Diseases affecting Oranges

THE GREENING DISEASE
Citrus fruits affected by the Greening disease
It is caused by a bacterium (Candidatus Liberibacter africanus). The disease is transmitted by citrus psyllids (Trioza erytreae) and through use of budwood obtained from diseased trees. The disease is not soil-borne. It is not seed-borne and it is not mechanically transmitted.
Symptoms on the leaves show mottling, yellowing of veins or zinc defiency (i.e. small leaves, interveinal chlorosis and brush-like growth). Zinc deficiency induced by greening is confined to one or several branches within a tree (sectoral infection).
Trees infected by greening are distributed within the orchard randomly. Affected branches bear few fruits and in some cases do not fruit.
The affected fruits are usually under-developed, reduced in size, lopsided, start to colour from the stem end instead of the stylar end as in the case with healthy fruits. Affected fruits drop prematurely. In seedy citrus varieties, seed abortion occurs. Severely diseased trees exhibit open growth, sparse chlorotic foliage, dieback of branches and severe fruit drop.
In Kenya, the disease is not found below 800 m above sea level because both the bacterium causing the disease and citrus psyllids are sensitive to high temperatures. Optimum temperature for symptom expression is 21 to 24°C; symptoms are masked above 26° C. The disease is especially destructive to sweet oranges and mandarins. It is less severe on lemon, grapefruit, citron and West Indian lime.
Rootstocks have no effect on greening disease.
CONTROL OF GREENING DISEASE
Use disease-free budwood
Strict control of citrus psyllids.
Very severely infected trees not producing economical yield should be up-rooted. If only a few branches are affected, they can be pruned out.
Diseased young citrus trees should be replaced, as they will never bear fruit
ANTHRACNOSE

Affected plants with anthracnose

There are three anthracnose diseases of citrus caused by Colletotrichum spp.

Post-bloom fruit drop
This affects flowers of all citrus species and induces drop of fruitlets and is caused by C. acutatum. Lime anthracnose, which attacks all juvenile tissues of only Mexican lime, is also caused by this Colletotrichumspecies. C. gloeosporioides causes a rind blemish on fruit, especially grapefruit, in the field.

C. acutatum infects petals and produces water-soaked lesions that eventually turn pink and then orange brown as the fungus sporulates. Infected fruitlets abscise at the base of the ovary, and the floral disk, calyx, and peduncle remain attached to the tree, forming structures commonly referred to as ‘buttons’. Leaves surrounding an affected inflorescence are usually small, chlorotic, and twisted and have enlarged veins. Warm, wet weather favours disease development

Lime anthracnose 
It affects only Mexican lime. It attacks flowers, young leaves, young shoots and fruits. Infected fruitlets abscise, and ‘buttons’ are produced as in postbloom fruit drop. In severe cases, young leaves become totally blighted and drop, and shoot tips die-back, producing wither tip symptoms. The fruit lesions are often large and deep, and cause fruit distortion. The disease is favoured by warm, wet weather.

Rind blemish on fruit 
The disease is caused by C. gloeosporioides. It is particularly severe on grapefruits. The blemish appears as a superficial, reddish brown discolouration, often in the form of tear stains, which usually appears following prolonged light rains in warm weather.

CONTROL OF ANTHRACNOSE

  • Avoid overhead irrigation (opt for under-tree sprinklers where feasible)
  • Wide tree spacing to reduce relative humidity within tree canopy
  • Pruning of dead tree tissues
  • Copper preventive sprays
 PHAEORAMULARIA FRUIT & LEAF SPOT
Citrus affected by phaeoramularia and leaf spot
The disease is caused by fungus Phaeoramularia angolensis. The disease is favored by wet, cool conditions. On leaves, the fungus produces circular, mostly solitary (single) spots that are up to 10 mm in diameterwith light brown or greyish centers. Each spot is usually surrounded by a yellow halo. Occasionally, the thin necrotic tissue in the centres of old spots falls out, creating a shot-hole effect.
The disease is caused by fungus Phaeoramularia angolensis. The disease is favored by wet, cool conditions. On leaves, the fungus produces circular, mostly solitary (single) spots that are up to 10 mm in diameterwith light brown or greyish centers. Each spot is usually surrounded by a yellow halo. Occasionally, the thin necrotic tissue in the centres of old spots falls out, creating a shot-hole effect.
During rains, leaf spots on young leaves often join together ending in generalized Chloris. Premature defoliation takes place when leaf petioles are infected. On fruit, the spots are circular to irregular in shape or joining together and surrounded by yellow halos. Most spots measure up to 8 mm in diameter. On young fruits, symptoms often start with nipple-like swellings without yellow halos.
Spots on mature fruits are normally flat, and often a dark brown to black sunken margin similar to anthracnose around the spots is observed. Fruits of more than 40 mm in diameter are somehow resistant to the disease. The disease has been observed on all citrus species including grapefruit, lemon, lime, mandarin, pummelo and orange. Grapefruit, mandarin, pummelo and orange are very susceptible. Lemon is less susceptible and lime is least susceptible. The disease can reduce yield by 50 to 100%
CONTROL.
The disease can be effectively be controlled by a number of fungicides including copper based products.
Successive use of coppers may cause stippling (dot-like marks) on the fruits.
The diseases listed above are not comprehensive, also there are a number of pests that affect the citrus fruit eg the citrus bud mite, mealy bugs, nematodes, citrus black flies extra extra,
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