THE WORLD’S ORANGE TREES ARE KILLING BY CITRUS GREENING

What is citrus greening?

One of the most extreme citrus plant diseases in the world is Citrus Greening (Candidatus Liberibacter asiaticus). It is also known as yellow dragon disease or Huanglongbing (HLB). There is no remedy if a tree is poisoned. Although the disease poses no threat to humans or wildlife, millions of acres of citrus crops across the United States and abroad have been destroyed. Citrus greening is transmitted by the Asian citrus psyllid (Diaphorina citri Kuwayama or ACP), a disease-infected pest, which has put the future of citrus in America at risk. Infected trees grow green, malformed and bitter fruits that are unsuitable for selling as fresh fruit or for juice. Within a couple of years, most infected trees die.

CITRUS GREENING DISEASE

One of the most serious citrus diseases is citrus greening, or Huanglongbing. Yields for all varieties of citrus have been diminished everywhere they occur in Asia, from China and the Philippines to the Arabian Peninsula and to Africa. It is one of the diseases from which the majority of the citrus-producing nations are protecting and bracing for their imminent expansion. Citrus greening signs consist of narrower leaves, yellowing of part of the leaves, or usually of the whole canopy of the trees, diminished foliage, and extreme twig dieback. However, the most characteristic signs are that infected trees grow fruit that is lopsided, does not mature, and stays green instead; giving juice derived from such fruit an undesirable taste.

Side effects of Huanglongbing (HLB) or citrus greening

The most serious citrus disease, currently crippling the citrus industry worldwide, is huanglongbing (HLB) or citrus greening. Candidatus Liberibacter spp., the suspected causative bacterial agent Tree health as well as fruit growth, ripening and citrus fruit and juice quality are affected. Fruit may be either symptomatic or asymptomatic from tainted orange trees. Tiny, asymmetrical and greener than healthier berries, symptomatic oranges are. In comparison, symptomatic oranges exhibit higher titratable acidity and lower soluble solids, total sugars, solids / acids ratio, and levels of malic acid. Ethyl butanoate, valencene, decanal and other ethyl esters are lower than the taste volatiles, but certain monoterpenes are higher in symptomatic fruit than in stable and asymptomatic fruit. An rise in secondary orange peel and pulp metabolites, including hydroxycinnamic acids, limonin, nomilin, narirutin, and hesperidin, is also caused by the disease. Juice made from symptomatic fruit is characterized as distinctly bitter, acidic, salty / umami, metallic, musty, and lacking in sweetness and fruity / orange flavor as a consequence of these chemical changes.

History

At the end of the 19th century, the citrus greening disease actually originated in China; it was named the "yellow shoot disease." The first epiphytotic conditions of CGD were noticed in the Chaoshan and Yuenchung districts of Fukien Province in 1925, according to Lin (1956). The disease was later identified as "yellow branch disease" in South Africa in 1929, and later renamed "greening," referring to the green colour of the fruit at the 2nd harvest. Since the discoveries of this disease, various names in different countries have been named the disease is associated with a decline in citrus plants and is expected to be prevalent in all the world's citrus producing countries. By shortening the active life cycle of plants, it has a great potential to harm citrus trees of all cultivars and do great damage to the citrus industry. Greening in many areas of Asia and Africa is believed to have incurred colossal damages.

How to control?

In order to grow safe plants and avoid the transport of contaminated nursery stocks, HLB management includes quarantine, clean inventory and certification systems. The vectors of psyllids must be regulated. During the winter months, when the psyllids are not as involved and only the overwintering adults are present, Florida and Texas have been using area-wide sprays to suppress the psyllid population. A three pronged approach to control is useful in areas where HLB is not already established: frequent surveys to detect early signs on trees that are then removed; control of the psyllid vector through survey and application of pesticides; and use of clean plant material for replanting. The identification of HLB-related bacteria is by PCR or real-time PCR. The monitoring of psyllids by real-time PCR for the presence of HLB-associated bacteria has been shown to provide an early notice of the presence of the disease in an environment where HLB is not already known.

Potential Citrus Greening Solutions

Short-Term Solutions

Advanced Nutritional Supplements 

Trace quantities of nutrients are added to the leaves or to the roots to delay the process of citrus greening in infected plants. Providing better nutrients helps trees combat citrus greening which encourages them to thrive for an prolonged period of time to yield healthy fruit. In reducing the effects of citrus greening, dietary therapies are continuing to be thoroughly studied.

Heat Treatment

By encasing them in plastic tents, heating HLB-infected trees in the sun delays or decreases the number of psyllids, which can theoretically prolong the productivity of trees. Steam treatment also demonstrates potential to extend the lifetime and fruit development capacity of the tree. This approach raises implementation problems with thousands of acres of citrus groves polluted with greening. Researchers are presently looking for how to achieve this on a broader scale.

Bactericides

Bactericides are a topical medication aimed at slowing down the greening of citrus-causing bacteria. Bactericides are not ingested into the fruit or the tree. Although this is a relatively recent treatment for citrus trees, pear and apple farmers have been using it to combat bacterial infections in their trees for many years. Citrus growers are optimistic that, as longer-term options are tested, bactericides can help control greening and increase overall tree quality.

Long Term Solutions

1.      Citrus Under Protective Screen (CUPS)

Researchers at the University of Florida are exploring new ways to grow citrus that give more psyllid protection. The citrus stays stable and unaffected by citrus greening by growing citrus in an enclosed environment, under screen. In order to decide if this can be achieved on a broad scale and whether costs may be minimized, further study is required.

2.      Biotechnologies

Researchers are studying how to grow a citrus tree breed immune to illness that is not susceptible to greening or HLB and may not become sick. As a consequence, the tree will stay healthy and begin growing quality citrus fruit. Orthodox cross-breeding and biotechnology are used in this study.

Work is ongoing, for example, investigating genetic alteration in citrus trees with the inclusion of a spinach gene that does not modify fruit or juice. In developing a more HLB-tolerant citrus tree, this alternative is showing promise and comprehensive research is being conducted. As recently as 2015, researchers discovered another promising solution by isolating a gene from the mustard family. Also after three years of being planted alongside infected plants, the resulting trees demonstrated greater resistance to greening. Although comprehensive analysis continues, a solution to greening is still years away.

By: Dua Mughal