Chemical control of bacterial fruit blotch of watermelon

In the spring of 1989, bacterial fruit blotch of water melon (Citrullus lanatus (Thunb.) Matsum. & Nakai), caused by a nonfluorescent pseudomonad, was first observed in Florida. While there are foliar symptoms of the disease, major losses are due to fruit symptoms that render mature fruit unmarketable. In field tests, cupric hydroxide alone and in combination with fosetyl-AI effectively controlled fruit blotch, reducing losses from 69% in the unsprayed watermelons to Florida Agricultural Experiment Station Journal Series No. N-00498. 270 9-10% in plants receiving weekly applications of these mate rials. Streptomycin and fosetyl-AI alone were ineffective. Mar ketable yields with unsprayed plants were 20-25% of the yields with plants that received cupric hydroxide weekly. Con trol of fruit symptoms was related to a reduction in the spread of foliar symptoms. Slight phytotoxicity was observed with the application of cupric hydroxide. The timing of 1 to 3 appli cations of cupric hydroxide appears to be critical to obtaining fruit blotch control without phytotoxicity. In the 1989 watermelon season, a new bacterial fruit rot occurred in commercial fields in Florida and, as the season progressed, the disease was observed in many other states in the Eastern USA (Hopkins, 1989; Latin and Rane, 1990; Somodi et al., 1991). In some fields, losses were up to 80% of the total marketable fruit. Symptoms on the fruit began as small, water-soaked areas of a few mm in diameter and usually developed into large, dark-green, water-soaked lesions with irregular margins. Initially, lesions did not exProc. Fla. State Hort. Soc. 104: 1991. tend into the flesh of the fruit but, as the lesions aged, the fruit surface cracked and decay sometimes followed. Necrotic lesions could be observed on the leaves, primarily along the major veins, but in most fields foliar symptoms were inconspicuous. A similar disease was observed in Au stralia and the Mariana Islands (Wall and Santos, 1988). The fruit blotch bacterium both from the Mariana Is lands (Wall and Santos, 1988) and the eastern U.S.A. (Latin and Rane, 1990; Somodi et al., 1991) was found to be a nonfluorescent pseudomonad that is very similar, but not identical, to a bacterium that caused a disease of watermelon seedlings of certain plant introductions in Georgia in the 1960s (Webb and Goth, 1965). This seedling bacterium was identified as Pseudomonas pseudoalcaligenes subsp. citrulli (Schaad et al., 1978), but this taxonomic classification may be incorrect (DeVos et al., 1985). Bacterial fruit blotch has occurred in a few Florida wa termelon fields in 1990 and 1991. While the disease has been devastating where it occurs, fruit blotch, fortunately, has been limited in occurrence to a small fraction of Florida's total watermelon acreage. Once the bacterium is present in the field there is no known control for the disease. Fruit of some cultivars have been reported to be more resistant to bacterial fruit blotch than that of other cultivars (Hopkins, 1990). However, the level of resistance does not appear to be sufficient to allow successful watermelon pro duction under conditions conducive to severe disease devel opment. The purpose of this study was to evaluate bactericides for the control of bacterial fruit blotch of watermelon in the field and to compare different application schedules for the most effective material. Materials and Methods Spring 1990 Test. 'Charleston Gray 133' watermelon was planted in test plots which were arranged in randomized blocks with 4 replications. Each plot consisted of 10 hills in 2 rows with 5 ft between hills and 6 ft between rows. Water melons were direct-seeded on 14 Mar. and were thinned to 2 per hill on 5 Apr. Inoculum was prepared by growing the fruit blotch bacterium on nutrient agar for 48 to 72 hr at 28C. On 11 Apr., plants in the center hill of each row were inoculated by misting with a 5 x 106 bacteria per ml water suspension of isolate WFB89-1, which had been iso lated from fruit from a commercial field in 1989. The in oculations were done in the late afternoon. Six weekly bactericide applications were made to the plants beginning on 1 May and ending on 5 June. The following bactericides were evaluated in 100 gals of water per acre: streptomycin at 100 ppm; fosetyl-Al (Aliette 80WP) at 2.0 lbs; fosetyl-Al at 4.0 lbs; cupric hydroxide (Kocide 101) at 1.0 lbs; and fosetyl-Al at 2.0 lbs plus cupric hydroxide at 1.0 lbs. Sprays were applied at 175 psi from 9 hollow cone (Teejet D2-25) nozzles spaced 15 inches apart on a 12-ft boom. Rainfall during the season included 1.91 inches in April, 1.14 inches in May, and 1.74 inches in June. Overhead irrigations of 0.75 inch each were applied 4 times during the season as needed. Foliar symptoms were rated using a Barratt-Horsfall system. Fruit symptoms were evaluated 2 weeks prior to fruit maturation and at harvest. Marketable yields were determined by weighing individual fruit at har