Asian Citrus Psyllid (diaphorina citri)
Female syllids lay bright orange eggs in the whorls of newest growth. The eggs hatch and yellow immature psyllid feed on new growth and suck cell sap. Immature psyllids go through five nymphal stars and excrete white wax from their hind ends.
Adult psyllids have piercing and sucking mouth parts and feed on citrus leaves at a 45 degree angle position. Under favorable conditions, Asian citrus psyllid may complete up to 30 generations.
Citrus greening, also known as yellow shoot disease or huanglongbing, is one of the most devastating diseases of citrus worldwide. It is a bacterial disease caused by Candidatus Liberibacter asiaticus (other strains of citrus greening bacteria also cause disease), that inhabits the phloem (or food conducting tissues) of the tree.
It affects all citrus varieties and certain other members of the citrus family. The bacterium is transmitted from infected to healthy trees by an insect, Asian citrus psyllid.
Citrus greening can also be transmitted by grafting diseased budwood. Once the tree is infected, it stays infected for the rest of its life and there is no cure.
Citrus trees affected y citrus greening may not show symptoms for years. Symptoms include blotchy molting of the leaves, thickening of the veins, yellowing of the shoots, twig dieback, stunted growth and lopsided fruits. Infected fruits do not ripe uniformly and some green color remains on the ripe fruits. Greening-affected fruit taste bitter, medicinal and sour. Seeds usually abort, and fruit set is poor.
There is no chemical control available for citrus greening. once the tree is infected, it stays infected for the rest of its life and there is no cure. The Asian citrus psyllid (vector of citrus greening) can be managed with insecticides.
Sweet Orange Scab
Sweet orange scab is caused by Elsino(E) astralis and occurs on sweet oranges, limes, lemons, and mandarins. In contrast to citrus scab, sweet orange scab produces circular, smoother and flatter pustles. These pustules give the fruit a corky or scurfy appearance.
Sweet orange scap requires moist conditions to reproduce and under optimal temperatures, will develop quite rapidly. The pathogens survive in older pustules on leaves and fruits and spread primarily with water splash and sometimes also with wind driven rain. Removal of infected leaves and fruits from the trees and/or the ground help reduce fungal spores. Minimize disease infection and spread by avoiding overhead irrigation and watering early in the morning to reduce extended periods of leaf wetness.
Among citrus varieties and rootstocks, citrus canker is most severe on grapefruit, Key/Mexican lime, lemons and Kaffire lime (C. hystrix). Citrus canker is a leaf-spotting and rind-blemishing disease, but it can also cause defoliation, shoot dieback and premature fruit drop when environmental conditions are highly favorable. On leaves, under optimal weather conditions, the lesions are visible about 7-10 days after the infection, principally on the underside of the leaf where most stomates are located. As an average, the lesions are 2-10 mm in diameter, but they can coalesce to form expanded diseased areas.
The causal bacterium of citrus canker survives season to season on leaves, stems, and fruits and even for several years on woody branches. Also the bacteria can survive for years in infected material that is dry and free of soil. A new significant advantage for disease spreading is wounding caused by Asian citrus leaf miner.