The Kaffir lime tree (Citrus hystrix) is a citrus tree that yields fruits and leaves used in Asian cuisine. It has a rough, warty green fruit and aromatic, distinctively shaped "double" leaves. At 2 inches wide, the Kaffir lime tree's fruit has a distinctively bumpy exterior. It does not usually have many pest problems, but may become susceptible to mites or scale and also suffer diseases affecting many citrus trees.
Greasy Spot Fungus
Caused by fungus Mycosphaerella citri, greasy spot is a common disease affecting citrus trees, including the Kaffir lime tree. Mycosphaerella citri is a leaf pathogen that originates from the orchard floor and released following rainfall or irrigation; it can also infect fruit.
Characterized by yellow spot appearing on the upper surface of the leaf, with the underside turning dark and slightly raised, greasy spot also gives a characteristic greasy appearance and texture, hence its name. Rust mite injury and humid weather can increase the susceptibility of the tree to greasy spot fungus.
Applying citrus oils and fungicides during the summer can help control the early onset of this fungal infection. Increasing the frequency of fungicide treatments to two to three times is ideal for heavily infected trees. Spraying thoroughly is essential to reach the underside of leaves. Removal of tree litters from the ground that come from infected trees will prevent transfer of fungus to healthy trees.
Phytophthora foot rot, also known as Phytophthora gummosis, is a disease affecting citrus trees caused by two soil-borne organisms—Phytohthora parasitica and Phythophthora citrophthora. In wet soils, both of these organisms produce motile spores (zoospores), which invade host roots where they germinate and infect root tissue.
The primary symptoms of foot rot are rotted roots and cracked bark, accompanied by gumming. The bark at the soil line appears water-soaked, reddish-brown to black. Discolorations of the tissue in the lower trunk along with yellowing and sparse foliage are additional telltale signs of foot rot. A tree can die from heavy foot rot infestation.
Using resistant rootstock, improving drainage and timely applications of effective fungicides can help control Phytophthora foot rot. Increasing drainage and careful irrigation will help prevent the production and spread of the zoospores.
Citrus canker, caused by Xanthomonas axonopodis pv. citri, is a highly contagious bacterial infection of citrus trees causing yellow halo-like lesions or scabs on the fruit, leaves and twigs. This disease can cause premature leaf and fruit drop. White spongy spots appear on newly infected leaves that will later turn gray or tan. The spots erupt into a crater-like formation that has oily margins or yellowish rings. The cankers are visible on both sides of the leaf. Old cankers sometimes fall out and leave behind holes.
Canker bacteria spread by strong winds, citrus leaf-hoppers, birds, humans and equipment that carry the bacteria. They also transfer from infected plant parts that touch the healthy plants (leaves or branches) when planted close to each other.
A variety of sprays designed to protect against infection, such as liquid copper fungicide, can provide preventative treatment for healthy citrus trees, especially when other trees already show signs of citrus canker. Destroying infected trees is the best and quickest way to slow down the spread of bacteria.