Pests and diseases management

1. Avocado root rot (Phytophthora cinnamomi)

This is the most serious disease in nearly all avocado producing areas of the world. Every effort has been made to rectify the situation by uprooting or treatment of affected trees. Hot water and fungicide treatment of seeds for propagation purposes is highly recommended and grafting on Phytophthora-tolerant and/or resistant rootstocks have been included. In areas subject to flooding and in poorly drained soils, trees of any size and age are likely to be infected by this fungus.

Trees affected by the disease lose vitality, become sparsely foliated and do not produce a good crop. Leaves are pale-green and wilted, branches usually become sun burnt and die back in advanced stages. Feeder roots get darkened and decayed and trees will eventually die prematurely. The fungus can be spread or introduced to new areas by infested planting material, soil and irrigation water.

Management:

• Use diseased-free seeds.
• Treat seed for planting in a hot-water bath at 48 to 50⁰C for 20 minutes. After the hot-water treatment, rinse the seed immediately with clean, cold running water and spread out to dry thoroughly on a clean surface not in contact with the soil.
• Use clean nursery soil for container-grown plants. The soil should be well drained and carefully irrigated to prevent excessive moisture.
• Use tolerant/resistant rootstocks (e.g. Duke No. 6 and 7; G 6).
• Remove diseased trees from the field.
• Avoid movement of soil or water from diseased to non-infested areas.
• Use cultivation equipment first in healthy portion of the orchard before use in diseased areas.
• Washed and dry cultivation equipment after use in diseased areas.

For chemical control of this disease, Ridomil (active ingredient Metalaxyl) a soil-applied granular formulation that works by killing the pathogen, is recommended.

 2. Anthracnose (Colletotrichum gloeosporioides )

Anthracnose infection is important only on fruits, where it is the most serious disease affecting the avocado. Dry spots, dark brown in colour, form on the skin, leading to abnormal development. In severe attacks the young fruits soon drop. On fruits at a later point of their development, the fungal spores germinate and then enter a dormant stage. The infection remains hidden for a long period until the fruit ripens. It is mostly a post-harvest disease as the fruits are more susceptible when mature.

Management:

• Remove dead branches and twigs since they harbour the fungus.
• Remove fallen rotten fruits from the field.
• Apply copper-based fungicides pre-flowering, at fruit formation, and after harvest. Also spray applications with chemicals such as: Benomyl, Mancozeb, Metiram, Propineb, Thiabendazole or Triforine are very successful.

3. Cercospora fruit spot (Pseudocercospora purpurea)

This is another important fruit disease impairing the quality of the avocado. The severity of infection varies from season to season and can cause losses of up to 60%. Small, light-yellow spots later changing to brownish appear on fruits and leaves which eventually become hard and crack, thus creating entry points for other decay organisms. Treatments are the same as those for anthracnose.

4.Scab

Scab is caused by Sphaceloma perseae is usually common in humid areas. The fungus readily infects young, succulent tissues of leaves, twigs and fruits. Lesions appear as small dark spots slightly raised, oval to elongated. These coalesce, giving a corky appearance to the surface of the fruits – impairing the appearance but not the internal quality of the fruit. Recommended treatments are again the same as for anthracnose.

5. Scale insects

Two main groups of scale insects include the “soft scales (Coccus sp.)” and the “armoured scales”. Scales are small, stationary brown greenish insects occasionally found sucking sap from avocado leaves.

Soft scales such as Coccus spp. excrete large amount of honeydew, which lead to the development of sooty mould on leaves, branches and fruit. Honeydew attracts ants, which while feeding on the honeydew protect scales from natural enemies. Armoured scales such as the coconut scale (Aspidiotus destructor) may encrust  young twigs, leaves and fruit. They do not produce honeydew.

Scales are usually not a problem in avocado orchards; however, damage can be serious on young tress and small twigs may be killed.  The presence of scales on the skin of fruit does not cause internal damage, it may lead to rejection of fruit, especially if grown for export.

Management:

• Conserve natural enemies. Scales are usually kept under control by parasitic wasps, ladybird beetles and lacewings, provided no broad spectrum pesticides are used and no ants are present.
• Chemical control recommendations are the application of systemic or contact insecticides like Imidaclopride.

6. Fruit flies (Bactrocera invadens, Ceratitis spp.)

Avocados, especially thin-skinned varieties might be attacked by various species of fruit flies.  Some fruit flies lay eggs under the skin of the fruit that is just beginning to ripen, but others attack young and old fruit. 

When the fruit reaches about the size of a golf ball a sting lesion appears as a slight puncture mark surrounded by a white exudate.  As the fruit develop the lesion becomes dry and turn into distinct star-shaped crack in the skin surface.

Management:

The few natural enemies of the fruitfly cannot control the problem effectively. There are 2 methods of control, namely:

• Eradication of unwanted host plants: eg. wild-growing guavas, etc. These plants should be eradicated in an area of up to 200 m around the orchards.
• Baiting: Using poisoned bait to lure the flies should be considered if damage exceeds 5 %.