ASHS 2015 Annual Conference

Brown rot on citrus fruit in Florida is primarily caused by Phytophthora palmivora and is most common during the fall rainy period. Control of brown rot was evaluated on four different varieties of harvested citrus fruit inoculated with P. palmivora about 2 days before treatment. The fruit were then dipped in ambient (22^oC) or heated (49^oC) solutions of potassium phosphite (1.2% KPhos^TM), calcium phosphite (1.2% Calci-Phite^R), and/or thiabendazole (0.025% TBZ) for 30 seconds. Control fruit were dipped in water alone. Treated fruit were then held at 29^oC with 90% RH until evaluated for brown rot and Diplodia stem-end rot (Lasiodiplodia theobromae from natural infections) after 7 and 14 days. ‘Murcott’ tangerine hybrids, ‘Navel’ and ‘Hamlin’ oranges, or ‘Ruby Red’ grapefruit treated with KPhos reduced brown rot incidence by 92%, 74%, 68% and 82%, respectively, compared to the control. Combination of KPhos and TBZ also significantly reduced the incidences of both brown rot and Diplodia stem-end rot. The use of Calci-Phite^R with TBZ did not significantly reduce brown rot incidence, but significantly reduced Diplodia stem-end rot incidence. Use of hot water alone (49^oC) significantly reduced brown rot incidence compared to the ambient control on ‘Murcott’ tangerine hybrids and ‘Ruby Red’ grapefruit. Mycelial growth of P. palmivora was significantly inhibited in vitro after 30 seconds exposure to 49^oC water, and completely inhibited after a 20 or 30 min exposure, even when evaluated up to 14 days after exposure. ‘Ruby Red’ grapefruit and ‘Murcott’ tangerine exposed to the 49^oC water dip treatments did not significantly differ in titratable acidity (TA), total soluble solids (TSS), TSS/TA ratio, vitamin C, or fruit weight loss and fruit respiration rate compared to those dipped in 22^oC water. The heated dip treatment also significantly inhibited subsequent color development and resulted in significantly higher juice glucose and fructose contents compared to the control. The results suggest that a postharvest fruit dip with KPhos^TM and TBZ can effectively control both brown rot and Diplodia stem-end rot on fresh citrus, and that heated (49^oC) solutions can provide even better control.