Postharvest Heated Fungicide Treatments to Control Citrus Black Spot (Guignardia citricarpa) on Citrus Fruits

Citrus black spot (CBS), caused by Guignardia citricarpa, is an invasive fungal disease that was first identified March 2010 in Florida and is spreading slowly. Although a USDA pest risk assessment determined that citrus fruit is not a good vector for the disease, lesions may develop on fruit after harvest and can be considered as a grade-lowering defect by receivers. Postharvest fungicides have been evaluated to control postharvest CBS symptom development, but with limited success. Heated fungicide solutions have been shown to enhance control of postharvest decay or allow lower fungicide concentrations to be used. Heat treatments can also induce resistance to disease and physiological disorders. The current preliminary experiment tested the effectiveness of dipping fruits from CBS-infected Valencia orange trees for 30 seconds into one of seven commercial postharvest fungicides. Solutions were at 25°C or 50°C. No pre-harvest fungicides were used on the trees in the field. After harvest, the fruits were sorted into two groups (non-symptomatic and symptomatic fruits) and evaluated separately. All fungicides were used at commercial recommended concentrations and control fruit were dipped in water alone. After dipping, fruit were kept at 25°C, with 80% relative humidity, 3 ppm ethylene, and continuous light to speed lesion appearance. Fourteen days after treatment, there was no significant reduction in lesion development on fruit that already showed CBS symptoms at harvest. However, fruits dipped in azoxystrobin or thiabendazole tended to develop 37% and 27%, respectively, fewer lesions during storage than the control. On fruit that were asymptomatic at harvest, all fungicide treatments significantly inhibited subsequent lesion appearance during storage. Compared to control fruit, thiabendazole, azoxystrobin, phosphorous acid, pyrimethanil, imazalil, fludioxonil, and sodium o-phenylphenate (SOPP) reduced the number of fruit lesions by 71%, 42%, 38%, 34%, 30%, 29%, and 17%, respectively. Heated solutions enhanced the efficiency of azoxystrobin and fludioxonil by 58% and 27%, respectively. Heated water alone also inhibited lesion appearance compared to control, decreasing the number of lesions on fruits by 28%. Future tests will provide more information and details for practical application.