Like shrubs, kumquat trees are slow-growing trees. They have thick branches, waxy leaves and produce a well-known citrus fruit. The trees are often planted to add color and texture to landscapes and are also grown indoors in containers. They produce aromatic white flowers in late spring and fruit in the fall. Unlike most citrus fruits, the peels are sweeter than the pulp.
Lack of Nitrogen
Kumquats that do not receive enough nitrogen are known to experience a variety of symptoms. Their oldest leaves, affected first, will being to fade from green to yellow and eventually fall from the tree. As trees lose their leaves, they are unable to produce enough food supply through photosynthesis and will see a large reduction in production of fruit.
Lack of Magnesium
With a lack of Magnesium, the leaf stems are first affected. Yellow spots begin to appear around the base of the stem and spread throughout the leave. As the leaves lose their color, they become more likely to fall off due to outside influences such as rain or wind. Eventually the tree's branches will also become affected, as well as fruit production throughout the tree.
Fixing Nutrient Deficiency
To bring additional nitrogen into the kumquat tree, give the tree a slow-release fertilizer that has an NPK amount of 5-1-3 every six to eight weeks once a deficiency has been detected. Spread the fertilizer evenly at the base of the tree and water thoroughly. Be sure to always follow the directions on the product's label. Discontinue fertilization before the year's first frost begins, so that the tree will enter dormancy naturally.
To bring additional magnesium use Epsom salt. Spread the salt around the base of the tree using 2 tbsp. to cover the area. This should be done a few inches from the tree. Do not allow the salt to touch the tree. Water thoroughly. Give your citrus trees Epsom salts every two to three months, stopping before the first frost of the year.
Alternaria brown spot is known to affect kumquat trees and can cause leaf drops. Affected trees need to be treated with copper fungicide when new leaves are ¼ to ½ expanded. Another application should be done once the leaves are fully developed. Another fungal disease that can cause leaf drop is greasy spot. Copper fungicide is also used to treat this in the same manner. Lastly, the soil-borne disease Phytophthora, or root rot, can cause leaves to drop from Kumquat trees. Avoid wrapping or mulching trees to prevent this problem.
Pests like citrus psyllid and citrus red mites can infest trees, causing leaf distortion and dropping. An option for gardeners is to use a variety of oil sprays, such as pyrethrum spray. The infestations can appear as waxy bumps on the surface of the tree that suck away its sap, draining nutrients and water directly from the tree.
Kumquat trees are sensitive to fluctuating temperatures and will often respond with leaf drop. Trees can recover from this affliction by placing them in an area with a steady heat source.