Monitoring Zasmidium citri-griseum Inoculum Production in Citrus Grown Under Protective Screens

Greasy spot, caused by Zasmidium citri-griseum, is a major foliar disease in citrus, leading to defoliation, yield loss, and reduced rind quality. The primary inoculum, ascospores, develop in decomposing leaf litter under alternating wet and dry conditions, with high humidity and extended leaf wetness promoting infection. Citrus Under Protective Screens (CUPS) is an innovative cultivation system designed to exclude the Asian citrus psyllid and reduce the impact of huanglongbing, however it can exacerbate some diseases such as greasy spot. Within these structures, the microclimate has higher than ambient relative humidity and temperatures, but the effect on Z. citri-griseum ascospore production is unknown. This study investigates the abundance of Z. citri-griseum ascospores within CUPS from March to October. Two Burkard volumetric spore traps were installed in a CUPS system planted with grapefruit: one at the center of the structure and one at the edge. Ascospore samples were collected weekly, with counts recorded every two hours by counting at 400× with a light microscope. Results revealed the number of ascospores at edge of the CUPS is approximately 57% higher than middle indicating that the majority of ascospores originate from outside the CUPS. The peak ascospore release occurred in the morning, likely influenced by irrigation events and morning dew.Comparing spore trapping between the middle and edge of CUPS helps pinpoint high-risk zones for targeted disease control. Additionally, predicting inoculum production aids in optimizing spray schedules.