Postharvest fungicide treatments and cold storage control citrus black spot infections

Citrus black spot (CBS) is caused by Phyllosticta citricarpa, which is regarded as a quarantine pathogen in certain countries. Pest risk assessments disagree on the risk of fruit as a pathway for introduction of the pest to new areas. Some countries accordingly regulate the movement of fruit from production regions where CBS occurs. Preharvest fungicide sprays are very effective in controlling CBS, but cannot consistently achieve complete control. The effect of postharvest treatments on CBS infections and on the reproductive ability of P. citricarpa in lesions that formed after these treatments was studied. Trials were conducted using naturally infected 'Eureka' lemon and Valencia orange fruit. Asymptomatic fruit were treated using commonly used packhouse sanitation and fungicide treatments, and cold storage (individually and combined), as well as alternative stand-alone treatments. After treatment, fruit were stored for 5 weeks (in the dark) at ambient temperature (20-22 °C) or −0.5-7 °C, whereafter they were incubated for a further 2 weeks at conditions conducive to expression of symptoms and formation of pycnidia. Individual treatments generally resulted in variable levels of control. The combination of packhouse treatments (including pre-packhouse drench with guazatine or propiconazole in combination with pyrimethanil, thiabendazole and 2,4-D; chlorine wash; dip treatment in imazalil; and brush application of a wax coating incorporated with imazalil, thiabendazole and 2,4-D) consistently showed moderate to high levels of control of CBS development from latent infections on lemons (32.4-43.4% for incidence and 61.6-68.3% for severity) and oranges (58.3-85.7% for incidence and 54.1%-88.8% for severity). However, cold storage subsequent to packhouse treatments (as is common shipping protocol) further improved the levels of control (44.0-58.3% for incidence and 78.1-82.5% for severity on lemons; 66.1-93.5% for incidence and 85.3-98.5% for severity on oranges). An average of 10% of new lesions on lemons and 15% on oranges formed pycnidia, indicating that P. citricarpa generally had a low reproductive capability in fruit lesions, which was in most cases further diminished by the combination of treatments followed by cold storage.