Sally Scalera

Brevard County Horticulture Extension Agent
3695 Lake Drive -- Cocoa FL 32926 -- 321-633-1702

Citrus greening

April 11, 2009

While many people are already familiar with Citrus canker the newest threat to our citrus industry and backyard citrus growing residents is Citrus greening which is also called Huanglongbing or HLB. Citrus greening is a serious disease of citrus because it affects all citrus cultivars and causes rapid decline of trees. Sadly, citrus greening has already been found throughout Brevard County and if infected trees are left to grow that will only help the spread of the disease into other yards and commercial citrus groves.

To give you a little background, citrus greening is caused by a bacterium that is spread by a psyllid insect. As the psyllid feeds on the sap of the citrus tree is passes the bacterium into the tree and then the tree is infected. Once a plant has become infected, the bacterium can move throughout the plant. That is, the infection is systemic, and thus cannot be removed by simply pruning away the part of the tree expressing symptoms.

Early symptoms of citrus greening are a leaf yellowing that may appear on a single shoot or branch. In early stages of infection, leaves may have a mottled or blotchy appearance. The yellowing spreads throughout the tree and affected trees show twig dieback and trees rapidly degenerate into a non-productive state in 2 to 3 years. In trees with advanced stages of the disease, the leaves are small and frequently show mineral (zinc or manganese) deficiency symptoms with yellow veining. Fruit are sparse, small, abnormal in appearance and fail to color properly, thus the name greening. The affected fruit often contain aborted seeds and have poor juice quality.

Here in Florida we have the Asian citrus psyllid which is adapted to warm humid areas and is the vector for citrus greening. Once infected, the psyllids remain capable of transmitting citrus greening for their entire lives. The psyllids feed and survive primarily on citrus and citrus relatives.

New infections of citrus greening in healthy trees may be signaled by the yellow shoot symptom but diagnosis is more difficult in older plants that lack vigor or are suffering from other problems. It is especially hard to detect in dooryard plantings when trees are often in poor condition from multiple causes such as inadequate fertilization.

Citrus greening is one of a few citrus diseases that can be considered a truly limiting factor for citrus production and control is difficult if infected trees are widespread and the psyllid vector is well established. The greening bacterium can infect virtually all citrus species, cultivars and hybrids, as well as several citrus relatives. Sweet oranges, mandarins (tangerines), and mandarin hybrids (tangelos) are highly susceptible to greening. Lemons, grapefruit, pummelos, and sour orange are also affected and are rendered non-productive when infected. Mexican lime, trifoliate orange and some trifoliate orange hybrids are more tolerant and may show only some leaf symptoms.

In addition to cultivated citrus and its relatives, greening can infect a number of citrus related ornamental plant species. The greening bacterium can infect and multiply in orange boxwood (Severinia buxifolia) and orange jasmine (Murraya paniculata) and they are also excellent host plants of the Asian citrus psyllid. Thus, having these ornamentals in the landscape can allow psyllid populations to build up and increase the risk of spreading the disease to other ornamental and citrus plants. Movement of these ornamentals is restricted under Florida regulations and they should not be moved out of areas where greening is present

Now for some things that you can do to protect your trees from citrus greening.

  • Go out and check your citrus trees for leaves that show a blotchy mottle. This is a variegated chlorosis or yellowing of the leaf that is not symmetrical about the center vein of the leaf. That is, the symptoms on the two halves of the leaf are not mirror images of one another, but rather are random across the leaf. An easy test to help determine if the symptoms are symmetrical or not is to draw two circles on the leaf surface on either side of the center vein. If the areas within the two circles have a similar appearance then that would suggest there is no citrus greening. However, if the areas within the two circles are not similar that could indicate greening. With time, leaves may develop what is known as vein corking. This symptom is typified by bright yellow leaf veins that are raised and have a corky appearance.
  • If a tree does have citrus greening it should be removed and the trunk ground up. The reason for grinding up the trunk is so that no root suckers sprout (remember they would still have the disease) which could continue to be spread by the psyllids.
  • If you have a lot of orange jasmine (Murraya paniculata) planted the recommendation is to remove it. Since orange jasmine can allow the bacterium and psyllids to multiply this will increase the spread the citrus greening.
  • Follow the fertilizer recommendations found in our bulletin titled Citrus Culture in the Home Landscape at http://edis.ifas.ufl.edu/pdffiles/HS/HS13200.pdf
  • For more information and pictures check out our bulletin Dooryard Citrus Production: Citrus Greening Disease at http://edis.ifas.ufl.edu/pdffiles/HS/HS38300.pdf
  • For extra protection, use a liquid seaweed spray on the foliage of your citrus tree. If your tree appears healthy and green do this monthly. If you tree is not a healthy green then spray it weekly for a couple of months and then go to a monthly spray schedule. For a sheet on using liquid seaweed you can call us at 633-1702 and give us your address or email me at sasc@ufl.edu .

Even though the news isn’t good, there are still things that you can do to be able to pick your own home-grown citrus!