RESUMO
Most commercial flue-cured tobacco cultivars contain the Rk1 resistance gene, which provides resistance to races 1 and 3 of Meloidogyne incognita and race 1 of M. arenaria. A number of cultivars now possess a second root-knot resistance gene, Rk2. High soil temperatures have been associated with a breakdown of root-knot resistance genes in a number of crops. Three greenhouse trials were performed from 2014 to 2015 investigate the effect of high soil temperature on the efficacy of Rk1 and/or Rk2 genes in reducing parasitism by a population of M. incognita race 3. Trials were arranged in randomized complete block design in open-top growth chambers set at 25°, 30°, and 35°C. Plants were inoculated with 3,000 eggs and data were collected 35 days post-inoculation. Galling, numbers of egg masses and eggs, and reproductive index were compared across cultivar entries. Nematode reproduction was reduced at 25°C and 30°C on entries possessing Rk1 and Rk1Rk2 compared to the susceptible entry and the entry possessing only Rk2. However, there were often no significant differences in reproduction at 35°C between entries with Rk1 and/or Rk2 compared to the susceptible control, indicating an increase of root-knot nematode parasitism on resistant entries at higher temperatures. Although seasonal differences in nematode reproduction were observed among experiments, relative differences among tobacco genotypes remained generally consistent.
RESUMO
Resistance to Meloidogyne incognita races 1 and 3 and race 1 of M. arenaria is imparted to flue-cured tobacco by the gene Rk1. Meloidogyne arenaria race 2 is not controlled by Rk1 and has become prevalent in Virginia. A second form of resistance effective against M. javanica, Rk2, is also increasingly available commercially. Greenhouse and field trials including a root-knot susceptible cultivar, cultivars homozygous for Rk1 or Rk2, and cultivars possessing both genes were conducted in 2018 and 2019 to investigate the effect of Rk1 and/or Rk2 on parasitism and reproduction of M. arenaria race 2. Plants were inoculated with 5,000 M. arenaria race 2 eggs in the greenhouse or infested by a native nematode population in the field. Data were collected after 28 days (greenhouse) or every 3 weeks following transplant until 18 weeks in the field and included root galling index, nematodes present in roots, egg mass numbers, and egg counts; reproductive indices were also calculated. We found that the combination of Rk1 and Rk2 provides greater resistance to M. arenaria race 2 than either gene alone. While the effect of either gene alone was inconsistent, we did observe some significant reductions in galling and reproduction associated with each gene relative to the susceptible control.
RESUMO
Chemical controls for root-knot nematodes are increasingly restricted due to environmental and human health concerns. Host resistance to these nematodes is key to flue-cured tobacco production in Virginia. Resistance to Meloidogyne incognita races 1 and 3, and race 1 of M. arenaria is imparted by the gene Rk1, which is widely available in commercial flue-cured tobacco. Rk2 imparts increased resistance to M. javanica when stacked with Rk1 and is becoming commercially available. The efficacy of Rk2 against M. arenaria race 2, which is increasingly prevalent in Virginia, is unclear. Greenhouse trials were conducted in 2017 to determine how potential resistance derived from N. repanda compares to the root-knot nematode resistance afforded by Rk1 and Rk2. Trials were arranged in a completely randomized block design and included an entry with traits derived from N. repanda, a susceptible entry and entries possessing Rk1 and/or Rk2. Data collected after 60 days included percent root galling, egg mass counts, and egg counts. Root galling and reproduction were significantly lower on the entry possessing traits derived from N. repanda relative to other entries, suggesting that the N. repanda species may hold a novel source of root-knot nematode resistance for commercial flue-cured tobacco cultivars.