Controlling Root Knot Nematodes: a New Screening Test Identifies Bacterial Antagonists

The root knot nematode (RKN) Meloidogyne incognita causes damage in field and greenhouse crops. Agroscope researchers have developed a new screening test to identify bacteria that antagonise this agricultural pest.

Plant-parasitic nematodes are major agricultural pests against which only a very limited number of permitted plant-protection products may be deployed. Found worldwide, the Southern root-knot nematode Meloidogyne incognita is a plant-parasitic nematode causing considerable damage in our moderate climate zones, particularly in greenhouse crops. The nematodes penetrate into the root tips of the plant, causing the root tissue to form knots or galls (Fig. 1).

Bacterial antagonists instead of chemical nematicides

In Switzerland, chemical nematicides are either strictly controlled or no longer permitted. It is hoped that a new screening test for antagonistic bacteria of M. incognita will identify highly promising isolates for future integrated management.

The new screening test allows nematodes to continue the natural infection process into the root after treatment with the bacteria. Hence, unlike the conventional practice, antagonistic activity is assessed based on root-gall formation on an indicator plant, rather than on the visual assessment of nematode motility under a microscope. This should facilitate the evaluation of antagonistic potential as well as the identification of suitable bacterial strains.

Structure of the screening test for bacterial isolates

A mixture consisting of nematodes and a bacterial isolate was incubated in water for three days in the dark at 20 °C. After the addition of an earth and quarz-sand mixture, a three-day-old cucumber seedling (Cucumis sativus cv. Sprinter F1) was planted. Root-gall formation in the cucumber plants was scored after a further three weeks’ growth (Fig. 2).

Selection of Swiss bacterial isolates active against nematodes

The screening test examined 44 Swiss bacterial isolates of beetroot roots. Five bacterial isolates – 102, 105, 108, 119 and 157 – repeatedly demonstrated an antagonistic effect towards M. incognita. These bacterial strains were then tested in the greenhouse under different soil conditions.

Selected bacteria as a root-ball application

The selected bacteria were especially successful in a root-ball application to cucumber plants cultivated in mineral wool, with four of the five strains causing a significant reduction in root-gall formation.

Further experiments and analyses

Various pot experiments were also conducted to study the potential for controlling.M. incognita of Pseudomonas spp. 105 and 108 – the bacterial strains achieving the best results in the experiments. Both strains significantly reduced root-gall formation, regardless of the application method used. The genomes of the strains were fully sequenced and annotated to aid in their identification and subsequent characterisation.