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GR Report 1999
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Status of Citrullus Germplasm Collection-Final Report

The purpose of this memorandum is to provide specific information to you on the Citrullus Germplasm Collection located at Griffin, Georgia. Over the years, most of you have heard that there is need for a major effort to get the collection into good shape. Dr. Bob Jarret, Curator at the Plant Genetic Resources Unit at Griffin, was in California at the Cucurbit Crop Germplasm Committee Meeting in conjunction with the Cucurbitaceae>98 meetings. He provided a very informative lecture on watermelon germplasm at the Plant Genetic Resources Unit. It is my intent to provide some additional information that may be useful in further establishing the status of the Plant Introduction watermelon (PI) collection. I have called several people from seed companies and public research to determine if they have been able to acquire the PIs they requested. For the most part, their response has been that they received those requested. Dr. Jarret confirmed that there were a number of requests made last year for the entire Citrullus collection, but all requests could not be fully met. However, this is not uncommon for any germplasm collection.

There are a total of 1,644 Citrullus PIs with 1,393 (85%) available for distribution. This would seem to indicate that the collection is Anearly@ in good shape. However, this is not the case! Of the 1,393 available, 121 accessions are down to an on-hand quantity of < 400 seeds. They are considered available, but inventories are below the critical replenishment level of 400 seeds. This does not take into consideration the likelihood that some seed lots may have low viability. Up until 1994, the Citrullus seed increases were open-pollinated. Consequently, the genes are still present, but it may be difficult to determine their original source. At present, there are approximately 100 cages for use at the Griffin Station with all increases being done in cages. Actually, Citrullus seed are presently increased at the ARS facility at Byron, Georgia (30 mi. south of Griffin). In 1998, approximately 70 PIs were planted for increase with approximately 20 lost due to soilborne disease. In the last 5 years, the increase of approximately 75 PIs per year have been attempted. Actual seed increases averaged only about 50 per year because of germination problems, plants didn=t flower, or disease. The bottom-line is that there is some progress being made but at a less than desirable pace. Seeds are being stored at -20EC at Griffin. Under those conditions, the watermelon germplasm should last for 30 to 40 years, maybe more.

The Citrullus heirloom varieties (more than 300 cultivars) are stored at Ft. Collins in the National Seed Storage Laboratory (NSSL). Many of the varieties have been identified as Aat risk@ because of low seed number with unknown seed germination percentage as well as others with known low seed germination. The heirloom varieties at NSSL are not officially included in the inventory at Griffin. Information on the heirloom varieties is scant and therefore little data is available in Genetics Resource Information Network (GRIN). The heirloom varieties are also the responsibility of the Griffin Station to increase. However, the heirloom varieties are not being regenerated at this time. There are no facilities for regeneration at Ft. Collins. The heirloom varieties should be given a PI number and duplicate samples placed at Griffin, provided that the rate of regeneration at Griffin can be increased. At the rate of 50 PI increases per year, it will take about 14 years to regenerate the seed stocks that are presently in low volume at Griffin and the heirloom varieties stored at Ft. Collins. We need to be achieving about 100 to 150 PI increases per year to adequately maintain the germplasm.

Dr. Jarret was contacted some years ago by Dr. Bernard DeWinter (Botanist) from South Africa. Dr. DeWinter had a Avery complete@ collection of watermelon germplasm native to South Africa (the center of origin and diversity for Citrullus lanatas). Dr. DeWinter was seeking assistance to regenerate his collection and offered samples of all accessions in return for that assistance. At our January meeting in Memphis, Tennessee, the Watermelon Research and Development Working Group demonstrated an interest in acquisition of the germplasm. It should be a high priority to procure this collection because it should contain some very valuable genetic material. Dr. Robinson donated two accessions of Citrullus ecirrhosus to the collection in 1998. Additional accessions of this species and accessions of Citrullus rehmii are needed. The current back-log of material awaiting regeneration may be negatively impacting efforts to take advantage of opportunities to introduce new and potentially valuable germplasm.

At present, data in the GRIN system is very incomplete, especially for disease resistance characteristics. If we want to find information on Fusarium wilt resistance, can we get it from the GRIN? Maybe! If you want PIs with multiple disease resistance, it may be impossible to get that information from the existing dbase. The GRIN system may never be complete for all desired characteristics. There is no back-log of data entry at Griffin. According to Dr. Jarret, he is not aware of any Citrullus evaluation data that has been submitted to Griffin for inclusion into GRIN in the last 5 years. This could mean that no PI screens were done during that time, that the data is not being properly submitted, or not being submitted through the correct channels. They do not compile data from published Journal reports. If more information was available in the GRIN system and data easier to access, there would surely be more requests for watermelon germplasm than present. According to the Cucurbit Crop Germplasm Committee Report-1996, there were 2,459 seed samples distributed from 1980 through 1988 (1,155 domestic requests and 1,304 international requests). This may be a relatively small number of requests but no reflection of the importance of the germplasm collection.

What is the risk to the watermelon industry if improvements are not made in the increase and maintenance of the watermelon germplasm collection? Our PI collection may be our only source for resistance genes to diseases and insect pests that emerge or are introduced in the future. This is especially true when adequate resistance is not already available in commercial cultivars. The Cucurbit Crop Germplasm Committee-1996 has classified what they consider to be critical areas for watermelon germplasm evaluation. Of the ten priority areas targeted for germplasm evaluation, seven were for disease or insect pests. In the last ten to fifteen years, we have experienced three watermelon diseases new to the United States that illustrate the importance of our germplasm collection. Each of the three diseases are potentially devastating to watermelon production. One thing is for sure. We can expect there will be additional new diseases to contend with in the future.

The three new diseases mentioned above include:

  1. Bacterial Fruit Blotch. Watermelon fruit blotch, caused by a seedborne bacterium, was first observed commercial production fields in 1989. The disease has been found in several watermelon production states, primarily in the eastern U.S. Fruit of some cultivars are more susceptible to bacterial fruit blotch than others which appears to be related to fruit color. The most susceptible fruit are those with a light green rind such as 'Charleston Gray'. Cultivars with a light and dark green stripe, such as >Crimson Sweet=, are more tolerant and the most tolerant are those with a solid dark-green color such as 'Sugar Baby'. However, the level of resistance in commercial cultivars is not sufficient when conditions favor fruit blotch development.
  2. Fusarium oxysporum f. sp. niveum race 2. Race 2 of the watermelon Fusarium wilt fungus was first reported in Texas in 1985. The fungus has since been found in Oklahoma and Florida. At present, race 2 is not widespread within watermelon production states, although the potential for further spread to other watermelon producing areas is great because the disease can be seedborne. There are no commercially acceptable hybrids or cultivars with resistance to the highly virulent race 2. Resistance has been reported in the PI-296341-FR. Consequently, PIs may be our only source of resistance to race 2.
  3. Yellow Vine. Yellow vine is a relatively new disease of watermelon, caused by a phloem-limited bacterium. Evidence indicates that insects (leafhopper) vector the disease. The disease was first observed in central Texas and Oklahoma in 1991 and has caused severe losses in early-planted watermelons in some years. In 1998, the disease was detected in watermelon and pumpkin in Tennessee. Consequently, production areas of Georgia, Florida, and other parts of the southeast U. S. may be at risk in the future. Presently, low levels of resistance or tolerance have been identified in a few open-pollinated and hybrid cultivars, although the mechanism of resistance is unknown. PIs will be evaluated in the future as a source of resistance genes to the yellow vine disease.

Specific recommendations will not be submitted because we are not familiar with the organizational structure. However, the Cucurbit Crop Germplasm Committee filed a report in 1996 with additional information and specific recommendations. For a copy of that report, you can contact Dr. Alan Stoner, USDA-REE-ARS-BA-PSI-NGR LAB, BLDG 003 BARC-WEST RM 101, 10300 Baltimore Blvd, Beltsville, MD 20705-2350. His e-mail address is: astoner@ars-grin.gov. In addition, Dr. Peter K. Bretting (USDA-ARS) is the National Program Staff (NPS) responsible for plant germplasm. Dr. Bretting is concerned about the status of the watermelon germplasm collection and wants to work with the Watermelon Research and Development Working Group and any other interested parties to improve the condition of the collection. If you have questions, he can be contacted at (301) 504-5541. His e-mail address is: pbk@ars.usda.gov.

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