Citrus of all types are heat lovers. The trees are considered sub-tropical and need to live in well-drained soil in a sunny location. Citrus are very easy to care for in the ground or in a pot. Citrus are prone to several fungal diseases and root rots that will produce growths on the trunk. The appearance of a green mold on the trunk is not related to any common citrus diseases. If the substance isn't moss there are a couple of other possibilities.
Foot rot is common in citrus and leaves gummy lesions that will leave bark looking slimy and waterlogged. Sooty canker (mold) is characterized by a black fungal growth below bark tissue. Dry root rot is a fungus that causes root tissue to decay and darken in color with loss of foliage. Rio Grande Gummosis causes cracks to form in the trunks and exude a yellowish gum which will form blisters on the tree. None of these diseases causes a green mold formation on the trunk.
Green Mold Post Harvest
It is not uncommon to find green mold on a citrus tree after harvest. The cause is the mold spore penicillin digitatum, which primarily affects fruit. The spores are dislodged when fruits are harvested and fly on a breeze to cling to the trunk or any other part of the tree. They can even remain lodged in the soil until conditions are right for them to ripen. They produce the classic mold look as seen on old bread. They will not kill the tree and usually the colder temperatures of winter will remove them.
Lichen are really two organisms. They are comprised of fungal filaments and green algae. The two organisms have a symbiotic relationship where they cannot live without each other. Lichens will not damage a tree. In fact they are indicators that the air is unpolluted as they do not thrive in polluted areas. There are crusty, leafy and shrubby lichen that look like some kind of mold and the green algae will lend its color to the lichen.
Salt lime develops on salty soils or soils that have been heavily fertilized. It is a dark green slime on the soil surface. It could spread to the lower part of the trunk. It can be growing in soil that has an orange cast to it, which is indicative of a high salt content. Citrus don't like salty soil, so there should be no more fertilizing and the salt lime should be carefully scraped away from the trunk and roots.
Spraying for molds has little effect. They are usually part of the tree organism, in the soil or airborne and they require certain conditions to bloom. Good care and regular maintenance is crucial to growing a healthy tree that can withstand the effects of mold. The correct soils and location will also help prevent molds and disease.