Origin

Development of ‘RKVL-318’ began in 1999 with greenhouse and field evaluations of the U.S. PI collection of Citrullus spp. for resistance to RKN. In the greenhouse study, 20 CLC PIs showed significantly higher RKN resistance compared with all other 400 Citrullus spp. PIs evaluated, including PIs representing cultivated [C. lanatus var. lanatus (CLL)] or desert-type watermelon [C. colocynthis (CC)] (Thies and Levi, 2003, 2007). These RKN-resistant CLC PIs had 3% to 19% of the root system galled. In contrast, all of the CC and CLL PIs were highly susceptible to RKN with 73% to 100% and 32% to 100% galling, respectively. After these greenhouse observations, we selected the most resistant CLC PIs with 3% to 13% galling and performed additional greenhouse and field evaluations of these PIs during 2000 through 2011 (Thies et al., 2010). A single plant of PI482324 that exhibited high RKN resistance (6% galling) was self-pollinated. The most RKN-resistant plant was self-pollinated in the greenhouse in three successive generations to produce S4 seeds.

Description

Rootstock performance of ‘RKVL-318’ was compared with commercial rootstocks and with non-grafted triploid watermelon cultigens in replicated field trials (2009 and 2010) in which soils were heavily infested with RKN (Table 1) (Thies et al., 2015). ‘RKVL-318’ roots had significantly less root galling (mean = 26% of root system galled) than standard commercial rootstocks ‘Emphasis’ (mean = 91% galled) and ‘Strong Tosa’ (mean = 97% galled) and non-grafted ‘Tri-X 313’ (mean = 69% galled) (Fig. 1A and B). ‘RKVL-318’ root systems (used as rootstocks for ‘Tri-X 313’ seedless watermelon) produced significantly more (P < 0.05) fibrous roots (mean = 87% of root system was fibrous) than the commonly used commercial cucurbit rootstocks, including ‘Emphasis’ bottle gourd (Lageneria siceraria) (mean = 22% fibrous roots), ‘Strong Tosa’ interspecific squash hybrid (Cucurbita maxima × Cucurbita moschata) (mean = 20% fibrous roots), or non-grafted control ‘Tri-X 313’ (mean = 42% fibrous roots). We have identified root fibrosity as a trait associated with RKN resistance in watermelon rootstocks. Overall, ‘RKVL-318’ rootstocks showed high graft compatibility with the triploid seedless ‘Tri-X 313’ scions and produced higher (P < 0.05) watermelon fruit yields than commercial cucurbit rootstocks (Yetisir et al., 2003). In fields infested with RKN, yields of ‘Tri-X 313’ seedless watermelon grafted on ‘RKVL-318’ rootstocks were an average 73% and 53% greater than ‘Strong Tosa’ and ‘Emphasis’ rootstocks, respectively, and an average 55% greater than non-grafted ‘Tri-X 313’.

Table 1.Percentages of roots galled by Meloidogyne incognita, numbers of M. incognita eggs per gram fresh root, percentages of roots with fibrous roots, and watermelon fruit yields and numbers for ‘Tri-X 313’ seedless watermelon grafted on rootstocks of RKVL-318 wild watermelon rootstock, selected commercial rootstocks, and self-grafted and non-grafted ‘Tri-X 313’ watermelon, Charleston, SC, 2009 and 2010.

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‘RKVL-318’ has a runner growth habit with an intermediate leaf blade type. The ‘RKVL-318’ plants are andromonoecious, having male and hermaphrodite flowers. The fruits have an oblate shape with average fruit size of 20.9 × 18.0 cm. The rind color of mature fruits is light green with darker green stripes, which have irregular margins; average rind thickness is 2.7 cm. The flesh color is light green to yellow with a firm texture and does not exhibit a hollow heart. ‘RKVL-318’ produced 1.3 fruits per plant (6.8 kg per fruit). Fruits are ready for harvest at 90 d post-planting. The seeds are moderately large and red in color.

‘RKVL-318’ should be useful for scientists and plant breeders interested in developing RKN-resistant watermelon rootstocks and/or in developing watermelon breeding lines or cultivars with enhanced RKN resistance.

Seed Availability

Small seed samples of ‘RKVL-318’ are available for distribution to interested research personnel and plant breeders. Written requests for seed should be submitted to Dr. Judy Thies, U.S. Vegetable Laboratory, 2700 Savannah Highway, Charleston, SC 29414-5334. Seed of ‘RKVL-318’ also will be submitted to the National Plant Germplasm System where it will be available for research purposes, including the development and commercialization of new cultivars. It is requested that appropriate acknowledgment of the source be given when this germplasm contributes to research or development of a new breeding line or cultivar.