There is some variability in citrus susceptibility to both the psyllid (Westbrook et al and Hall et al) and the bacterial infection (Shokrollah et al and Stover and Mc Collum). It is not fully clear at this point which of these species would be best to include in a breeding program, but both scions (http://www.concitver.com/3erEIIC2011/Viernes_23_sep/Ed_Stover/Ed_Stover.pdf) and rootstocks (http://www.flcitrusmutual.com/files/4cbb1e3c-1e1f-4b04-a.pdf) are being evaluated.
Some varieties like Australian Finger Lime (Microcitrus australasica) might show more resistance/tolerance to HLB than other species. At the UC Citrus Variety Collection website, it's possible to see the wide range of citrus species that are available for breeding and the commercial availability of those species. The collection is a view into the different materials that breeders can select from, in order to improve resistance to this citrus disease.
Citrus Variety Collection
Australian Finger Lime Collection and Availability
Susceptibility to Infestation by Asian Citrus Psyllid
Catherine J. Westbrook1, David G. Hall2, Ed Stover, and Yong Ping Duan
D. Hall, C. Westbrook, Y.-P. Duan, E. Stover, R. Lee and M. Richardson http://citrusagents.ifas.ufl.edu/events/fl_citrus2011/Hall%20ACP%20IR%20Citrus%20Expo%202011.pdf
Susceptibility to HLB
Hajivand Shokrollah, Thohirah Lee Abdullah, Kamaruzaman Sijam,Siti Nor Akmar Abdullah and
Nur Ashikin Psyquay Abdullah
Ed Stoverand Greg McCollum
Australian Finger Lime Fruit
Australian Finger Lime Tree
Australian finger lime fruit
Australian finger lime tree
Citrus trees may ‘eat their spinach' to ward off huanglongbing – from AgProfessional.com
In a landmark step in the fight against citrus greening disease, also known as huanglongbing (HLB), the U.S. Environmental Protection Agency has approved an application from Southern Gardens Citrus of Florida for an Experimental Use Permit under the Federal Insecticide, Fungicide and Rodenticide Act. This allows researchers to move forward in the development of the possible use of a spinach protein to help control this devastating disease.
Ricke Kress, president of Southern Gardens, said this latest development is a milestone in efforts to fight off HLB.
“A final solution to eliminating this disease may still take some years,” Kress said, “but the latest EPA action and continuation of all research projects are major steps in the right direction.”
Research conducted by Dr. Erik Mirkov, a plant pathologist at the Texas A&M AgriLife Research and Extension Center at Weslaco, resulted in the production of proteins that appear to provide effective control of citrus greening disease.
“Citrus greening is a bacterial disease that affects the vascular system of the tree,” Dr. Mirkov said. “It basically shuts off the tree's ability to take up and use water and nutrients, causing the tree to die. We were able to improve the transgenic trees by having the genes express themselves in the vascular system.”
HLB is the most serious citrus disease in the world. It was first identified and confirmed in Florida in 2005. HLB is now found in every Florida county where citrus is grown commercially. There are no successful control programs yet available for this disease. (NOTE – HLB has been detected just once in California, at a residential property in Los Angeles County in 2012)
Consistent with the conditions established by EPA, researchers may now move forward with field tests to evaluate the efficacy of the spinach protein against HLB in citrus plant tissues and continue generating the environmental, health and safety data that are required under federal law to support a fully registered product for commercial use.
Mark Hoddle of UC Riverside Entomology Department has intorduced a second species of natural enemy of Asian Citrus Psyllid (ACP), Diaphoencyrtus aligarhensis, imported from Punjab, Pakistan. It was officially released December 16, 2014 at the Biological Control Grove at UCR. It is anticipated that this natural enemy will be complimentary to Tamarixia radiata, a parasatoid that also attacks ACP nymphs that was released in California in 2011. It's thought that it might occupy slightly different environments where it might be more successful than Tamarixia.
Recently, I read an article in "Fresh Plaza" about the arrival of large amounts of 'Kinnow' mandarin fruit from Pakistan. http://www.freshplaza.com/article/117470/Discover-Pakistani-kinnow-mandarins-at-Fruit-Logistica-2014#SlideFrame_1
This is a country that is surrounded by countries with huanglongbing. It's also the country where Mark Hoddle, Biocontrol Specialist from UC Riverside, collected Tamarixia radiata , the tiny wasp that is helping control Asian Citrus Psyllid. Pakistan has invested heavily in juice plants just for this industry which was established with this selection that came out of the Citrus Research Station breeding program in the 1930's. It makes me wonder if there might be a significant tolerance to this citrus disease in this mandarin variety. There has been a lot of work by both USDA and U. of Florida evaluating citrus varieties for tolerance to HLB. A wide range of tolerances have been noted. Fred Gmitter along with others are involved with this work and find that under different climatic conditions, resistance can vary. In the 'Kinnow' variety, it looks like there is hope in finding a variety that can be used to breed tolerance into other varieties. The fruit itself is noted for its juiciness and sweetness. But as you can see from the photo, it's got a lot of seeds.
University of Florida research Jude Grosser has been working with a new breeding technique that creates tetraploid rootstocks that are showing significantly improved resistance to Huanglongbing. This is done with conventional breeding and is not based on genetic engineering. He takes citrus rootstocks that have shown some resistance but because of their genetic makeup, it has not be possible to interbreed them. This new technique permits these crosses that were before not possible. He and his group have created new rootstocks that are now being field trialed.
Also on another front, Richard Lee and Manjunath Keremane at the USDA Citrus Germplasm Repository in Riverside have been working with University of Hawaii and a private company – Diagenetix – to develop a field test for identifying HLB infected psyllids. LAMP (loo-mediated isothermal amplification) is a faster, cheaper method than the traditional PCR (polymerase chain reaction) method. It would allow for rapid identification of infected psyllids and a more rapid identification of a potential quarantine area. Conventional PCR would still need to be performed to legally identify infected insects. The technology has been used on other disease organisms such as powdery mildew in grape and bacterial infections in stone fruit trees. Literally anything that carries DNA can be identified by this new technique.
ACP adult and nymph