Citrus Tree Fungus
Fungus is a common threat for citrus trees and most of the trees will be infected silently. This means that the trees can be infected for a long time before they get discovered. The fruits produced will be decreased to a great extent and the health of the tree will be severely weakened. Also, the tree infected by insects may also be susceptible to the fungi infection. The best method for your tree to be resistant to fungi is to take good care of them instead of the removal of fungi when they are infected.
The best way to make citrus trees to resist fungi is to provide excellent maintenance for them. You should leave adequate space between two adjacent individual trees, ideally more than 20 feet apart. Large space between two trees will lower the risk of fungi spreading from tree to tree. Also the improved aeration ability will lower the moisture and thus prevent the existence of fungi. Most fungi infections will occur on leaves. So, it is a good idea to avoid spraying on leaves. Excessive water on the surface of leaves will increase the moisture, and thus increase the risk of fungi infection.
Sooty mold is a common mold which typically appears on the surface of leaves. Soapy water can be used to wash off the sooty mold. The honeydew substances produced by insects can cause the fungi infection and you should get rid of them as soon as possible. Some pesticide is also available to do the same job. If unattended, the roots of trees will rot due to the infection. The rot roots will cause the leaves to turn yellow. You should stop over-watering your tree immediately and remove the rot areas as quick as possible to prevent the spread.
Image provided By Tomwsulcer (Own work) [CC0], via Wikimedia Commons