Avocado (Persea Americana) can be grown outdoors as a large fruit tree in U.S. Department of Agriculture plant hardiness zones 9 through 11, or indoors as a houseplant, where it may or may not bear fruit. Avocados need the right amount of watering and are especially sensitive to overwatering. Young avocado trees may develop soft stems when overwatered and chronically overwatered avocados often develop fungal diseases. Because symptoms of overwatered and under-watered avocado plants are similar and sometimes the same, it is necessary to check the soil before watering.
Moist or Muddy
Moisture can be present in the soil when the surface looks dry. For an indoor avocado, insert your finger into the soil up to the second knuckle. Lightly moist soil means the avocado tree does not need watering. Wet, muddy or soggy soil means it has been overwatered. For an outdoor avocado, dig 6 inches down and gather a handful of soil. If it sticks together in a ball, the tree has enough water; if it is too wet to stick together, the tree has been overwatered.
Thirsty in a Sea of Water
Too much water in the soil forces oxygen out of the soil. Avocados need oxygen in the soil to properly draw water in through their roots. The tree can actually die from lack of water planted in soaking-wet soil. Roots that cannot function properly can break down and die. When this happens, the avocado tree can no longer absorb enough water or nutrients from the soil.
Wet Soil Leads to Rot and Fungus
Overly wet soil creates comfy conditions for fungal infections that cause wilted, small and sickly pale green leaves on avocado trees and can also kill small branches. New leaves stop growing and foliage becomes sparse. If they are detected early, you can fumigate small fungal spots. If avocado root rot is not detected early, the tree must be cut off at ground level and outdoor soil must be fumigated. If grown indoors, place the potted plant in a bag and dispose of it.
Armillaria Root Rot
Another fungus that attacks avocados is armilaria or oak root rot. In addition to small yellowing leaves, the tree will stop growing and drop leaves prematurely. During the winter months, clusters of fan-shaped mushrooms will form at the base of the avocado tree a few days after rain. Indoor plants will have a mushroom-like odor to the soil. There may not be visible symptoms of armilaria root rot until the fungus is well established in the avocado roots. Once armilaria root rot is established, it is very difficult to save an avocado tree. It is much easier to prevent fungus infection by watering properly.