Powdery mildew (Podosphaera xanthii) can cause severe damage to cucurbit crops grown in open fields and greenhouses. In recent years, there has been an increased interest in the United States in grafting watermelon plants onto various cucurbit rootstocks. Bottle gourd plants (Lagenaria siceraria) are being used throughout the world as rootstocks for grafting watermelon. Although gourd plants are beneficial, they may still be susceptible hosts to various soilborne and foliar diseases. Bottle gourd plant introductions (PI) resistant to diseases and pests can be a valuable source of germplasm in rootstock breeding programs. We evaluated 234 U.S. PIs of L. siceraria for tolerance to powdery mildew in two greenhouse tests. Young seedlings were inoculated by dusting powdery mildew spores of melon race 1 on the cotyledons. Plants were rated 2 weeks after inoculation using a 1 to 9 scale of increasing disease severity. Although none of the L. siceraria PIs were immune to powdery mildew, several PIs had significantly lower levels of powdery mildew severity compared with susceptible watermelon cultivar Mickey Lee. The experiment was repeated with 26 select PIs on whole seedlings and cotyledon disks. Significant variability in the level of resistance to powdery mildew on plants within PI was observed. Moderate resistance in several PIs to powdery mildew was confirmed. PI 271353 had consistently lower ratings in the various tests and can be considered the most resistant to P. xanthii race 1 among the L. siceraria accessions evaluated in this study. A few other PIs with moderate resistance to powdery mildew included PI 271357, PI 381840, and PI 273663. These results suggest that novel sources of resistance could be developed by careful selection and screening of several of the PIs with moderate resistance described in our study.
Chandrasekar S. Kousik, Amnon Levi, Kai-Shu Ling, and W. Patrick Wechter
Karen R. Harris, Kai-Shu Ling, W. Patrick Wechter, and Amnon Levi
Zucchini yellow mosaic virus (ZYMV) is one of the most economically important viruses affecting watermelon [Citrullus lanatus (Thunb.) Matsun & Nakai var. lanatus] in the United States. The ZYMV-Florida strain (ZYMV-FL) is considered a major limitation to commercial watermelon production in the United States. Inheritance of resistance to ZYMV-FL is conferred by a recessive gene. This report describes the identification of single-reaction, polymerase chain reaction-based markers linked to the ZYMV-FL resistance gene in watermelon. In this study, we identified a marker ZYMV-resistant polymorphism (ZYRP) linked to the ZYMV-FL resistance gene locus (genetic distance of 8 cM) in an F2 population, and in a backcross one to the resistant parent population (BC1R) (genetic distance of 13 cM). The identification of a single nucleotide polymorphism within the ZYRP marker for the parental genotypes allowed the development of a sequence-characterized amplification region marker linked to the ZYMV-FL resistance gene. Experiments using a BC2F2 population derived from the U.S. Plant Introduction 595203 (C. lanatus var. lanatus) and the recurrent parent ‘Charleston Gray’ indicated that the ZYRP marker can be used in marker-assisted selection to identify genotypes containing the gene conferring ZYMV-FL resistance in watermelon.
Dennis N. Katuuramu, W. Patrick Wechter, Marcellus L. Washington, Matthew Horry, Matthew A. Cutulle, Robert L. Jarret, and Amnon Levi
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 cm2, 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.
Howard F. Harrison Jr, Trevor R. Mitchell, Joseph K. Peterson, W. Patrick Wechter, George F. Majetich, and Maurice E. Snook
Caffeoylquinic acid compounds are widespread in plants. They protect plants against predation and infection and may have several beneficial functions in the human diet. The contents of chlorogenic acid and the 3,4-, 3,5-, and 4,5- isomers of dicaffeoylquinic acid (DCQA) in the storage root tissues of 16 sweetpotato [Ipomoea batatas (L.) Lam.] genotypes were determined. Averaged over genotypes, the contents of the four compounds were highest in the cortex, intermediate in the stele, and lowest in the periderm. Among the genotypes, chlorogenic acid contents ranged from 16 to 212 μg·g−1 in periderm, from 826 to 7274 μg·g−1 in cortex, and from 171 to 4326 μg·g−1 in stele. The 3,5-DCQA isomer comprised over 80% of total DCQA. In most genotypes, 3,5-DCQA and chlorogenic acid contents were similar in cortex and stele tissues, but chlorogenic acid was lower than 3,5-DCQA in periderm tissue. Among the 16 genotypes, total DCQA contents ranged from 0 to 1775 μg·g−1 dry weight in periderm, from 883 to 8764 μg·g−1 in cortex, and from 187 to 4768 μg·g−1 in stele. The large differences found in a small germplasm collection suggest that selecting or breeding sweetpotato genotypes with high caffeoylquinic acid content is possible. The four caffeoylquinic acid compounds comprised over 3% of the dry weight of storage roots of the sweetpotato relative, bigroot morningglory [Ipomoea pandurata (L.) G.F.W. Meyer], indicating that it may be a good source for the compounds. The effect of DCQAs isolated from sweetpotato and I. pandurata tissue and caffeic and chlorogenic acid standards were tested in proso millet (Panicum milliaceum L.), Fusarium solani (Sacc.) Mart., and bacterial growth bioassays. Caffeic acid, chlorogenic acid, and 3,5-DCQA were most inhibitory in millet and F. solani bioassays, but 3,5-DCQA was the least inhibitory compound in bacterial growth bioassays. Their activity in the bioassays suggests that the caffeoyl quinic acid compounds contribute to the allelopathic potential and resistance to root diseases of some sweetpotato clones.
Amnon Levi, Alvin M. Simmons, Laura Massey, John Coffey, W. Patrick Wechter, Robert L. Jarret, Yaakov Tadmor, Padma Nimmakayala, and Umesh K. Reddy
Citrullus colocynthis (CC) is a viable source of genes for enhancing disease and pest resistance in common cultivated watermelon [Citrullus lanatus var. lanatus (CLL)] cultivars. However, there is little information about genetic diversity within CC or the relationship of CC accessions to C. lanatus. In this study, we examined genetic diversity and relationships among 29 CC accessions collected in northern Africa, the Middle East, and Asia, and their relationships to 3 accessions and 3 cultivars of CLL, 12 accessions of citron melon [C. lanatus ssp. lanatus var. citroides (CLC)], and 1 accession representing the desert perennial Citrullus ecirrhosus (CE). Twenty-three high-frequency oligonucleotides-targeting active gene (HFO–TAG) primers were used to produce a total of 431 polymorphic fragments that target coding regions of the genome. Cluster and multidimensional scaling plot analysis, separated the CC into five groups, in general agreement with their geographic origins. CC genotypes admixed with CLL and CLC also were identified. Major reproductive barriers resulted in significantly reduced fertility in CC × CLL hybridizations. However, several of the U.S. PIs of CC were successfully crossed with watermelon cultivars using traditional breeding procedures, and the seeds produced from these crosses were viable. This suggests that CC can be a viable source to introduce biotic and abiotic stress resistance genes into cultivated watermelon.