Clemson University, Coastal Research and Education Centre, Charleston, SC 29414-5332, USA

Background and objectives
The fungus Didymella bryoniae (=Mycosphaerella citrullina), anamorph Phoma cucurbitacearum (=Ascochyta cucumis), causes gummy stem blight (foliar phase) and black rot (fruit phase) on cucurbits. In the south-eastern USA, gummy stem blight is one of the most destructive foliar diseases of watermelon (Citrullus lanatus). Because of public concern about fungicide use on food crops, it is often necessary to demonstrate a concomitant yield or quality increase. In my previous studies, at least six fungicide applications were necessary to prevent loss of marketable watermelon fruit under heavy gummy stem blight pressure [1]. In other studies, yield in fungicide-treated plots was not significantly greater than that in untreated plots. The effect of gummy stem blight on sugar content of watermelon fruit has not been examined directly. Sugar content of watermelons affected with both downy mildew and gummy stem blight was reduced by 10% on plants which were not sprayed [2], but it was not possible to determine which pathogen was responsible for the reduced quality. My objectives were (1) to measure the changes in watermelon yield and quality under different fungicide application schedules and (2) to determine if gummy stem blight severity and the number of fungicide applications influenced yield and quality of watermelon fruit.

Materials and methods
In the summer of 1996 and 1997, watermelon cv. Royal Star seedlings were transplanted 0.9 ;m apart into single rows 15.2 ;m long and 3.3 ;m apart. The experimental design was a split-plot with five replications. Fungicide served as the main plot and application intervals as the split plots. The protectant fungicides mancozeb and chlorothalonil were applied with a tractor-mounted hydraulic sprayer. Plots within rows were separated by 2.7 ;m of unsprayed vines. In 1996, fungicide applications began at anthesis or when gummy stem blight was first detected, then at 7- or 14-day intervals until first or last harvest. In 1997, fungicide applications began at anthesis or when disease was first detected, then at 7-, 10- or 14-day intervals until first harvest. Per cent leaf area diseased was assessed eight times from late August through mid-October each season. Ripe fruit were harvested two (1996) or three (1997) times in early October; all remaining fruit were collected in a final harvest. Fruit were graded into marketable (healthy, unblemished fruit weighing at least 6.35 ;kg) and cull classes, counted, and weighed. Soluble solids content (% Brix) was measured with a hand-held refractometer from both halves of two marketable fruit from each plot at each harvest.

Results and conclusions
The number of fungicide applications ranged from four to nine in 1996 and three to eight in 1997. Weight of all fruit and marketable-quality fruit was greater in sprayed treatments than the unsprayed control both years (P<0.05). In 1996 but not in 1997, weights were greater with 7-day than with 14-day frequencies. The total number of healthy, unblemished fruit of all sizes was linearly related to the number of fungicide applications. Soluble solids content was greater with 7-day than with 14-day frequencies in 1996, and greater with 7-day frequencies than with 10- and 14-day frequencies in 1997. Likewise, the percentage of fruit with Brix readings of 10.0% was higher for all sprayed treatments than for the unsprayed control and for 7-day frequencies than for 14-day frequencies (data from both years combined). Initiation and termination times for fungicide applications did not affect yield or quality. Yields did not differ significantly between the two fungicides. Although many yield and quality variables were highly correlated with disease ratings in 1996, few such correlations were statistically significant in 1997. Regular fungicide applications are necessary to prevent yield and quality losses in watermelon fruit whenever environmental conditions favour development of gummy stem blight and black rot.

1. Keinath AP, 1995. Plant Disease 79, 354-58.
2. Young JY, Kirkpatrick TL, Henry RN, 1995. Biological and Cultural Tests 10, 154.