Reproduction of the root-knot nematode Meloidogyne arenaria on flue-cured tobacco possessing resistance genes Rk1 and/or Rk2 and the impact of parasitism on the accumulation of nicotine in conventional and low-alkaloid tobacco

Host resistance has become a cornerstone of sustainable production of flue-cured tobacco in regions where root-knot nematodes present a threat to growers. Resistance to races 1 and 3 of M. incognita, historically the most significant root-knot nematode threat to tobacco production, is now widely available in commercially available flue-cured tobacco varieties, and is imparted by the gene Rk1. The same gene also provides resistance to race 1 of M. arenaria. The widespread deployment of this resistance has fostered a shift in root-knot nematode population dynamics, as a result of which M. arenaria race 2 has become the predominant root-knot nematode threat in Virginia. A second resistance gene known to impart resistance to M. javanica, Rk2, has also been incorporated into numerous released cultivars in combination with Rk1. This combination has been demonstrated to impart increased resistance to M. incognita and M. javanica relative to either gene alone. In the present work, eleven greenhouse trials conducted from 2017-2019, as well as two trials conducted in 2018 and 2019 on a cooperating farm, investigated the efficacy of this stacked resistance against M. arenaria race 2 and compared the effect of stacking both resistance genes to the effect of either gene alone relative to a susceptible cultivar. We also evaluated how these forms of resistance compare with resistance possessed by a breeding line with resistance reportedly derived from N. repanda to determine if additional, novel sources of resistance to root-knot nematodes previously identified from other species in the genus Nicotiana could play a role in expanding the genetic diversity of germplasm available for the refinement of host resistance in flue-cured tobacco. Additionally, in light of potential new rule making from the FDA mandating reduced nicotine content of cured tobacco leaf, we investigated the relationship between alkaloid (nicotine) content of flue-cured tobacco and root-knot nematode parasitism, while also evaluating nematode parasitism effects on carbohydrate content. Despite considerable variability in our results, particularly under field conditions, our results demonstrate that stacking Rk1 and Rk2 imparts greater resistance to M. arenaria race 2 than either gene alone, but that an entry possessing resistance reportedly derived from N. repanda exhibited significantly greater resistance to root-knot nematodes than the combination of Rk1 and Rk2 based on root galling, and egg mass and egg production. The alkaloid content of flue-cured tobacco did not appear to have an effect on root-knot nematode parasitism under greenhouse or field conditions, but the presence of the nematode did lead to increased accumulation of nicotine in the roots of plants, while translocation of nicotine to leaves was reduced. Conversely, root-knot nematode parasitism was reduced accumulation of carbohydrates in roots, while having no significant effects on leaf carbohydrate content.