Differences in parasitism of root-knot nematodes (Meloidogyne spp.) on oilseed radish and oat

Evaluation of summer and winter cover crops for variations in host suitability for Meloidogyne incognita, M. arenaria and M. javanica

Root-knot nematode (RKN) is an important pathogen on vegetables; therefore, planting a non- or poor host cover crop following a susceptible vegetable crop is a promising management option. This study builds upon previous studies and evaluates the variations in host status of cover crop candidates for reducing the reproduction of RKN populations (Meloidogyne incognita, M. arenaria and M. javanica) in Georgia, USA, to shed light on previous inconsistencies regarding the host status of cover crops and effectiveness in the field. Two glasshouse trials tested the host status of 14 plant species and 18 cultivars plus susceptible tomato ‘Rutgers’. Sixty days after inoculation, roots were evaluated for galling (GI) and egg mass index (EI). Gall formation was not a reliable indication of RKN reproduction for many cover crops, which had higher EI than GI. Based on GI, all cover crops were either non-hosts, ranging from non-hosts to poor hosts or poor hosts to all three RKN species, except blue lupine and hairy vetch, which were susceptible to all three RKN species and had a GI and EI equal to the susceptible tomato control. Based on EI, only bahiagrass, bermudagrass, marigold, millet and velvetbean were either non-hosts or ranged from non-hosts to poor hosts. Eleven cover crops varied in host status to the three RKN species screened, ranging from either non-host to poor hosts or poor hosts to susceptible, which could explain inconsistencies in glasshouse and field trials.

Identification of Four Populations of Meloidogyne incognita in Georgia, United States, Capable of Parasitizing Tomato-Bearing Mi-1.2 Gene

Meloidogyne incognita, the southern root-knot nematode (RKN), is the most predominant plant-parasitic nematode species of tomato and causes significant yield loss. The Mi-1.2 gene confers resistance in tomatoes to M. incognita; however, virulent RKN populations capable of parasitizing resistant tomato cultivars have been reported from different regions in the world. Four naturally occurring virulent populations of M. incognita were found in vegetable fields from four counties in Georgia with no history of tomato cultivation of the Mi gene. Two consecutive greenhouse trials showed that all four virulent RKN populations reproduced on tomato cultivars, including Amelia, Skyway, and Myrtle, with the Mi-1 gene, while an avirulent population of M. incognita race 3 was unable to overcome host resistance. Virulent RKN populations varied in reproduction among resistant cultivars, with Ma6 population having the greatest reproduction potential. No difference in penetration potential of the virulent (Ma6) and avirulent populations was found on susceptible and resistant tomato cultivars. However, virulent Ma6 population females were successful at egg-laying, whereas avirulent female development was arrested in the resistant cultivars. The virulent Ma6 population also induced feeding sites in the roots of resistant cultivars, whereas the avirulent population did not. To our knowledge, this is the first report of resistance-breaking populations of M. incognita in Georgia and the second state in the United States after California.