Blue and green mold fungi are mainly problems during fruit storage but can also occur on injured fruit on the tree.

Identification

Disease from infection by blue or green molds initially appears as a circular, soft, water-soaked lesion on rinds called clear rot. This early symptom is easily overlooked unless fruit are carefully inspected. Disease becomes obvious once whitish mycelia and the blue or green mold spores develop on the fruit surface, usually after harvest.

Life Cycle

Blue mold (Penicillium italicum) and green mold (P. digitatum) usually occur together. These fungi are common in soil and on decaying plant materials. The fungi can spread via airborne spores that decaying plant material can produce in abundance.

When blue and green mold affect fruit on the tree, it is because their rinds split or fruit were otherwise injured by another cause. Split fruit develops after tree stress, such as that caused by inappropriate irrigation, potassium deficiency, or unseasonably high temperatures or other extreme weather.

Damage

Blue and green mold cause an odorous, soft, watery decay that makes citrus fruit inedible. After harvest when fruit is stored, blue and green mold can infect healthy fruit if it is stored with injured fruit where the pathogens first developed then spread.

Solutions

Provide trees with a good growing environment and proper cultural care to keep them healthy and reduce plant stress. Provide proper nutrition, including adequate potassium. Appropriate irrigation is especially important because too little or too much soil moisture can stress trees. The amount of water applied and how frequent of irrigation must be adjusted according to the weather and trees' changing need for moisture. For example, when hot and windy weather is expected trees will need more water.

References

Adapted from Integrated Pest Management for Citrus, University of California Statewide Integrated Pest Management Program (UC IPM).