Name: Acidovorax Willems et al. Etymology: L. Acidovorax, a new genus for Pseudomonas facilis, Pseudomonas delafieldii, E. Falsen EF group 13, EF group 16, and several clinical isolates, with the species Acidovorax facilis comb.
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Top of page The disease cycle in most host crops apparently begins with contaminated seed Latin and Hopkins, Much of the watermelon production in the USA is still from direct-seeded plants, but transplants are rapidly gaining in popularity.
A warm, humid environment favourable for disease development is maintained in transplant facilities used for raising watermelon seedlings. Bacteria from infested seed infect the developing seedling as the cotyledons emerge from the seed coat. Most transplant facilities employ overhead irrigation, which effectively splash-disperses bacteria to neighbouring seedlings. This secondary spread in the transplant house can result in high numbers of infected seedlings reaching the field.
Some of these transplants may harbour the bacterium, but show no symptoms. Because a single transplant house may supply plants to several growers over a large geographical area, transplants can be the mechanism through which the bacterium is dispersed. The fruit blotch bacterium may be introduced into a field from infested seed, infected transplants, contaminated volunteer crop cucurbits, or infected wild cucurbits.
In the field, symptom development and spread of A. These conditions are common during the summer. In addition to wind-driven rain, the bacterium can be spread by contact with farm workers, irrigation, and other cultivation equipment D. The bacterium does not appear to spread as rapidly during cool, rainy weather. With favourable weather, a few primary infection sites in a field can result in infection of all plants by the time of harvest.
Leaf lesions in the field do not result in defoliation, but are important reservoirs of bacteria for fruit infection. Under some environmental conditions, foliar symptoms may not be very conspicuous and the watermelon grower may not realize that there is a problem until fruit symptoms render the crop unmarketable.
Watermelon fruits are most vulnerable to bacterial infection 2 to 3 weeks after anthesis Frankle et al. The bacterium invades the fruit through stomata on the fruit surface. Small, water-soaked lesions develop days later. Mature watermelon fruit are covered with a wax layer that plugs stomata and prevents the entry of bacteria into the fruit. Once the wax layer forms, mature watermelons can be invaded by the bacterium only after wounding.
Thus, the bacterium must invade the fruit seven days, or more, before maturity, but the symptoms continue to develop as long as the fruit is attached to the plant. Surface lesions cease to enlarge after harvest. Wild cucurbits can become infected with A. Volunteer cucurbits that grow from previous crops that had fruit blotch can be naturally infected with the bacterium.
These infected wild cucurbits and volunteer crop cucurbits may be important overseasoning hosts for the bacterium that helps to perpetuate the disease in an area Latin and Hopkins, Production of cucurbit crop seed in areas with these alternate hosts of A. However, the infection level generally has been low, because in heavily infected seed lots the bacterium would have been detected and seed would not have been sold. One commercial seed lot was reported, in a grow-out test, to have one infected seed in Rane and Latin, Walcott et al.
Using immunomagnetic separation and PCR, A. The ability to penetrate watermelon flowers was not unique to A. The data indicate that watermelon blossoms are a potential site of ingress for fruit and seed infestation by A. These data indicate that watermelon blossoms are a potential site of ingress for fruit and seed infestation by A.
Lessl et al. The authors also showed that low levels of A. Subsequently, using a constitutive green fluorescent protein mutant of A. This mechanism of seed infestation provides a possible explanation for the production of contaminated seedlots from fields with no visible BFB symptoms.
In addition, recently, it was reported that blossom or stigma inoculation resulted primarily in bacterial localization in the seed embryo, whereas pericarp inoculation resulted in bacterial deposition under the seed coat Dutta et al. These aspects may have implications on bacterial survival in seed and efficiency of pathogen detection by seed health testing.
Effect on Seed Quality There does not appear to be any effect of A. Seed appearance, germination and seedling vigour are unaffected. Pathogen Transmission The outbreak of bacterial fruit blotch of watermelon in the USA appeared to be initiated by contaminated seed. In Indiana, initial symptoms in diseased seedlings in commercial greenhouses were limited to distinct foci Rane and Latin, Many of the fruit blotch infected greenhouses and fields were associated with a seed lot that was later shown to be infested with A.
Other studies have corroborated transmission of the pathogens by seeds. In Australia, tests showed A. In a recent study, it was reported that A. Researchers observed a sigmoidal relationship between A. One-hundred and As disease transmission was observed for seedlots with just one seed containing as few as 10 c.
It was also demonstrated an increase in spatial spread of BFB in the greenhouse with increase in A. Infested seed is the primary way in which A. Alternative hosts can also serve as the source of inoculum into cucurbit fields.
The conditions in a transplant house are highly conducive for BFB development Walcott, Infested seeds are normally the primary source of inoculum, and secondary spread of bacteria is due to the splash-dispersal facilitated by overhead irrigation that generates bacterial aerosols.
Bacteria landing on uninfected seedlings penetrate cotyledons and leaves via stomata and multiply rapidly in intercellular space. Numerous secondary infection cycles can take place due to splash dispersal. Infected seeds and seedlings are the most important primary sources of inoculum in commercial fruit production field.
However, there may be other endemic sources of inoculum like debris from infected fruit or foliage tissue, volunteer watermelon seedlings, or cucurbitaceous weeds Black et al. In the field, BFB development is heavily dependent on rainfall and relative humidity. Secondary dispersal of A. When A. Although, there is currently no evidence of systemic A. This infection court is accessible for weeks after anthesis, and as fruits mature, wax becomes deposited over the stomata preventing further bacterial entry.
Despite this, A. Seeds from infected fruits may become buried in the soil where they may serve as an inoculum for subsequent crops. While fruit infection is initiated at anthesis, fruit symptoms do not develop until harvest maturity. Alternative hosts can also serve as the source of inoculum in cucurbit fields.
Seed Treatments Several studies have been conducted on the control of bacterial fruit blotch of watermelon with seed treatments. Sowell and Schaad reported that soaking seed known to be infested with A.
However, this treatment has not been developed into an effective commercial treatment. In a later study, treatments of infested seed in 0. They also showed that, among bactericide treatments, antibiotic preparations were more effective than copper compounds and other pesticides. Moreover, low concentration and lengthy soaking treatments were far superior to high concentration and short treatments. However, bactericide treatment under vacuum was not as good as the normal treatment.
Although bactericide treatment reduced disease incidence, it did not eradicate the pathogen. Fermentation for hours plus treatment with HCl was most effective in eliminating seed transmission. These treatments are effective on diploid watermelon seeds, but germination of triploid seed can be reduced by the treatments. Fermentation of freshly harvested seed in watermelon fruit juice is a common practice routinely employed in commercial seed production. The process involves incubation of seeds in fruit juice for h followed by rinsing and drying.
Hopkins et al. However, fermentation cannot be used for certain watermelon hybrids and other cucurbits as it can adversely affect seed germination Walcott, Many seed treatments have been reported to reduce BFB seedling transmission.
Treatments with 0. Currently, wet seed treatment with peroxyacetic acid is a standard practice by cucurbit seed producers. There have been many other attempts to develop seed treatments for BFB. For example, Feng et al. Seed treatments with NaOCl at 50oC for 20 min, peroxyacetic acid for 30 min and ACA at 50oC for 20 min eradicated the pathogen, seed germination was negatively affected. Among the seed treatments, only AEW eradicated A.
The watermelon seed industry is currently testing their seed lots by grow-out test of 10, to 50, seedlings per seed lot. In this test, seeds are planted in a sterile potting mix in a greenhouse that is free from other sources of A.
Seedlings are watered by overhead irrigation to promote foci of symptomatic plants that are easily visible. After days, each seedling is carefully inspected for symptoms. Isolations are made from seedlings showing any symptoms of disease and A.
This method takes days to complete. There is zero tolerance for contaminated seed and any infested seed lot is rejected. Elaborate precautions must be taken to ensure that cross-contamination of seedlings does not occur in test procedures.
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