Impact of Common Diseases on Lychee Trees and Their Control Methods
Do you grow lychee? If you do, then you must pay attention to lychee pests and diseases. Let's first take a look at the common diseases of lychee!
1. Downy blight
Caused by Peronophythora litchi, it can damage flowers, leaves, young stems and fruit, but fruit is the most commonly affected; during rainy periods, irregular brown spots appear on the rind of fruit near the ground or in dense branches and leaves, and dense grayish-white powdery fungal growth forms on them. The diseased spots expand very quickly in diameter, often reaching more than half of the fruit, and affected fruit drops severely. After the weather clears, the fungal growth on the spots of diseased fruit still hanging on the tree disappears, but the fruit has already lost its commercial value. This is one of the biggest causes of lychee fruit loss, especially in continuous rainy seasons. The spots produce zoospores of the pathogen with flagella, which can quickly spread to other fruit or other lychee plants through water droplets. According to reports, these zoospores need only 5 minutes to invade the fruit epidermis, and after 48 hours they can again produce cysts on the fruit epidermis and release zoospores, causing even greater spread and infection. However, once dry conditions occur, the pathogen on the fruit surface dies and cannot spread, so a major outbreak of this disease can only occur after many consecutive rainy days. Fruit harvested during rainy periods can also develop downy blight during packing, transport and sale, causing losses.
Control methods:Before the rainy period arrives, refer to the plant protection manual and rotate fungicides such as fosetyl-aluminum plus cymoxanil, zinc manganese mancozeb, or cymoxanil for control. To avoid phytotoxicity, do not apply pesticides under strong sun and high temperatures. Early use of a 1,000-fold solution of phosphorous acid mixed with potassium hydroxide can also increase the disease resistance of lychee.
2. Brown root rot
The pathogen is Phellinus noxius. This basidiomycete has more than 130 known host species in Taiwan, China, including many forest trees, street trees and fruit trees. This disease is currently the most serious disease of lychee. Diseased plants can show two types: chronic wilt and rapid wilt. In the former, tree vigor declines, leaves become sparse, yellow and thin, and the tree dies after 1 to 2 years. In the rapid wilt type, branches and leaves droop and discolor, and the whole plant dies in about 1 week. The base of the plant and some thick roots turn black. When the bark is peeled off, the xylem at the stem base can be seen browning. If dead for a longer time, the xylem is white with interspersed brown streaks, and yellowish-brown fungal growth may be present outside the bark.
Control methods:At present, no suitable control chemicals are recommended for use. Barrier trenches should be set between diseased plants and healthy plants to avoid spread through root contact; applying more alkaline organic fertilizer can reduce pathogen activity and lower infection.
3. Sour rot
The pathogens are Geotrichum candidum and G. luddwigii, which cause browning of the fruit rind. At cracked areas of the fruit, a yogurt-like milky white slimy fungal growth forms and gives off a sour, foul odor. During rainy periods, spread by splashing rainwater can cause most fruit to become diseased rapidly. After the rain stops, the milky white fungal growth can still remain for some time, which can distinguish it from downy blight. Small animals such as fruit flies, ants or snails can all serve as vectors.
Control methods:No recommended chemicals are yet available for use. Attention should be paid to water management during the fruit-setting period to reduce fruit cracking, and fruit flies should be controlled to reduce opportunities for contamination and spread of the pathogen.
4. Leaf gall disease
Caused by the lychee rust mite (Eriophyes lichii), commonly known as the hairy spider or hairy mite, it produces fuzzy raised outgrowths on the epidermis of young lychee leaves, inflorescences and young fruit. The raised areas accumulate and thicken and turn brown, and the affected parts become deformed and curled, especially on the lower leaf surface of young leaves, where damage is most commonly seen.
