Citrus trees of all types are susceptible to certain diseases, whether grown indoors or outdoors. Some types of citrus resist disease more efficiently than other types. The best way to keep disease at bay, which will save money in the long run, is through prevention. A healthy, robust tree can resist disease much better than a neglected tree.
Greasy spot looks like irregular, oily spots on the leaves and fruit of the citrus plant. It is collapsed tissue caused by the greasy spot fungus. This disease occurs in the summer and causes leaf drop over time as well as diminished plant vigor. The dropped leaves spread the infection to the plant, and possibly to other plants, for subsequent seasons. To stop its spread, gather infected leaves and fruits and throw them away. Do not compost them, or you may spread the disease.
The most obvious symptoms of melanose are small, sand-paper textured brown spots on the leaves and fruit of the plant. Melanose affects citrus trees most often after the spring rain, when water spreads the disease from dead twigs to the leaves and fruit. The disease does not cause leaf drop and really only causes superficial damage to the fruit. To control it, prune away dead and infected twigs and avoid watering from overhead.
Alternaria Fruit Rot
Alternaria fruit rot, also called black rot, is a fungal disease caused by Alternaria alternata. It usually arises in storage but can also occur on fruits of citrus plants still in the ground, where it causes premature fruit drop. Symptoms of this fungal infection are mostly invisible from the outside. In splitting the fruit open, however, you will find a black or brown rotted area where the fruit attaches to the tree. Sometimes the fungus remains there, while other times it spreads through the core of the fruit.
Citrus scab appears as light-colored warts or scabs on the fruit of the tree as well as on the leaves and stems. In young fruits, it can distort the appearance seriously, but does not usually affect the inner quality of the fruit. This is an aggressive disease that begins in early spring and continues throughout the year. Avoid watering from overhead for prevention. In order to control the disease, you must remove infected fruits to stop the spread.
Blue and Green Fruit Molds
Blue and green fruit molds, caused by strains of the Penicillium fungus, most often affects fruits post-harvest. The disease appears first as a soggy, moist spot on the fruit. Within a day it spreads and begins growing green or blue mold. In a very short amount of time the mold covers the whole fruit and easily spreads to others. You can prevent this by making sure the rind of the citrus is not injured during harvest.