Root traits are an important component for productive plant performance. Roots offer immediate absorptive surfaces for water and nutrient acquisition and are thus critical to crop growth and response to biotic and abiotic stresses. In addition, roots can provide the first line of defense against soilborne pathogens. Watermelon crop performance is often challenged by inclement weather and environmental factors. A resilient root system can support the watermelon crop’s performance across a diverse range of production conditions. In this study, 335 four-day-old watermelon ( Citrullus spp.) seedlings were evaluated for total root length, average root diameter, total root surface area, and total root volume. Total root length varied from 8.78 to 181 cm (20.6-fold variation), total surface area varied from 2 to 35.5 cm ² , and average root diameter and total root volume had an 8- and 29.5-fold variation, respectively. Genotypes PI 195927 ( Citrullus colocynthis ) and PI 674448 ( Citrullus amarus ) had the largest total root length values. Accessions PI 674448 and PI 494817 ( C. amarus ) had the largest total root surface area means. Watermelon cultivars ( Citrullus lanatus ) had a relatively smaller root system and significantly fewer fibrous roots when compared with the roots of the other Citrullus spp. Positive genetic correlations were identified among total root length, total root surface area, and total root volume. This genetic information will be useful in future breeding efforts to select for multiple root architecture traits in watermelon. Germplasm identified in this study that exhibit superior root traits can be used as parental choices to improve watermelon for root traits.

Weeds negatively affect organic vegetable crop growth and profitability. Weed management is the greatest challenge for vegetable organic growers since control options are limited for organic vegetable production. Anaerobic soil disinfestation (ASD) is a novel non-chemical pest management technique that creates anoxic conditions in the topsoil layer for a limited time. ASD is primarily based on the addition of labile carbon sources to topsoil to promote anaerobic conditions driven by microorganisms in moist soil mulched with polyethylene film (polyfim). Field studies were conducted in the summer–fall of 2020 and 2021 to determine the efficacy of warm season cover crops used as carbon sources for ASD and their role in weed management. The study used a factorial experimental design with four cover crop residue treatments (sorghum-sudangrass, sunn hemp, both, or none) in two soil aeration conditions (aerated or non-aerated). Cover crops were grown for 75 days, incorporated into the soil, and sealed with totally impermeable film (TIF) clear mulch, followed by a 4-week ASD process. All incorporated cover crop treatments in non-aerated conditions generated moderate to higher anaerobic conditions (0–150 mV) and provided significantly higher (p < 0.05) weed control than all the other treatments tested or controls. Tomato plants transplanted in non-aerated, cover crops incorporated plots were more vigorous and produced higher yields than aerated plots. No phytotoxicity was observed on tomato plants following ASD treatment in any of the treatments tested. This study demonstrated that warm season cover crops could potentially serve as a carbon source for ASD in organic tomato production.

Meloidogyne enterolobii is a virulent species of root-knot nematode that threatens watermelon (Citrullus lanatus) production in the southeastern United States. There are no known sources of root-knot nematode resistance in cultivated C. lanatus. Specific genotypes of a wild watermelon relative, C. amarus, are resistant against M. incognita but the genetics that underly this resistance are still unknown and it is not clear that this same resistance will be effective against M. enterolobii. To identify and characterize new sources of resistance to M. enterolobii, we screened 108 diverse C. amarus lines alongside a susceptible C. lanatus cultivar (Charleston Gray) for resistance against M. enterolobii. Different C. amarus genotypes ranged from resistant to susceptible for the three resistance phenotypes measured; mean percent root system galled ranged from 10 to 73%, mean egg mass counts ranged from 0.3 to 64.5, and mean eggs per gram of root ranged from 326 to 146,160. We used each of these three resistance phenotypes combined with whole-genome resequencing data to conduct a genome-wide association scan that identified significant associations between M. enterolobii resistance and 11 single-nucleotide polymorphisms (SNPs) within the C. amarus genome. Interestingly, SNPs associated with reduced galling and egg masses were located within a single quantitative trait locus (QTL) on chromosome Ca03, while reductions in nematode eggs per gram of root were associated with separate QTL on chromosomes Ca04 and Ca08. The results of this study suggest that multiple genes are involved with M. enterolobii resistance in C. amarus and the SNPs identified will assist with efforts to breed for M. enterolobii resistance in watermelon.

Watermelon (Citrullus lanatus) is an important vegetable fruit crop, widely grown in many countries in the tropical and subtropical regions around the world. The narrow genetic base of cultivated watermelon makes it susceptible to many pests and diseases, especially viruses. However, genetic sources of resistance are available in wild relatives that could potentially be used to incorporate the resistance genes into watermelon cultivars. Ribosome-inactivating proteins (RIPs) are prevalent in many plant species and are known to have an important role in defense against bacteria, fungi, and viruses. The objective of this study was to identify RIP-like sequences in the watermelon genome and to conduct comparative measurement of RIP gene expression between the potyvirus-resistant Citrullus amarus ‘PI 244019ʹ versus the susceptible watermelon (C. lanatus) ‘Charleston Gray’ in response to infection by papaya ringspot virus (PRSV). Taking advantage of the recently available genome sequences of ‘PI 244019ʹ and ‘Charleston Gray’, in this study we mined out watermelon RIP genes and conducted comparative sequence and expression analyses of RIP-I and RIP-II between the genotypes. The RIP gene expression analysis revealed elevated RIP gene expression in resistant PI 244019 plants, but not in susceptible ‘Charleston Gray’ plants, in response to inoculation with PRSV, indicating a potential association of RIP gene expression and resistance to potyviruses in watermelon. This elevated expression of RIP genes in association with potyvirus resistance opens a new avenue to investigate the underlying mechanism of potyvirus resistance in watermelon.