Fungal infections are a danger to most forms of trees and plants, although several varieties of fungi pose a special threat to citrus trees. Fungi everywhere thrive in humid conditions and in damp, moist, cold soil. Their spores are often borne by irrigation water but they can be blown by the wind, spread by water splash, or carried on infected pruning tools and packing material.
Foot rot, also called root rot, ordinarily starts in the roots and spreads upward through the vascular system of the tree, infecting roots, leaves, blossoms and fruit.
Varieties of the fungus Phytophthora cause feeder roots and lateral roots to rot. A gum often exudes from lesions on soaked bark. These lesions, which can become inactive during dry weather, will eventually encircle the tree and kill it. This fungus is sometimes called brown rot because it causes brown spots on the fruit.
A soil-borne fungus, Fusarium solani, causes dry decay to develop in fibrous roots and the trunk just at or below the surface of the soil. The wood is stained a dark color. The leaves wilt and turn yellow. The tree can die with the leaves still on.
Infections of Limbs & Bark
Several varieties of fungus, ordinarily wind-borne, cause cankers on bark and limbs to wilt.
The fungus Hendersonula toruloidea causes a sooth black growth to develop under the bark, killing the sapwood. Leaves on infected branches turn brown and die.
The fungi Anhtrodia sinuosa and Coniophora eremophia infect broken or injured limbs of lemon trees. These fungi cause wood to rot and limbs to fall off so all that remains is a stump.
Several fungi, collectively called gummosis, can cause gum to form on the trunk and branches of citrus trees. Blisters on the trunk will ooze a pink-orange gum.
Infections of Leaves
Some forms of fungus attack the leaves. If not removed or treated they can spread, causing wart-like lesions on the fruit.
Alternaria brown spot, produces brown to black lesions surrounded by a yellow halo on twigs, leaves and young fruit, especially tangelos and tangerines.
Elsinoe fawcetti, or citrus scab, is a special problem with lemons and grapefruits. Small orange spots appear on the leaves, turning into wart-like lesions. Severely infected leaves become distorted and stunted.
The fungus Mycoisphaerella citri produces greasy spot, causing premature loss of foliage in the fall and winter. A yellow mottle appears on the bottom of the leaves. Blisters form, producing yellow and brown spots. Ugly spots form on the fruit rinds.
Infections of Fruits
Airborne molds attack citrus fruit after they are harvested. A watery, discolored spot from ¼ to ½ inch in diameter appears, growing one to two inches wide in one or two days. Penicillium italicum, produces blue mold; Penicillium digitatum causes green mold. The spore enters damaged spots on the rinds.
Alternaria stem-end rot, caused by Alternaria citgri, a fungus found in litter and borne by the wind or water splash, causes rot on the stem ends of oranges.
Conditions for Infection
Fungal infections can be caused by heavy rain, hurricanes, poor drainage, high water table, salt in the water, and freezing weather. In humid, wet areas, fungi in the soil can infect the fruit and upper trunk. In dry areas, they usually only infect the roots and lower bark.
Precautions and Treatment
Buy rootstocks that are resistant to fungi. Do not plant citrus in low areas that can't drain. To avoid leaf spot, make sure trees are spaced wide enough to they can receive air. Do not over-water your citrus trees or use too much fertilizer containing nitrogen. Remove infected branches. Use clean pruning tools and packing material.
Some fungal infections respond to fungicides; others do not. Your local agricultural extension service can help you select one that works.