Department of Microbiology and Plant Pathology, University of California, Riverside, CA 92521.
Plant Dis. 2021 Apr;105(4):972-977. doi: 10.1094/PDIS-06-20-1414-RE. Epub 2021 Feb 17.
, , , and less commonly are causal agents of brown rot of citrus fruit in California. The chronic disease occurs during the winter season, requires annual management, and has limited California citrus exports because of quarantines in some markets. Potassium phosphite (KPO) is registered as a pre- and postharvest fungicide in the United States to manage Phytophthora brown rot. We evaluated the in vitro toxicity of KPO to 65, 60, and 38 isolates of , , and , respectively, that were obtained from major growing regions of California. Frequency distributions of effective concentrations to inhibit mycelial growth by 50% (EC values) were not normally distributed, with skewness values of 1.84, 1.60, and -0.51 for each species, respectively. Isolates considered sensitive (EC values <25 µg/ml), moderately resistant (EC values from 25 to 75 µg/ml), or resistant (EC values >75 µg/ml) were identified for each species. The majority of (83.1%) and (78.3%) isolates were sensitive, whereas most isolates (86.8%) were moderately resistant or resistant. Resistance factors were calculated as 65, 19, and 10 for the three species, respectively. In preharvest field trials, KPO (2,280 g/ha) applications were not effective in reducing citrus brown rot incidence when orange fruit were inoculated with a resistant (EC = 161.9 µg/ml) isolate of , demonstrating the potential for field resistance. Oxathiapiprolin (32.6 g/ha), however, was highly effective, indicating the absence of multidrug resistance. Postharvest treatments with KPO were only effective in reducing brown rot caused by the resistant isolate of to a low incidence when high rates (8,000 µg/ml) were used in heated (54°C) applications. The sensitive and moderately resistant isolates were managed using rates of 4,000 µg/ml, but heated treatments at this rate were needed to reduce brown rot to commercially acceptable levels when decay was caused by a moderately resistant isolate.
溃疡疫霉、恶疫霉和终极腐霉是加利福尼亚州柑橘果实褐腐病的病原体。这种慢性疾病发生在冬季，需要每年进行管理，并且由于一些市场的检疫，限制了加利福尼亚州的柑橘出口。亚磷酸钾（KPO）在美国被注册为一种用于防治疫霉褐腐病的采前和采后杀菌剂。我们评估了 KPO 对从加利福尼亚主要种植区获得的 65、60 和 38 个溃疡疫霉、恶疫霉和终极腐霉分离株的体外毒性。抑制菌丝生长 50%的有效浓度（EC 值）的频率分布不是正态分布，每个种的偏度值分别为 1.84、1.60 和-0.51。对于每个种，确定了敏感（EC 值<25µg/ml）、中度抗性（EC 值 25-75µg/ml）或抗性（EC 值>75µg/ml）的分离株。大多数溃疡疫霉（83.1%）和恶疫霉（78.3%）分离株是敏感的，而大多数终极腐霉分离株（86.8%）是中度抗性或抗性。三个种的抗性系数分别为 65、19 和 10。在采前田间试验中，当橙子果实被抗性（EC=161.9µg/ml）溃疡疫霉分离株接种时，KPO（2280g/ha）的施用量不能有效降低柑橘褐腐病的发病率，表明田间可能存在抗性。然而，恶唑菌酮（32.6g/ha）的防治效果非常高，表明不存在多药抗性。KPO 的采后处理仅在使用高剂量（8000µg/ml）进行加热（54°C）应用时，才能有效降低由抗性终极腐霉分离株引起的褐腐病的发病率。对于敏感和中度抗性的分离株，使用 4000µg/ml 的剂量进行管理，但当由中度抗性分离株引起腐烂时，需要以该剂量进行加热处理，才能将褐腐病降低到商业上可接受的水平。
