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Research

Bacterial Speck Causing Larger Lesions

The Tomato Magazine
February 2006

The bacterial speck pathogen this past spring was associated with much larger lesions on tomatoes in Florida than has been observed in the past, according to Jeffrey B. Jones with the University of Florida/IFAS Plant Pathology Department in Gainesville.

Speaking during the annual Florida Tomato Institute last September in Naples, Fla., Jones noted the change, indicating that all strains to date have been identified to be race O (Race 0 is the typical race that was originally present. The fruit size is not a result of the race change).

In Florida, bacterial spot is considered the most destructive bacterial disease when high temperatures are accompanied with excessive moisture, Jones told the group. Bacterial leaf spots associated with tomato have primarily been bacterial spot incited by Xanthomonas campestris pv. vesicatoria and bacterial speck incited by Pseudomonas syringae pv. tomato.

Bacterial spot is considered a warm weather pathogen and thus in the spring is not as aggressive as in the fall, the researcher pointed out. Hence, bacterial leaf spot in Florida is not a significant concern to spring crops of tomato because of typically drier weather then.

Likes Cool Weather Conditions
Bacterial speck, however, is favored by relatively cool weather conditions such as occurred in Florida tomato fields in the spring of 2005, Jones said. Therefore, it was considered “the more likely candidate” to be associated with a spring epidemic.

“Bacterial speck has been a problem in Florida tomato production during periods of significant rainfall during spring production,” Jones said, adding that one such period was 1983, when bacterial speck proved a major problem in a spring tomato crop in several fields in Manatee County. That particular year was cool and extremely wet.
In the past, bacterial speck has been a chronic problem in the Homestead area during winter tomato production, he said. Researchers, however, attribute much of the problem to the use of overhead irrigation.

Looking at the 2005 spring crop, Jones said Florida weather conditions were “unusually wet,” and, hence, were conducive for bacterial diseases.

“A number of fields were affected by what was thought to be bacterial leaf spot,” he said. “The fruit was also affected and was unusual in appearance. The lesions were large, sunken and distinctly black. They resembled bacterial speck more than bacterial spot, although the lesions were atypical in their rather large size (Figure 1a) unlike the ‘speck’ size lesions normally observed on the fruit.

“Large fruit lesions caused by the bacterial speck pathogen, although rare, have been observed previously in Florida by Dr. K.L. Pernezny (personal communication),” Jones explained. “Although bacterial speck does occur frequently on fruit, only very young developing fruit are susceptible. Furthermore, it requires optimal conditions and inoculum for infection, as demonstrated by Getz et. al. (1983) for bacterial speck and by Scott et. al. (1989) for bacterial spot. In both studies, it was demonstrated that fruit infection only occurs during a short time-frame following anthesis.”

To conclusively show that the problem was associated with bacterial speck, Jones said isolates were taken from lesions in samples from three fields.

All Lesions Were Infected
“ In all the lesion samples, a bacterium was present which was characteristic of the bacterial speck pathogen (Pseudomonas springae pv. tomato), and in a number of isolations we recovered two types of organisms, the bacterial speck pathogen and another one that was characteristic of the bacterial spot pathogen (Xathomonas campestris pv. vesicatoria),” he said. “The strains that were suspected of being the bacterial speck pathogen were typical P. syringae pv. tomato strains….”

During additional research, Jones and colleagues inoculated some very young fruit in the greenhouse to determine if large fruit lesions could be reproduced. This effort “conclusively demonstrated that P. syringae pv. tomato can cause large speck lesions,” he reported.

“Control of bacterial spot and bacterial speck requires an intensive program,” the researcher cautioned. He recommended the following:

• Clean Transplants: Make sure they are free of bacterial diseases going into the field.
• Good Isolation: All fields need to be isolated from production fields from the previous season given that bacterial cells can be transported long distances to infect healthy crops.
• Reduced inoculum: It is important to eliminate volunteers or adjacent fields as sources of inoculum.
• Bactericides: These are an important component of an integrated approach for controlling bacterial diseases of tomato. Copper has been used for many years and has been shown many times to be significantly more effective if applied in a tank mix with an EBCD compound.
• Activators: A second group of compounds that have proven effective are plant activators. Actigard has been shown in many studies to reduce bacterial spot and bacterial speck disease severity (Louws et. al., 2001).
• Bacteriophages: A very promising approach for controlling bacterial spot has been to use bacteriophages (Flaherty et. al., 1999; Balogh et. al., 2001; Obradovic et. al., 2004).

“We have demonstrated in the field that bacteriophage applications result in significantly higher fruit yields compared to the control or other standard treatments,” Jones said. “A similar strategy should be effective for controlling bacterial speck.”

Editor’s note: The research reported here was conducted by J.B. Jones, Ellen R. Dickstein and R.E. Stall, all with the UF/IFAS Plant Pathology Department, Gainesville, Fla. Jones can be contacted by e-mail at: jbjones@ ufl.edu.

© 2006 Columbia Publishing

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