Research Project: Characterization of Host Resistance and Biology of Diseases and Nematodes in Vegetable Crops

Location: Vegetable Research

2022 Annual Report

Objectives
1. Identify and characterize disease and nematode resistance in vegetable crops, and develop molecular tools to enhance breeding efforts for resistance. 1.A. Determine genetic basis of resistance to powdery mildew (PM) and Phytophthora fruit rot in watermelon and develop molecular markers linked to resistance genes. 1.B. Fine map quantitative trait loci (QTL) conferring resistance to watermelon vine decline (WVD) caused by Squash vein yellowing virus (SqVYV) and develop molecular markers. 1.C. Develop markers for and characterize the molecular mechanisms of resistance to root-knot nematode in cucumber. 2. Identify and develop vegetable germplasm and breeding lines with enhanced resistance to diseases and nematodes. 2.A. Identify sources of resistance to the whitefly-transmitted Cucurbit leaf crumple virus (CuLCrV) in watermelon. 2.B. Develop pepper breeding lines with pyramided nematode resistance genes. 3. Characterize population dynamics to improve biological understanding of existing and emerging fungal and nematode pathogens in the southeastern United States. 3.A. Characterize powdery mildew (Podosphaera xanthii) isolates collected from cucurbits in southeastern U.S. 3.B. Characterize the influence of temperature on the development of the guava root-knot nematode (Meloidogyne enterolobii).

Approach
This approach will identify and develop resistant germplasm and breeding lines for managing Phytophthora fruit rot(PFR),powdery mildew(PM),whitefly-transmitted Squash vien yellowing virus(SqVYV), Cucurbit leaf crumple virus(CuLCrV), and root-knot nematodes(RKN). Modern reisstance phenotyping and breeding techniques will be utilized to accomplished our objectives. A recombinant inbred line(RIL) population will be developed by crossing a multiple disease resistant watermelon line (PFR and PM) and a susceptible line. The RIL population will be phenotyped for resistance to PFR and PM to determine genetics of resistance. Analysis of differentially expressed genes by RNA-Seq resulting from Phytophthora and watermelon fruit interactions will be used to further elucidate genetics of fruit rot resistance. Red fleshed RILs with resistance to PFR and/or PM will be evaluated for fruit quality and selected for release. Similarly, a RIL population developed from a cross of SqVYV resistant watermelon line and a susceptible cultivar will be phenotyped to detremine genetics of resistance and develop molecular markers for use in breeding programs. We will phenotype cucumber lines from a mapping population to fine map the mj resistance gene. The resulting candidate resistance genes will be cloned and characterized to provide pathologists with a better understanding of the biological underpinnings of RKN resistance and provide molecular tools to assist breeders in developing resistant cucumbers. Agrobacterium infectious clones of CuLCrV have been developed for phenotyping watermelon and other susceptible vegetable crops for resistance. The clones will be used to phenotype the watermelon core collection available with USDA and sources of resistance will be identified for use in breeding programs. We will cross the M. enterolobii resistant pepper line PMER-2 with the M. incognita resistant 'Charleston Belle' and develop populations to assess the inheritance pattern of M. enterolobii resistance. If resistance from PMER-2 is simply inherited, wewill be able to select M. enterolobii resistant breeding lines. Development of new RKN resistant pepper lines with resistance against both the invasive M. enterolobii and the endemic RKN species will provide valuable tools to help pepper growers in southern U.S. to manage these pests in the field. We will conduct side-by-side developmental embryogenesis assays with M. enterolobii and M. incognita over a range of temperatures to compare the thermal developmental profiles of these two species and for the first time calculate an estimate of the thermal base temperature and optimal thermal temperature for M. enterolobii. A greater understanding of the thermal factors that underlie the development in M. enterolobii will provide insight into the growing regions in the U.S. that may be threatened by this nematode as it continues to spread. We have a large collection of isolates of cucurbit powdery mildrew pathogen collected from accross the U.S. This collection will be genotyped using simple sequence repeat(SSR) markers developed based on an available sequence and used to characterize diversity among the pathogen population.

Progress Report
New project plan started 4/12/2022. This project is related to the old, terminated project: 6080-22000-029-000D, Biology, Etiology and Host Resistance in Vegetable Crops to Diseases and Nematodes. Please see annual progress report for the related project 6080-22000-029-000D. Research has been initiated and is on track to fully complete all projected 12-month milestones. Short summary of terminated project (6080-22000-029-000D). Despite setbacks that arose from COVID 19 pandemic during this five-year cycle, significant progress was made toward developing new resources of resistance to help stakeholders manage diseases and root-knot nematodes in vegetable crops. The project with 2 SY, successfully published 31 peer reviewed journal manuscripts and released 9 disease resistant vegetable germplasm lines, which included four powdery mildew resistant watermelon lines, a phytophthora fruit rot and powdery mildew resistant watermelon line, 2 powdery mildew resistant bottle gourd lines, and root knot nematode resistant pepper and sweetpotato lines. In addition disease resistant pepper and watermelon germplasm was provided to several national and international seed companies and universities through 15 outgoing material transfer agreements (MTA). The project also developed molecular markers linked to resistance genes for powdery mildew of watermelon. We completed quantitative trait loci (QTL) analysis to determine the chromosomal regions where potential resistance gene are located for PM, Phytophthora fruit rot (PFR) and watermelon vine decline (WVD). We mapped resistance to the Javanese root-knot nematode in cucumber. We developed new methods for detecting root-knot nematodes in sweetpotato roots. And we identified new sources of resistance to the guava root-knot nematode in both sweetpotato and pepper. All of this work as well as new avenues of research will be continued into the next five-year project plan cycle to continue to help stakeholders manage these pests within their fields.

Accomplishments