Citrus Yellow Dragon Disease

Community Contributed

By Glenn I. Teves, County Extension Agent, UH CTAHR Molokai 

Citrus Greening disease, a bacteria, is one of the greatest threats to citrus worldwide and it has been spreading in the U.S. First reported in Florida in 2005, over 60 percent of the citrus crop has been impacted with over 7000 jobs and $4 billion in revenues lost. Since then, it has spread to Louisiana, South Carolina, Alabama, California, Texas, and Georgia. In 2015, the disease was found in Guam, and it’s just a matter of time before it reaches Hawaii. It’s important to keep this disease off of Molokai. To do this, do not bring citrus plants to Molokai without State Department of Agriculture inspection from the island of origin!

Also known as Citrus Huanglongbing meaning yellow dragon in Chinese, Citrus Greening disease gets its name from the pale yellow color of new shoots on diseased plants. 

The disease usually starts on one part of the plant then spreads throughout the entire canopy. However, this symptom is very common with other citrus diseases. Fruits look small and lopsided, and often remain green on the bottom and have a sour flavor. Seeds will abort and turn brown. Trees will also fruit out of season, defoliate, drop fruit, and eventually succumb to the disease. Leaves of infected trees display a blotchy chlorotic mottling, but can easily be confused with nutritional, fungal, or insect infestation or some viral diseases. Nutritional symptoms usually show the same pattern on both halves of the leaf while HLB has different symptoms on two halves. 

The bacteria Candidatus liberibacter has three different strains, and the Asian strain is considered the most aggressive and spreading in the more tropical areas since it tolerates warm temperatures. The symptoms are inconsistent which makes it difficult to identify and must be done by submitting a sample to a laboratory for a DNA-based test. 

This disease is spread by the Asian citrus psyllid and this insect has been found in Hawaii since 2006 and is now widespread. This disease can be spread by grafting an infected piece onto a new plant. Some citrus species are more susceptible than others, but sweet orange, grapefruit, and tangerine are the most susceptible. Acid citrus such as lemon and lime appear to be more tolerant to the disease. In South Africa, pruning the symptomatic tree limbs in conjunction with pest control has been effective in slowing its spread. 

The most likely introduction of this disease to Hawaii will be infected plants and budwood brought in by humans. Once introduced, the most effective strategy will be monitoring and removal of infected trees, replacing it with clean disease-free plants, and controlling the psyllid, and this strategy has been effective in China along with proper nutrition. But for now, the take home message here is any citrus plants brought to Molokai must be first inspected by the Dept. of Agriculture!