| IN THIS ISSUE: Federal Disaster Declaration Approved Reminder of 1997 Texas Produce Convention Bacterial Fruit Blotch of Watermelon Is Still a Problem Squash Leaf Curl Virus on Watermelon |
Federal Disaster Declaration Approved: Jason Johnson
United States Secretary of Agriculture Dan Glickman has approved the Federal Disaster Declaration emanating from the untimely spring eather for producers in Cameron, Willacy and Hidalgo Counties. Word of approval for the disaster declaration encompassing Starr County is anticipated soon. This action enables producers to apply for disaster assistance through various governmental agencies such as the Small Business Administration and the Farm Service Agency. Any questions or claims for assistance can be directed to these agencies.
Reminder of 1997 Texas Produce Convention
The Texas Produce Convention & Expo `97 will be held during September 10-13 in Corpus Christi. Hosting organizations include the Texas Produce Association, Texas Vegetable Association, Texas Citrus Mutual, Texas Fruit Growers Association, Texas Association of Apple Growers, Texas Blueberry Growers Association and the Texas Certified Farmers Market Association.
The general sessions will be held on Thursday and Friday, September 11 and 12 and will cover topics of interest for the entire produce industry. Scheduled presentations include: Water conservation and planning for the future of Texas by Barry McBee, FQPA: Status and implications for the Texas produce industry by Steve Johnson.
Commodity sessions will offer over 40 presentations on all sorts of vegetable and fruit topics. Thursday's vegetable session will focus on Texas produce in the 21st century: challenges and opportunities.
Saturday will include workshops on marketing, soil and plant tissue testing, food safety and farmers market techniques. Put this all together with over 100 exhibitors from across the nation showing every thing from equipment to products to services and you have quite an opportunity to learn and gather information on a number of topics.
Information about convention exhibits, presentations and registration may be obtained from the Texas Vegetable Association or Texas Produce Association offices in Mission or by calling (956) 584-1772.
Bacterial Fruit Blotch of Watermelon Is Still a Problem: Tom Isakeit
Bacterial fruit blotch (BFB) of watermelon is a vicious disease usually noticed on fruit only at harvest time - too late for control. Typically, the pathogen is present on the plant throughout the whole growing season, producing minor or no symptoms at all. The pathogen is seedborne, but its detection on seed is difficult and is the focus of continuing research. The standard assay method involves growing several tens of thousands of seeds of a lot and examining them for symptoms. Although this approach screens out heavily-contaminated seed, it is not 100% reliable as cases of BFB from screened seed continue to be documented. A very low level of contamination, beyond the limits of detection, can increase and spread to other plants during the growing season, and especially in transplant greenhouses.
This disease has occurred in most watermelon growing areas of the southeastern United States. This year, a handful of outbreaks of BFB have been reported in several areas of Texas, with losses ranging from insignificant to 100% of a field. However, growers are reluctant to report this disease, fearing a reduced grade of non-affected fruit, or even the loss of potential customers (particularly if competitors learn of an outbreak). This is unfortunate, because the disease does not cause a post-harvest or food safety problem and infected fruit are easily culled at the packing shed.
Seed is the most important means to spread the BFB pathogen. Once it occurs in a field, growers need to prevent it from becoming established. The pathogen can carry over in seed from infected fruit, so volunteer seedlings must be controlled in subsequent seasons. Other cucurbits can become infected with bacteria from these volunteers. For example, a field of honeydew was infected with BFB in south Texas in 1996. The source of this infection is not known, although fruit blotch occurred on nearby watermelons in the previous season, so there was the possibility of carry-over via infected volunteers.
As a side issue, is BFB a new threat to honeydew production, or other cucurbits, for that matter? Probably not. This was the only recorded occurrence of this disease on honeydew and it did have some link to BFB on watermelon. In comparison, the frequency of BFB on watermelon, as recorded over the past eight years, is substantially greater.
Growers who have had a history of BFB should rotate out of watermelon or other cucurbit crops for at least a year. They should not only control volunteers, but also citron melons and other cucurbit weeds. Citron can become infected and experiments have shown that the infection can carry over in seed. I recently documented BFB in citron fruit growing in a cowpea field. The field had been previous cropped with watermelon.
Will the pathogen survive in a field in the absence of any cucurbit hosts? My preliminary experiments indicate that the bacterium can not survive without a plant host in soil for more than a few weeks when the soil experiences Texas summertime temperatures. Essentially, when the rind rots, the pathogen disappears. There is still no published evidence that the pathogen can carry over on non-cucurbit plants; for example, peanuts. It's conceivable, but field observations by scientists in many locations over several years have not suggested that non-cucurbit hosts are of any importance.
Disease development and spread is favored by high temperatures, and wetness of foliage caused by rain or overhead irrigation. The critical time for infection is during flowering and fruit set. There is a need for a study on how the presence of bacteria on symptomless plants at the time of flowering links to fruit blotch symptoms several weeks later.
To minimize the risk of BFB, growers need to plant seed tested for BFB. Transplants should be monitored for symptoms, particularly leafspots, although symptoms don't always develop on infected plants. As a protective measure, copper sprays should be applied starting at the time of flowering and, if transplants are used, earlier. Although Don Hopkins in Florida reported that copper sprays effectively controlled BFB in fields planted with highly-contaminated seed, I have observed a 50% incidence in one field in spite of copper applications at the recommended rate. Since bacteria isolated from fruit were (fortunately) not resistance to copper, it is likely that the sprinkler irrigation created a severe disease pressure that overwhelmed the effectiveness of copper applications.
Bacterial fruit blotch is not a disease that will hit all watermelon growers. Even if contaminated seed or transplants are used, the disease will not occur unless the environmental conditions are favorable for its development. However, it does have the potential to cause serious losses with individual growers. Although practices such as overhead irrigation can increase the risk of a BFB epidemic, the grower does have the ability to reduce some of the risk.
Squash Leaf Curl Virus on Watermelon: Lynn Brandenberger
Squash leaf curl virus (SLCV) can be an extremely limiting factor in fall watermelon production in the South Texas area. A member of the gemini viruses, SLCV is vectored by the sweet potato whitefly (Bemisia tabaci) and has a wide host range within cucurbits and can also infect several other plant groups. Once SLCV is acquired by a whitefly feeding on infected plants, it is transmitted to other plants by subsequent feeding and can be spread throughout the remainder of whitefly's life.
Symptoms on watermelon occur both on the foliage and fruit. Tom Isakeit, TAEX plant pathologist states that foliar symptoms include a blistered appearance to the leaf, a curling under of the outer lobes of the leaf into a tight cluster around the stem and wide-spread yellowing. The leaves may also exhibit some pale green mottling and vines may turn up on the ends to give a "snake head" appearance. Plant growth is stunted, yields can be reduced from 75 to 100% and much of the fruit is unmarketable due to poor plant growth which can result in fruit that is poorly developed and malformed.
Control for squash leaf curl on watermelon is not possible once plants are infected with the virus. Because of this, watermelon producers will need to consider management practices that will work to prevent the start of SLCV and reduce it's spread. Late planting following the natural reduction of whitefly populations in the fall will help reduce the spread of the virus, but this often conflicts with the need to hit fall marketing windows for the crop. Control of the whitefly vector is normally not very effective because whiteflies spread the disease prior to being controlled, regardless of the chemical control being used. To date, the best potential for a control strategy includes:
- Use of clean virus free transplants to allow the crop time to establish prior to infection with SLCV.
- Planting as late as possible and still early enough to hit the market window.
- Minimum of at least a four week break between fall and spring watermelon or other cucurbit crops.
- Good sanitation practices including the destruction of crop debris, volunteer plants and other cucurbit hosts near the proposed production site at least four weeks prior to planting.
- Selection of production sites away from other crops that act as a source of whitefly infestations in the fall i.e. cotton fields.
- Use of appropriate chemical controls for whiteflys to reduce the spread of SLCV by this virus vector.
If you would like more information concerning squash leaf curl virus I would suggest the following two references. Compendium of cucurbit diseases published in 1996 by the American Phytopathological Society, St. Paul, MN. The other reference is: The squash leaf curl virus a whitefly-transmitted pathogen infecting watermelons and squash, written by Dr. Thomas Isakeit February, 1995 at the Texas A&M Research and Extension Center, Weslaco, Texas. Available upon request (956) 968-5581.
Lynn Brandenberger/Editor
The information given herein is for educational purposes only. References to commercial products or trade names are made with the understanding that no discrimination is intended and no endorsement by the Cooperative Extension Service is implied.