Avocados are native to Central and South America and are grown commercially in the United States, mostly in California. They prefer tropical or semi-tropical climates and don't tolerate cold winters. Avocados are affected by several diseases, most prominently in humid, wet climates. Good cultural practices, such as planting avocados in well-drained soil and providing adequate space between trees for good air circulation will minimize the chance of disease.
Tip Burn and Marginal Necrosis
Tip burn and marginal necrosis are signs of water and salinity stress. Alkaline soils in Texas and Arizona have high levels of sodium, which can lead to salt burn in trees. Signs include dry, burnt leaf tips and black spots. Water avocado trees uniformly and deeply to help eliminate this problem.
Root rot is a problem in wet humid areas and is caused by the soil fungus "Phytophthora cinnamonomi." Young roots turn black and brittle and dark brown cankers form on the trunks, close to ground level. Cankers may exude white sap. Cut out cankers and spray the tree with a fungicide.
Anthracnose is a common disease of avocado trees. Brown spots appear on the leaves, followed by small brown and black spots on the fruit. The fruit falls to the ground and is inedible. Treat anthracnose by spraying with a fungicide after the tree has flowered. Harvest avocados only when they are mature so they ripen quickly.
Verticillium wilt is carried by plants such as tomatoes and potatoes. Avoid growing avocados in soil previously planted with these crops. Affected avocado trees have one or more branches with wilting leaves. Cut out the affected branches. Most trees recover on their own.
Soft rot is caused by a fungus and treated as you would treat anthracnose. Affected fruit has a dark, metallic skin. The fruit inside is brown and soft and has a rancid odor.
Sooty blotch is a fungus spread by airborne spores. Black fungus grows on the fruit, leaves and branches of the trees. The fruit is still edible, but aesthetically unappealing. Treat sooty blotch as you would anthracnose.