Potential distribution of citrus black spot in the United States based on climatic conditions

Abstract

Citrus black spot (CBS), caused by Phyllosticta citricarpa McAlp Van der Aa, was recently detected in southern Florida in the US. In addition to infected plant propagation materials, movement of infected citrus fruit poses a concern for potential spread of the disease out of the current quarantine zone, because lesions with pycnidia and conidia could develop after harvest. The conditions conducive for mycelial growth and development of pycnidia and conidia are not well known. Therefore, effects of temperature and relative humidity on growth and conidial production of P. citricarpa were determined and used as parameter inputs in CLIMEX to predict potential establishment of CBS in North America. Colony growth and conidial production in vitro were optimal at 27 °C, whereas there was no growth below 4 °C and above 37 °C. On fruit, lesion development and conidial production were observed at 4 °C, though at a low rate, indicating a greater versatility of the fungus on fruit. More full pycnidia were produced on the CBS lesions at 91 % RH compared to 84 %. Input parameters for CBS risk in CLIMEX obtained from literature, which reflected conditions for infection in spring/summer in Florida, predicted potential establishment in Florida but not in California. However, altering the parameter values to account for survival of the pathogen in leaf litter in winter predicted potential establishment in California as well as Florida. Thus, P. citricarpa could possibly establish beyond Florida if this organism is transported outside of the current quarantine zone to other citrus production areas.