This virus-like disease, known locally as likubin in Taiwan, leaf mottle yellows in the Philippines, citrus dieback in India, and citrus vein phloem degeneration (CVPD) in Indonesia, has been devastating all citrus-growing areas in Asia except Japan, and is also widespread in South Africa.
The South African greening organism belongs to a heat sensitive form, which induces severe symptoms at cool temperatures (22-24°C). The Asian greening organism produces symptoms in either warm temperatures (27-32°C) or a cool climate, and is classified as a heat-tolerant form.
The Asian form of greening has been spread rapidly by Asian psylla, and is causing great damage to the citrus industry by shortening the lifespan of trees.
SYMPTOMS
Although the disease syndrome differs to some extent according to citrus variety, common symptoms are yellowing of the veins and adjacent tissues, followed by yellowing or mottling of the entire leaf, occasionally with corking of the veins .
This is following by premature defoliation, dieback of twigs, decay of feeder rootlets and lateral roots, decline in vigor, and ultimately, the death of the entire plant. Diseased leaves become hardened and curl outward, while young leaves developed after premature defoliation are small and slender, with symptoms of zinc deficiency ( Fig. 2 ).
Trees affected with greening become stunted, bear multiple off-season flowers, most of which fall off, and produce small irregularly shaped fruit with a thick, pale peel ( Fig. 4 ).
PATHOGEN
The fastidious bacteria (FB) which causes greening exists in sieve tubes ( Fig. 3 ). Electron microscopy, using serial sections and a three-dimensional configuration can confirm the presence of mature forms of the pathogen, generally rigid rods measuring 350 _ 550 x 600 _ 1500 mm, surrounded by a two-layered envelope, 20 _ 25 nm thick. However, FB bodies are pleomorphic, and produce flexible elongated rods which grow into new organisms (100-250 x 500-2,500 nm), while when they are old they form spherical bodies 700 _ 800 nm in diameter with a thin cytoplasm. Multiplication is generally accomplished by budding, less frequently by binary fission or beading.
INDEXING
Identification of citrus greening is mainly based on the diagnostic symptoms in citrus trees described above, and electron microscopy of the pathogen bodies in phloem sections. Additional indexing can be done by inoculating grafts of more susceptible citrus varieties particularly sensitive to the pathogen, such as Ponkan mandarin seedlings or Tankan tangor seedlings, with the suspect buds, to test for the presence of the pathogen according to symptom expression.
