Avocado farming has grown in popularity in Kenya, and holds a lot of promise for farmers. However, to ensure a high quality fruit and maximum yield, it is important to understand the pests and diseases that afflict the crop.Most avocado diseases are associated with poorly drained soils, poor agronomic practices, or growing trees that carry pathogens
.Common pests include:
- Spider mites: They cause brown spots on leaves and fruits, which increase leaf falls. Proper pruning and irrigation can control them. Washing leaves using high-pressure hoses also helps reduce mite populations. Irrigate and fertilise heavily infested trees to maintain the flush of new growth after leaf shed.
- Insect borers: They tunnel into avocado trees and lay eggs causing branches to weaken and fall off. To prevent their spread, cut off infected branches.
- Scales: These are small, stationary brown-greenish insects often found sucking sap from the leaves. Although they do not cause internal damage, their presence on the skin could lead to rejection of fruits, especially if grown for export. Natural enemies like the parasitic wasp and ladybird keep the insects in check.
- Fruit flies: Fruit flies lay eggs under the skin of the fruits. This can happen at any stage of the fruit development. When the fruit grows, a lesion appears as a slight puncture mark surrounded by a white substance. As the fruit develops, the lesion becomes dry and turns into a distinct star-shaped crack on the skin surface. The fruits may fall pre-maturely. Fruit flies can be controlled by proper orchard sanitation, monitoring, and using baits.
- Lace bugs: These damage leaves and cause yellow spots on them. Weakened leaves fall out, exposing fruits and the wood to destructive ultraviolet rays. Horticultural oils are used for their management.
- Thrips: They cause leathery scars on fruits. Organic mulch discourages their survival.
- False codling moth: Their caterpillars damage the fruit, flowers, and leaves. Sprays made using Bacillus thuringiensis (Bt) are effective. When spraying, pay special attention to the folded leaves where the caterpillars tend to thrive.
Common diseases include:
A. Fungal diseases
- Avocado root rot: A serious and common disease caused by Phytophthora cinnamomi, which is a fungal pathogen that thrives in moist, poorly drained areas. It causes pale wilted leaves and very small fruits. When planting, hot water treatment of seeds and grafting can help prevent this.
- Scab: This disease is caused by Sphaceloma perseae, a fungus which attacks fruits, twigs and leaves. The symptoms are oval or irregular spots on the fruits. Poor drainage accelerates growth and spread of the fungus. Remove dead branches and twigs and clear fallen fruits from the field. Apply copper-based fungicides such as Bordeaux mixture (a solution of copper sulfate, lime, and water)
- Anthracnose: This is caused by pathogen Colletotrichum gloeosporioides. It affects the fruits, leaves, and flowers, leaving dark spots and defoliation. For prevention, prune the dead branches of the affected tree before the fungi produce spores. Also store the harvested fruits in a dry, clean and well ventilated place.
- Avocado black streak: This infection causes black streaks on the bark of the tree. It can be prevented through proper fertilisation and irrigation.
- Cercospora fruit spot: Pseudocercospora purpurea fungi affect fruits and leaves, leaving small light yellow spots that eventually turn brown. This disease occurs due to poor field sanitation.
B. Viral diseases
- Algal leaf stain: Caused by the virus Cephaleurors virescens, it affects the trees, causing red spots on the leaves, twigs and branches. Preventive measures include proper weeding and pruning.
- Sun blotch: This is a serious condition that causes stunted growth of trees, discolouration of fruits and leaves and reduced yields. Once contracted, it cannot be cured. Maintain proper field sanitation. The disease can be transmitted through grafting. Therefore, farmers must purchase certified seedlings.
C. Bacterial diseases
- Bacterial soft rot: Causes a dark, stinking rot on the fruit. Once contracted, the disease cannot be cured. Farmers must always buy certified seeds or seedlings and maintain a high level of sanitation.
- Wilts and blights – They are characterised by dead areas in trees, especially when only a part of the tree is affected. Unsurprisingly, these cause sudden wilting of leaves and death of branches. Pruning the symptomatic tissues from trees and providing good support can help your avocado recover.