Front Range Seed Analysts
1995 Seed Forum Volume 9 Number 1
WATERMELON FRUIT BLOTCH
By Darrell A. Maddox
STA Laboratories
630 S. Sunset
Longniont, CO 80501
Bacterial fruit blotch of watermelon (WFB), caused by a bacterium (currently namedAcidovorax avenae subsp. citrulli.; AAC), is a relatively now economic disease of watermelon. Though a bacterial disease causing similar symptoms in watermelon seedlings was described in the 1960's and again in 1978, economic losses due to WFB were not reported until 1989. WFB has occurred in the United States in every year since 1989, resulting in significant crop losses in most fields with the disease.
The bacterium causing WFB is known to be seedborne, and can develope and spread rapidly under favorable environmental conditions of high relative humidity and warm tempertures. Alternate hosts, such as other cticurbits, volunteer watermelons and wild citron have been identified, but relatively little else is known about the epidemiology of WFB. Since 1991, extensive research into the various aspects of WFB have been undertaken, including spread, survival, detection in plants and seeds, and control.
WFB can appear as a seedling blight, leaf blight and fruit rot. It is most devastating on seedlings and the fruit, though leaf lesions are sources of bacteria for fruit infection. On seedling cotyledons and leaves symptoms first appear as dark water soaked areas. The infected areas become necrotic and are usually black, but may be reddish brown. The leaf lesion typically follows the veins and midrib of the cotyledons and leaves. On fruit, lesions begin as watersoaked, greasy areas on the upper surface of the outer rind. Eventually, these spread to large, dark, watersoaked areas throughout the rind. Advanced fruit lesions turn brown, crack and occasionally develop a whitish gmy bacterial ooze. Fruit is most susceptible during the first 3 weeks of development, when stomata are active.
Once WFB is diagnosed in a field or green house, control is difficult. Routine copper sprays have been shown to slow the spread of the disease. As with most bacterial diseases of plants, prevention is most effective. Sanitation, crop rotation and weed control am important to prevent introduction of AAC into a watermelon fleld. Since AAC can be seedborne, the use of healthy watermelon seed is very important. Healthy watermelon seed is produced by practicing the above production control measures, producing seed in an and environment and not under overhead irrigation. Seed crops am routinely inspected for WFB symptoms and after harvest, a sample can be tested for AAC. However, a reliable, reproducible laboratory technique for the detection of AAC in watermelon seed has not been developed, though research is continuing. To this date, a seedling growout test of at least 10,000 seeds, under controlled conditions can be used to determine if a seed sample has AAC. Based on the available research. maintaining relative humidity above 55%, temperatures between 25 C (77 F) ard 32 C (90 F), and 12 hour light period are necessary to induce sypmtoms of WFB. The seedlings must be grown under these conditions for at least 16 days. High pressure, overhead watering should be avoided to limit the potential of cross contamination between seed samples. The soil should be pasteurized, and the planting trays and greenhouse benches should be disinfested prior to planting. Wood benches are not recommended, because the bacterium may survive for several months in the wood pores. A seedlot that is known to be contaminated with WFB bacteria should be planted as a positive control, but isolated from the test seed samples, to prevent cross contamination.
Seedlings with dark spots and water-soaked or greasy areas surrounding the spots on leaves or cotyledons, should be taken back to a plant pathology laboratory for final identification. Other pathogenic (Pseudomonas syringae py. laclyymans, angular leaf spot) or opportunistic bacteria may produce leaf spots, or other diseases such as gummy stem blight (caused by a fungus, Didymella bryonlae) or may be present Laboratory identification procedures include isolation of the bacterium on one of the semi-selective media, purification on general media, biochemical tests and rinally pathogenicity on clean watermelon seedlings to confirin disease potential. The whole process from planting to final confirmation may take up to 30 days.
In 1994, the National Watermelon Association recommended that all seedlots
to be planted the United States should be tested by this grow
out method and that no evidence of WFB bacteria be found in at least
10,000 seeds. By recommending this important control measure and implementing
other control strategies, especially in the early (trarisplant) stages,
hopefully, WFB will be a minimal problem in the future. The American Seed
Trade Association and the National Watermelon Board continue to cooperate
in research efforts and through publications such as the Bacterial Fruit
Blotch of Watermelon pamphlets, journals and grower group meetings, will
sufficiently inform the watermelon industry of new control strategies as
they become available.
References:
1. Crall, J.M. and Schenck, N.C. 1969. Bacterial fruit rot of watermelon
in FloridL
Plant Dis. Rep. 53:74-75.
2. Hopkins, D., Stall, R., Kucharek, T, Gay, D., Gitiatis, R, Cook,
W., Kcinath, A., and Latin,.R. 1994.
Bacterial Fruit Blotch of Watermelon.
3. Latin RX and Rane, K. 1990. Bacterial Fruit Blotch of Watermelon
in Indiana. Plant Dis. 74:331
4. Schaad, N.W., Sowell, G. Jr., Goth, R.W., Cowell, R.R. and Webb,
R.8. 1978. Pseudomonas
pseudoalcaligenes subsp. cilrulli subsp. nov. Int. J. SySL Bacteriol.
28:117-125.
5. Webb, R.E. and Goth, R.W. 1965. A seedborne bacterium isolated from
watermelon.
Plant Dis. Rep. 49: 818