Greening! Serious Threat to Citrus & Export
Muhammad Shahzad Zafar
Horticultural Research Station, Sahiwal
Greening also known as yellow shoot disease or Huanglongbing greatly reduced the production of citrus in Pakistan and rest of the world also. The disease was first described in 1929 and first reported in China in 1943, where it is still widespread. In Pakistan, the Area under Citrus is 199400 hectare while the Production is 2294500 metric ton. Greening will reduce both the area under citrus cultivation and its production side by side. Citrus greening is a deadly bacterial disease that affects all citrus varieties, and is spread by an insect, the Asian citrus psyllid. The causative agents are fastidious phloem-restricted, gram-negative bacteria in the gracilicutes clade. The Asian form, L. asiaticus is heat tolerant. This means the greening symptoms can develop at temperatures of up to 35ºC. The African form, L. africanum, is heat sensitive and in its case, symptoms only develop when the temperature is in the range 20-25ºC. The bacteria are transmitted by the psyllid vectors and by graft transmission. Although Trioza erytreae is the natural vector of African citrus greening and Diaphorina citri is the natural vector of Asian citrus greening, either psyllid can in fact transmit either of the greening agents under experimental conditions. This disease is distinguished by the common symptoms of yellowing of the veins and adjacent tissues; followed by yellowing or mottling of the entire leaf; followed by premature defoliation, dieback of twigs, decline in vigor; and followed by, ultimately, the death of the entire plant. Affected trees have stunted growth, bear multiple off-season flowers (most of which fall off), and produce small, irregularly-shaped fruit with a thick, pale peel that remains green at the bottom, reduced amount of fruit and fruit from these trees tastes bitter. After infection, the first symptoms may appear in six to 18 months, with relatively fast progression of the disease throughout an orchard. As the severity of the disease increases, citrus yield drops and could make the orchard’s production uneconomical in seven to 10 years after planting. If bacteria-infected trees can be found and removed sooner than through visual detection, the number of infected Asian citrus psyllids and spread of the disease should drop significantly.
The most powerful long-term management tool
likely will be the cultivation of citrus trees resistant to the bacteria that cause citrus greening and to the Asian citrus psyllid. Genetic engineering holds the greatest hope for generating trees with these traits. Orchard test plots consisting of infected trees that show no symptoms, as well as ones with symptoms, also should be established to evaluate new scouting and therapeutic methods.
Until these approaches can be implemented, other high-priority actions that could sustain citrus production, including: Developing “citrus management areas” to facilitate mitigation of citrus greening and other threats to citrus production, Integrating efforts to improve insecticide control of the Asian citrus psyllid. Expanding extension efforts that emphasize removal of infected trees in groves. Encouraging homeowners to remove backyard citrus trees, particularly trees infected with citrus greening. Biological control can also be a powerful tool against citrus greening.
It is also found that greater use of insecticide sprays, as currently required for successful suppression of the Asian citrus psyllid population, runs the risk of the insect developing resistance to the insecticides, the number of beneficial insects decreasing, and the groundwater being contaminated. More information on Asian citrus psyllid behavior and citrus greening disease are needed to improve the insect’s suppression, and research should aim to develop alternative Asian citrus psyllid management strategies. There is no cure for Huanglongbing and efforts to control the disease have been slow because infected citrus plants are difficult to maintain, regenerate, and study.