Nihat Guner, Todd C. Wehner, and Zvezdana Pesic-Van Esbroeck
The purpose of this experiment was to develop a method for evaluation of watermelon for resistance to papaya ringspot virus-watermelon strain (PRSV-W), and to use that method to screen the watermelon germplasm collection.
To date, we have developed a reliable method, and are half way through the germplasm screening. The screening of the germplasm will be finished by early summer, 1999. A retest of the highly resistant cultigens is underway. The most resistant plants from the most resistant cultigens will be self-pollinated. That work should result in the development of resistant inbreds for use by plant breeders.
- Nihat Guner, graduate research assistant
- E. Bruton Strange, graduate research assistant
- Todd C. Wehner, professor, horticultural science
- Zvezdana Pesic-Van Esbroeck, researcher, plant pathology
- Collect watermelon cultigens for use in screening
- Collect isolates of PRSV-W for methods development
- Develop a method for screening the germplasm
- Screen the germplasm for resistance to PRSV-W
- Retest additional plants of the most resistant cultigens
- Release most resistant lines for use by the industry
- Determine inheritance in resistant x susceptible crosses
- Develop elite inbred lines with high resistance to PRSV-W
Research from this project will provide effective screening methods for use by seed company personnel in testing their cultivars for resistance to PRSV-W. More important, resistant accessions identified could be used in breeding programs to develop improved watermelon cultivars. Resistance to PRSV-W would provide growers with protection against loss by this disease, which is otherwise difficult to control.
Much has been learned about PRSV-W, including the development of an efficient method for screening for resistance. The germplasm screening involving 4 replications of 1275 cultigens is complete. Now, we are screening all plant introduction accessions that were unavailable or had incomplete data in the initial screening. A collection of old cultivars and land races has been obtained, the seeds increased, and is being screened for resistance.
Concurrent with the screening effort, we are testing the most resistant 17 cultigens with larger numbers of plants to permit selection within and among cultigens. The most resistant plants of the most resistant cultigens will be released as inbred lines for use by the industry.
Finally, we are crossing the most resistant with the most susceptible cultigens to determine the inheritance of resistance. The most resistant selections will be released as inbred lines for use by the industry. The most resistant selections will be backcrossed into elite cultivars and made available to the industry.
The best method for use in screening the watermelon germplasm collection was as follows:
- Isolate – 2682 (highly virulent and uniform)
- No. replications – 4
- No. plants/rep – 1
- Application – rub (spray is as good)
- Plant stage – first true leaf (better than cotyledon)
- No. days to rating – 2 weeks (rate weekly 4 additional times)
All plants are inoculated on the same day with checks randomly placed throughout to measure uniformity of the inoculation procedure. Chenopodium is also used as an indicator of virus in the inoculum. Inoculum is obtained from host ‘Gray Zucchini’ squash plants that were previously inoculated with the virus isolate. The host plants are grown in aphid-proof cages to prevent contamination of the plants by other viruses.
Two weeks after inoculation, plants are rated weekly for five weeks. The rating is on a scale of 0-9 (0=no symptoms, 9=dead plant). All plants that show resistance to the virus are inoculated again four weeks after the initial inoculation. Also plants that did not emerge by the first inoculation are inoculated at that time. Plants that are still alive after five ratings are tested with ELISA, a chemical analysis that determines if there is virus present in the plant tissue. The most resistant plants are moved to another greenhouse to be self-pollinated for seed production.