Early symptoms include the development of a soft, water-soaked area on the peel. After a few days, a circular and superficial spot of white mold grow on the original lesion, often several centimeters in diameter. Over time, the mold spread superficially on the skin and older parts at the center become bluish or greenish. The neighboring tissues become soft and water-soaked or colonized in turn by a broad band of white mycelium. The fruit rapidly spoils and collapses, or in lower humidities shrinks and mummifies.
Two species of fungi belonging to the genus Penicillium cause these destructive rots of citrus fruits. P. italicum and P. digitatum grow on the fruit's skin as bluish mold and greenish mold respectively. The lesions of the former usually spread more slowly than those caused by the latter. Their development is also characterized by a band of younger white mycelium that surrounds the older growth in the center. These fungi are opportunist and take advantage of wounds on the fruit surface to start their life cycle. The spores germinate with the release of water and nutrients from the site of injury. At the optimal temperature of 24 °C, the infection takes place within 48 hours and initial symptoms become visible within 3 days. Transmission can occur mechanically or via the water- or air-borne dispersal of spores. These spores often reside in the soil but can also be found in the air of contaminated storage spaces.
Biological control of the fungi can be achieved with the use of formulations based on the Pseudomonas syringae strain ESC-10. Extracts from the plant Ageratum conzyoides are also effective against the molds. An 'essential oil' from the herb Thymus capitatus and neem oil have a similar effect. Tea saponin is regarded as a safe compound and has also been used to control postharvest decay of citrus fruit.
Always consider an integrated approach with preventive measures together with biological treatments if available. Washing harvested fruits at 40-50°C with detergents or weak alkali solutions, usually including some fungicides, reduces fruit decay. Recommended fungicidal compounds are imazalil, thaibendazole and biphenyl.