils and lack of organic matter: root-knot nematodes are fearsome soil parasites that can be controlled with good soil management, especially in organic farming.

Nematodes are worm-like creatures, small in size (from a half to a few millimetres), with varying physical characteristics and behaviours according to the species.

While parasitic nematodes can be a cause of great concern for various crops that require careful strategy to both prevent and fight, there are also highly useful nematodes in organic and integrated agriculture, such as the Steinernema and Heterorhabditis genera that can parasitize the larvae of harmful Coleoptera, Lepidoptera, Diptera and Hymenoptera.

Nematodes are generally of two separate sexes, although cases of parthenogenesis and hermaphroditism are known. Among the most dangerous for cultivated plants there are Nematodes belonging to the Meloidogyne genus, also known as “root-knot nematodes” because they stimulate the formation of knots on the roots of the plants they attack. From a morphological point of view, adults of the Meloidogyne species show a marked sexual dimorphism with pearly white pear-shaped females and worm-like males.

Nematodes of the Meloidogyne genus are endoparasites that penetrate the tissues of the host—using enzymes to facilitate the breakdown of plant cell walls—in which they cause a disorganization of the cortical parenchyma cells and the production of knots that are externally visible. Together, these morphological alterations are responsible for inefficient water and nutrient absorption, causing yellowing leaves and reduced growth. In crops with roots, taproot bifurcation and anomalous development of secondary roots can also occur, while drastically lowering the quality of the product. In the case of trees, younger plants experience greater damage.

There are four species in the Meloidogyne genus to pay close attention to, as they are potentially harmful to agriculture, that are present in different areas of Italy. M. incognita and M. arenaria can be found from north to south, while M. hapla can be found primarily in the north and M. javanica primarily in the south. Damage occurs both in open field and protected crops.

While M. icognita and M. javanica cause damage mainly to vines, M. arenaria, M. hapla and M. artiella are more cause for concern for horticultural or industrial crops such as tomato, carrot, bell pepper, eggplant, potato, pumpkin, salads, asparagus, strawberry, beet, tobacco, but also to fruits such as kiwi and citrus plants or ornamental flowers, such as the rose. In addition to the four species mentioned above (the most common in Italy), we should also note Meloydogine graminicola, with a more limited spread that causes damage to rice. Tests carried out by Agribios in 2021 in rice fields in the province of Pavia have shown the effectiveness of treatments with Nemag R, containing the fungal antagonist Pochonia Clamydosporia, in promoting the recovery of root mass in affected plants, with obvious positive effects on production.

In the species of the Meloidogyne genus, eggs resulting from sexual reproduction are laid by the female in a sack on the back part of her body. After hatching, larvae which have already completed their first moult inside the eggs continue developing and moulting to reach the adult stage. Adults are equipped with a stylet they use to penetrate the host’s roots to begin to feed. The complete cycle, in optimal conditions, is completed in 3-4 weeks and, depending on the latitude and the type of crop that is affected (herbaceous crops or trees), the species can produce 6-7 generations in a year.

During the winter, or in periods in which host plants are lacking, the adult females and larvae encyst and enter a dormant state that allows them to survive in adverse conditions for up to three years, at varying depths in the soil—even up to 150 cm—making it difficult to eliminate these parasites from cultivated land.

The optimal conditions for the development of root-knot nematode infestations—which always start in localized areas of plots and then extend if left unchecked—are light and sandy soils (which explains their prevalence in coastal areas), humid, and characterised by low levels of organic substance. With poor mobility, nematodes are sometimes transported by drainage, tillage or man. Nematodes also love mild temperatures, which means that infestations tend to appear in spring and disappear in autumn, thus peaking between May and September.

The control of root-knot nematodes relies primarily on systemic products based on Oxamyl or other natural substances such as thymol and geraniol. Also highly effective are also microbiological products based on antagonistic fungi from the Paecilomyces, Pochonia and Arthrobotrys genera or bacteria with adulticidal activity, such as Bacillus firmus or Streptomyces avermitilis.

In both integrated and organic farming, however, agronomic practices are essential for an effective control of root-knot nematodes, in particular to eliminate dormant forms or to create soil conditions that are unfavourable to their development. Useful techniques include:

– planting certified and healthy nursery material

– using resistant or tolerant rootstocks

– rotations with less sensitive species such as cereals

– solarization and vaporization (mainly applicable in greenhouses)

– cultivating and green manuring of biocidal plants (brassicaceae) or distributing panels or pellets of brassicaceae seeds

– destroying residual infected vegetation

– eliminating stagnant water

– using organic substances

The Agribios Terrapiù line includes three fertilisers suitable for preventing damage from root-knot nematodes and to prevent them from spreading in cultivated soils in three different ways:

• by introducing microbial consortia to improve root activity: NEMAG R is an inoculum of mycorrhizal fungi that contains—in addition to an organic matrix with high protein content—fungi of the Pochonia genera and Artrobotrys, a consortium of microorganisms with strong ovo-adulticidal activity
• by supplying microbial consortia to solubilize microelements (B, Fe, Zn) that encourage vegetative activity and antagonistic microorganisms: FIRAV R is a liquid product based on Bacillus firmus and Streptomyces avermitilis, which exert adulticidal activity
• by supplying highly humified organic fraction and substances that exert containment action on the development of soil nematodes and parasitic insects (NEMAKIL 330, with Nitrogen, Phosphorus and Calcium; NEMAKIL 400, with Nitrogen, Iron and Calcium)