No abstract

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.

Summary 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.

Plant-parasitic nematodes (PPN) limit yields in vegetable production in the United States. During the spring and fall cropping seasons of 2018, 436 fields in bare ground and plastic bed cropping systems were randomly sampled from 29 counties in Southern Georgia. The incidence (%), mean and maximum relative abundance (nematodes per 100 cm3 of soil ) of the 10 different PPN genera detected in 32 vegetable crops in bare ground and plastic bed cropping systems include Meloidogyne spp. (67.3, 292, 14144), Nanidorus spp. (49.4, 6, 136), Mesocriconema spp. (39.6, 17, 340), Helicotylenchus spp. (31.6, 20, 1152), Pratylenchus spp. (20.1, 2, 398), Rotylenchulus spp. (5.9, 1, 116), Hoplolaimus spp. (12.6, 1, 78), Heterodera spp. (2.3, <1, 60), Tylenchorhynchus spp. (0.9, <1, 12) and Xiphinema spp. (0.2, <1, 2). A Non-metric Multidimensional Scaling analysis (NMS or NMDS) indicated that most environmental and geological factors (i.e. longitude, precipitation, soil moisture, sand + silt content, and soil electrical conductivity) had no apparent relationship with nematode counts, except for latitude, soil pH and temperature. Multi-rank Permutation Procedure (MRPP) followed by Indicator Species Analysis (ISA) and non-parametric Kruskal-Wallis Analysis of Variance (KW ANOVA) indicated that Meloidogyne spp. was the predominant PPN associated with plastic beds in the South region sampled. The South region consisted mainly of commercial fields that rotated multiple vegetable crops through the same plastic beds. All other PPNs were associated with bare ground beds in the North region that are commonly rotated with row crops. This study validates that Meloidogyne spp. is the most important PPN in vegetable fields of Southern Georgia and suggests that cropping systems have a greater effect on PPN population dynamics than the environment.

Root-knot nematode (RKN; Meloidogyne spp.) is the most prevalent plant-parasitic nematode in vegetable fields of Georgia, with an incidence of 67.3%. Since aggressive RKN species are reported in the southeastern United States, molecular-based identification of RKN species was conducted on soil samples taken from a nematode surveillance study in 2018 from 292 RKN-infested vegetable fields in South Georgia. The RKN-infested soil was potted with tomato cv. Rutgers and individual nematode females were isolated from galled roots and subjected to species-specific PCR and mitochondrial haplotype-based RKN species identification. The incidence (%), mean, and maximum relative abundance (second-stage juveniles per 100 cm3 of soil) of the 5 RKN species identified consisted of M. incognita (91.9, 486, 14144), M. arenaria (36.0, 707, 14144), M. floridensis (2.2, 909, 5264), M. javanica (5.5, 352, 1488), and M. haplanaria (0.7, 8, 14). A large proportion of fields (29%) had mixed populations of M. incognita and M. arenaria which may reflect the region’s long history of cotton and peanut cultivation. For unknown reasons, mixed populations of M. incognita and M. arenaria were associated with higher population densities. Meloidogyne incognita is the most important RKN species in vegetable fields followed by M. arenaria, therefore, pure or mixed populations of these species should be addressed in nematode management programs. Although at a lower incidence, the newly detected species, M. floridensis and M. haplanaria, have the potential to become a major threat since they reproduce on vegetables with Mi-resistant genes.