Field survival of Xanthomonas cucurbitae, the causal agent of bacterial spot of pumpkin, and efficacy of selected chemicals and biocontrol agents for control of the diseases
Bacterial spot, caused by Xanthomons cucurbitae, has become one of the most important diseases of pumpkins in Illinois. This research was conducted to assess survival of X. cucurbitae in commercial fields, and to evaluate efficacy of selected chemicals and biocontrol agents for control of bacterial spot of pumpkin. To assess survival of X. cucurbitae in the field, a factorial experiment was setup at the University of Illinois Fruit and Vegetable Research Farm in Urbana, IL. Field location, plant tissue, burial depth, and recovery date were the experimental factors. The experimental unit was either five infected leaves or two infected fruit pieces (10 cm × 10 cm), which were placed in fiber glass mesh and buried either at 0-10 cm or 10-20 cm deep on 7 October 2011. The samples were recovered on 7 April, 7 July, and 7 October 2012; and 7 January, 7 April, and 7 October 2013 and processed for presence of X. cucurbitae. The location of the field did not significantly (P = 0.7895) affect the number of X. cucurbitae CFUs recovered from the samples. However, survival of X. cucurbitae was significantly affected by burial depths (P = 0.08), the plant tissue (P = 0.001) and time period of plant tissue in soil (P = 0001). X. cucurbitae survived for 24 months in both infected pumpkin leaf and fruit tissues in the soil. Seventeen chemicals and five biocontrol agents were tested in laboratory and field for their efficacy for control of bacterial spot. Ten isolates of X. cucurbitae were tested in nutrient broth (NB) and casitone yeast extract broth (CYE). Laboratory studies of chemicals were conducted to determine effective concentrations for 50% and 100% reduction in cell multiplication (EC50 and EC100) of X. cucurbitae. The EC50 values of the chemicals ranged from 0.17 ppm mancozeb (Dithane) to 64.53 ppm cuprous oxide (Nordox) in NB, and from 0.23 ppm mancozeb (Dithane) to 38.87 ppm cuprous oxide (Nordox) in CYE. Similarly, the EC100 values of the chemicals ranged from 1 ppm mancozeb (Dithane) to 175 ppm cuprous oxide (Nordox) in NB, and from 1 ppm mancozeb (Dithane) to 125 ppm cuprous oxide (Nordox) in CYE. The Laboratory tests of biocontrol agents were conducted using the filter-disc assay method. All five tested biocontrol agents prevented cell multiplication of X. cucurbitae cells around the filter disc dipped in biocontrol agents. The field trials were carried out at the University of Illinois Vegetable Research Farm in Champaign, IL. Jack-o-lantern pumpkin ‘Howden’ was used in the field studies. Plants were inoculated with an equally mixed inoculum of 10 X. cucurbitae isolates (5 × 107 CFU/ml). Spray-application of chemicals began either pre- or post-inoculation of plants. Spray-application of all biocontrol agents began pre-inoculation. All compounds were applied at 7-day intervals. Incidence and severity of bacterial spot in leaves were significantly lower in treated plots than those of untreated control plots. Treatments with Badge, Cuprofix, Mycoshield, Phyton, Kocide plus ActiGard, and Kocide plus Tanos were more effective in reducing incidence and severity of bacterial spot on both leaves and fruit than other chemicals. Treatments with biocontrols Regalia, Serenade, and Serenade plus Milstop were more effective in reducing incidence and severity of bacterial spot on both leaves and fruit than other biocontrol treatments
