Judy A. Thies

Analysis based on RAPD and ISSR markers reveals closer similarities among Citrullus and Cucumis species than with Praecitrullus fistulosus (Stocks) Pangalo

A cucurbit species named Praecitrullus fistulosus (Stocks) Pangalo, which thrives in India, is considered to be a distant relative of watermelon. Recent experiments indicated that it has mild resistance to whiteflies (Bemisia tabaci). However, our attempts to cross various US plant introductions (PIs) of P. fistulosus with watermelon or other Citrullus PIs have not been successful. Thus, to determine genetic relatedness among those species, phylogenetic analysis [based on simple sequence repeat (SSR)–anchored (also termed ISSR), and randomly amplified polymorphic DNA (RAPD) markers] was conducted among PIs of P. fistulosus, Citrullus lanatus var. lanatus (watermelon), C. lanatus var. citroides and the wild Citrullus colocynthis. Phylogenetic relationships were also examined with Cucumis melo (melon), Cucumis sativus (cucumber), and wild Cucumis species including C. africanus, C. metuliferus, C. anguria, C. meeusei, and C. zeyheri. Wide genetic distance exists between Citrullus and Cucumis groups (8% genetic similarity). Phylogenetic relationships among Citrullus species and subspecies are closer (25–55% genetic similarity) as compared with those among most Cucumis species (14–68% genetic similarity). P. fistulosus appeared to be distant from both Cucumis and Citrullus species (genetic similarity between P. fistulosus and Cucumis or Citrullus groups is less than 3%). Although wide genetic differences and reproductive barriers exist among cucurbit species examined in this study, they are still considered as potential germplasm source for enhancing watermelon and melon crops using traditional breeding and biotechnology procedures.

High frequency oligonucleotides: targeting active gene (HFO-TAG) markers revealed wide genetic diversity among Citrullus spp. accessions useful for enhancing disease or pest resistance in watermelon cultivars

There is a continuous need to enhance watermelon cultivars for disease and pest resistance. Different U.S. Plant Introductions (PIs) of Citrullus lanatus subsp. lanatus var. lanatus [also known as C. lanatus (Thunb.) Matsum. et Nakai subsp. lanatus var. citroides (Bailey) Mansf. ex Greb.] (CLC) collected in southern Africa are a useful source for enhancing disease or pest resistance in watermelon cultivars. They are also valuable as rootstocks for grafted watermelon, particularly in fields infested with root-knot nematodes or Fusarium wilt. However, there is little information about genetic relationships among these PIs. In this study, genetic diversity was examined among 74 CLC PIs collected from their center of origin in southern Africa. Also, 15 Citrullus lanatus subsp. lanatus (CLL) PIs and the American heirloom cultivars Charleston Gray and Black Diamond (Citrullus lanatus subsp. vulgaris (Schrader ex Eckl. et Zeyh.) Fursa) (CLV) and five Citrullus colocynthis (L.) Schrader (CC) PIs collected in different locations throughout the world were used as out-groups in the phylogenetic analysis for the CLC PIs. Twenty-three high frequency oligonucleotides—targeting active gene (HFO-TAG) primers were used in polymerase chain reaction (PCR) experiments to produce a total of 562 polymorphic markers among the Citrullus PIs and cultivars. Cluster and multidimensional scaling plot analysis produced distinct groups of CLC, CLL, and CC PIs. Several PIs that were designated as CLC or CLL were in transitional positions, indicating that they are the result of gene flow between the major Citrullus groups or subgroups. Population structure analysis indicated that CLC comprises two subgroups; each containing a set of unique alleles. Also, unique alleles exist in the CLL and the CC genotypes. Overall, broad genetic diversity exists among the Citrullus PIs. The data in this study should be useful for identifying PIs with a wide genetic distance between them that could be used in breeding programs aiming to develop heterotic F1 hybrid rootstock lines for grafted watermelon.

Genetic diversity among Lagenaria siceraria accessions containing resistance to root-knot nematodes, whiteflies, ZYMV or powdery mildew.

In recent years, there has been an increased interest in Europe and in the USA in grafting watermelon onto bottle gourd, Lagenaria siceraria (Mol.) Standl. In this study, genetic diversity and relationships were examined [using 236 sequence-related amplified polymorphism markers] among 56 United States plant introductions (PIs) of L. siceraria and PIs of important cucurbit crops [including Cucurbita maxima Duchesne (winter squash), Cucurbita pepo L. (squash and pumpkin), Citrullus spp. (watermelon), Cucumis melo L. (melon) and Cucumis sativus L. (cucumber)]. The analysis showed that L. siceraria is distinct and has similar genetic distances to the cucurbit species examined herein. The L. siceraria PIs were assembled into two major clusters. One cluster includes groups of PIs collected mostly in South Asia (India) and a few PIs collected in the Mediterranean region and in Northeast Africa. The second cluster includes groups of PIs collected mainly in Southern Africa and in North, Central and South America, and PIs collected in China, Indonesia and Cyprus. All L. siceraria PIs in this study were susceptible to the southern root-knot nematode (RKN) [ Meloidogyne incognita (Kofoid and White) Sandground]. However, several PIs, among them a group of closely related PIs collected in Mexico and Florida, were less infected with southern RKNs. All L. siceraria PIs were infested with whiteflies [ Bemisia tabaci (Gennadius)], while several PIs were less infested than others and need further evaluation and selection for developing breeding lines that may be less appealing to this pest. Most of the PIs that showed resistance to zucchini yellow mosaic virus and tolerance to powdery mildew were collected in India and belong to the same phylogenetic groups (PGs). Experiments with L. siceraria PIs representing different PGs showed similar grafting compatibility with watermelon. Findings from this study should be useful for the development of superior L. siceraria rootstock lines with enhanced resistance to diseases and insect pests of cucurbit crops.

Comparison between the N and Me3 genes conferring resistance to the root-knot nematode (Meloidogyne incognita) in genetically different pepper lines (Capsicum annuum)

Genetic resistance to Meloidogyne incognita in pepper (Capsicum annuum) has been well characterised for the N and Me3 resistance genes. However, there are no studies comparing the effects of these two genes directly or investigating the combined effects when both genes are present together. Several studies were undertaken to investigate the relationship of the N and Me3 gene systems to one another and to assess whether these two genes are allelic or truly separate genes. Two genotypes homozygous for the N gene (‘Carolina Wonder’ and ‘Charleston Belle’) and two genotypes homozygous for the Me3 gene (HDA 149 and PM 687) were compared in a replicated greenhouse test for reaction to M. incognita race 3. There were no significant differences between the resistant reactions of genotypes possessing the N or Me3 gene. Allelism tests were performed using the F2 populations of the parental genotypes HDA 149 × ‘Charleston Belle’ and HDA 149 × ‘Carolina Wonder’. The results of these studies clearly show the N and Me3 genes to be distinct, separate dominant resistance genes conferring resistance to M. incognita race 3 and not alleles of the same gene.

USDA-ARS Research on Practices Compatible with Organic Agriculture for Management of Plant-Parasitic Nematodes on Vegetable Crops

Abstract The market for organically grown fruits and vegetables has been increasing in recent years, and research is vital for obtaining optimal quality and yields in organic production systems. Scientists at the United States Department of Agriculture-Agricultural Research Service (USDA-ARS) are investigating methods for managing plant-parasitic nematodes on these crops, and studies that involve practices appropriate for organic vegetable production are reviewed in this paper. The projects summarized here focus primarily on suppression of root knot nematode species, including Meloidogyne arenaria (Neal) Chitwood, M. hapla Chitwood, M. incognita (Kofoid and White) Chitwood and M. javanica (Treub) Chitwood. Projects from Florida include investigations of plant growth-promoting rhizobacteria (PGPR) and chitin amendments for management of nematodes on pepper (Capsicum annuum L.), muskmelon (Cucumis melo L.), and tomato (Solanum lycopersicum L.). In South Carolina, research programs focus on the identification, characterization, and development of host plant resistance to root-knot nematodes in bell and hot peppers (Capsicum L.), southernpea [cowpea; Vigna unguiculata (L.) Walp.], and watermelon [Citrullus lanatus (Thunb.) Matsumura and Nakai]. Collaborative research in Georgia and South Carolina concentrates on the utilization of root-knot nematode-resistant bell pepper for managing root-knot nematodes in double-cropped squash (Cucurbita pepo L. cv. Cougar) and cucumber (Cucumis sativus L.). Research conducted in Maryland involves the use of rye (Secale cereale L.) and velvetbean (Mucuna Adans.) cover crops as nematotoxin-producing soil amendments, and application of beneficial microbes and their metabolites for suppression of root-knot nematodes on bell pepper, cucumber, tomato, and muskmelon. This research contributes to development or improvement of nematode management strategies that do not rely on the use of synthetic nematicides.