Yellow leaves on an avocado tree signal a potentially serious, even fatal, problem. The Hass avocado tree is susceptible to the same cultivation and disease problems that may affect other avocado species. Native to California, the Hass avocado tree is named for Rudolph Hass, who successfully grew a found seed in his yard to produce the fruit-bearing tree. Yellow leaves aren't just unsightly, they're a problem. Cultivation deficiencies, virus and disease may be to blame for yellow leaves.
Water stress makes the leaves of avocado trees turn yellow and wilted at the tips. Most common in hot and dry weather, water stress occurs when the Hass avocado tree isn't receiving enough moisture. If weather is exceedingly hot, it doesn't take an active drought to put water stress on your plant. Establish a routine watering schedule in times when normal rainfall isn't great enough to keep the tree healthy. The tree may lose its leaves completely when water stress become severe, but new leaves will grow when it begins receiving enough moisture again.
Sun-blotch sounds like a cultivation problem, but it's actually a viral infection that appears as yellow or white lines and spots on fruits, twigs and leaves of the Hass avocado. Sun-blotch is a rare problem, a good thing because infected trees must be destroyed as soon as possible. No control methods work to eliminate sun-blotch virus, which will be transmitted to other trees through seeds, buds and root-grafting.
Powdery mildew is named for its appearance. The fungus looks like small specks of white powder. As it progresses, the disease appears as dark yellow and brownish spots. Powdery mildew appears in moist weather and does not cause fatal damage to avocado trees. Usually, fungicide is not necessary to control the fungus. Algal leaf spot, another fungus, appears on leaves as yellowish or reddish spots in summer and fall. Copper fungicides are effective at treating and managing the algal leaf spot fungus.
Good cultivation is the best line of defense against the problems and pathogens that cause yellowing leaves in Hass avocado trees. Do not grow avocado trees on wet or poor-draining soil. Excess moisture in soil encourages fungal growth. Standing water should not be present around the tree. If Hass avocado trees are kept healthy and their water needs are met (not exceeded), they will stay healthy and produce fruit for many years.