1.1. Alternaria Brown Spot (Alternaria alternata or A. citri)
Conidia are released by rain events or sudden changes in relative humidity. Optimum temperatures for diseases development are 23-27°C. Plants can get infection between 17-32°C. Infection can occur with as little as 4-6 h of leaf wetness but disease severity increases with leaf wetness.
1.2. Melanose (Diaporthe citri)
The disease causes lesions on fruit and leaves. All citrus are susceptible but grapefruit and lemons are the most susceptible. Optimum temperature for the disease development was determined to be 24-28 °C.
1.3. Pseudocercospora Fruit and Leaf Spots
(Pseudocercospora angolensis; Syn.: Phaeoramularia angolensis)
This is a quarantine disease affecting all Citrus but the most susceptible are grapefruit, oranges and mandarin; while the less susceptible is lemon. Yield loss due to this disease ranges between 50-100%.
Young leaves are highly susceptible to infection from lesions older tissues. Young fruit up to golf ball size are also highly susceptible.
The disease spread is so far restricted to humid tropics of Africa between 800-1500 m and is favoured by prolonged wet weather followed by dry periods with temperatures between 22-26°C. Leaves are the main source of inoculum. The disease is long distance spread by windborne conidia. Infected planting material may also contribute to long distance spread. Within the orchard, spread is by splash dispersed conidia.
Fruit lesions are circular to irregularly shaped. Young fruit has nipple‐like lesions with yellow halo. And can become mummified. Mature fruit lesions are dark brown to black and generally flat or sunken with a yellow halo.
Leaf symptoms are circular to irregularly shaped lesions that can coalesce. Leaf lesions with brown or grayish center surrounded by a yellow halo. Young flush can be killed and leaf drop can occur.
This is a quarantine disease affecting all Citrus but the most susceptible are grapefruit, oranges and mandarin; while the less susceptible is lemon. Uield loss due to this disease ranges between 50-100% .
1.4. Citrus greening
Citrus greening disease is a major cause of crop and tree loss in many parts of Asia and Africa. Before it was identified as one disease, it became known by various names: yellow shoot (huanglungbin) in China; likubin (decline)in Taiwan; dieback in India; leaf mottle in the Philippines; vein phloem degeneration in Indonesia; and Yellow branch, blotchy-mottle, or greening in South Africa. As it became clear that all these were similar diseases the name "greening" was widely adopted.
Nature of the causal agent:
The demonstrations that greening is a graft- and insect-transmissible disease led to the conclusion that a virus was responsible. In China some researchers believed tristeza virus to be the cause. In South Africa it was shown that tristeza and greening could readily be distinguished since the aphid Toxoptera citricidus transmitted tristeza but not greening, and psylla vice versa. Other researchers suggested that it should be classified as a true bacterium.
Citrus greening is caused by phloem-limited bacteria. The African form of the disease is heat sensitive, and the Asian one is not. The bacteria are transmitted by insect vectors and by grafting.
Losses due to greening are not easy to assess. Sometimes only sectors of a tree are affected and losses are small, but in other cases the entire tree is infected and crop loss is total.
To date, there is nowhere in the world where citrus greening disease occurs that it is under completely successful management. In every place where the disease occurs, life expectancy of citrus trees is vastly reduced and production losses are significant.
1.5. Aphids: Black citrus aphid (Toxoptera aurantii), Cotton or melon aphid (Aphis gossypii), Spirea aphid: Aphis spiraecola
Aphids feed on buds and on the underside of leaves , causing leaves to curl toward the stem. Spirea aphid, black aphid and cotton aphid can all transmit citrus tristeza virus. Insecticide applications for aphids are not recommended because it is difficult to prevent transmission of citrus tristeza virus by controlling aphids with insecticides unless an area wide treatment program is conducted.
1.6. Citrus mealybugs (Planococcus citri)
The citrus mealybug is a sporadic, and extremely damaging and difficult to control pest of citrus. They prefer humid conditions. In citrus, mealybugs spread by crawling from tree to tree, wind, on bird’s feet, machinery, and labor crews.
On hatching, the nymphs are light yellow, but they soon excrete a waxy covering. There are 2-3 generations per year.