One disadvantage of copper-based fungicides is that they have a contact mode of action; that is, the chemical works only on the surface where it is applied. With such applications, it is possible for the bacterium to survive and multiply on surfaces of the plant that are not covered by the chemical. This could be prevented by using a systemically-acting chemical. Another disadvantage of copper-based fungicides is that their repeated use could lead to development of resistance. This has not happened so far with the BFB pathogen, but it has occurred with other bacterial pathogens on other crops. One approach to preventing this problem is to use other chemicals in alternation or combination with copper-based fungicides. The use of different fungicides in a spray program is recommended to prevent development of fungicide resistance with diseases such as late blight of potato and powdery mildew of cucurbits.

Experiments done by T. Isakeit with greenhouse-grown transplants have shown that Actigard^TM, Starner⁷, and Messenger⁷ reduce BFB symptoms and bacterial population growth under temperature and moisture conditions very conducive to disease (data not shown). However, further experiments are needed to optimize application rates and eliminate questions about phytotoxicity or effectiveness for use of these chemicals in transplant production. Actigard^TM and Starner⁷ are not currently labeled for watermelon. Messenger⁷ is labeled for use on cucurbits, but the label does not specifically promise control of BFB.

The objective of this experiment was to test the activity of three experimental chemicals for BFB control under field conditions, in comparison with copper hydroxide. Two of the chemicals, Actigard^TM (acibenzolar-S-methyl) and Messenger⁷ (harpinEA), are systemic and do not act directly on the pathogen but, rather, stimulate host defense mechanisms. These chemicals are most effective when they are applied to the plant several days prior to exposure to the pathogen. Starner⁷ (oxolinic acid) interferes with the ability of the bacteria to multiply, and has a contact mode of action.

The experiment was conducted at the Texas A&M University Research and Extension Center in Weslaco. On 3/13/02, just prior to planting in the field, Actigard^TM and Messenger⁷ were sprayed onto watermelon transplants at the 4 - 5 true leaf stage (first application date, see Table 1). On 4/8/02, when vines were 12 - 18 inches long, a suspension of the BFB pathogen was sprayed onto the foliage, using a concentration to 10¹⁰ cells/ml. The pathogen was applied between the second and the third chemical application dates (see Table 1). After inoculation, the sprinklers ran overnight to maintain constant leaf wetness following inoculation. Further sprinkler watering was done weekly, for 12 - 18 hours.

Harvest dates were 6/4/02, 6/7/02, and 6/13/02. Fruit in each plot were counted, weighed, and categorized as: marketable, obvious fruit blotch symptoms, and culls (shape and/or size defects). Data was analyzed for statistical differences.

Although plants were inoculated early in the season with massive quantities of the pathogen, hot, dry weather conditions resulted in low disease pressure. The mean incidence of BFB among treatments ranged from 0 - 19 percent, while the mean proportion of weight of melons with BFB ranged for 0 - 24 percent (Table 1). The severity of symptoms was slight, and some of the fruit with obvious symptoms might be considered marketable. None of the treatments significantly (P=0.05) affected the mean fruit number, total fruit weight, or marketable fruit weight (Table 1). In other trials where there was greater incidence of BFB, there was usually a significant difference in marketable fruit weight between the control and a copper fungicide treatment. Hot, dry, and windy weather conditions made transplant establishment difficult. Poor plant growth likely accounted for the great variation in fruit number and weight observed among replicates of treatments.

The bottom line of this study indicates that Copper hydroxide (as Kocide⁷ 2000) was the most effective treatment for reducing the incidence of bacterial fruit blotch (BFB) of watermelon in a field experiment conducted in Weslaco, Texas in the spring of 2002. When applied singly, three experimental chemicals did not reduce BFB, although BFB was reduced when two of the chemicals were tested in combination with Kocide⁷ 2000. The results of this one trial do not necessarily preclude the effectiveness of these experimental chemicals under other conditions or environments. For prevention of BFB, growers should apply copper-based fungicides such as copper hydroxide. In this experiment, applications were made every two weeks, which was appropriate for the low disease pressure that occurred. Under greater disease pressure, for example, when frequent rains occur, the full label rate should be applied weekly.