Need for Genetic Diversity to Combat Citrus Greening

University of Florida (UF) researchers have analyzed the genome sequences of 10 diverse citrus varieties for the first time, and are calling for genetic diversity among the fruits to help combat the ongoing epidemic of citrus greening.

Sweet and sour oranges, mandarins and pummelo varieties were among the fruits investigated as a part of this decade-long study.

Oranges have a history that is over 5 million years old, and by tracing back this ancestry and better understanding how they came to have "very narrow genetic diversity," researchers may be able to genetically turn back time and create strains that more effectively fight off crop diseases.

"Citrus has incestuous genes - nothing is pure. Now that we understand the genetic structure of sweet orange, for example, we can imagine reproducing early citrus domestication using modern breeding techniques that could draw from a broader pool of natural variation and resistance," Fred Gmitter of the Institute of Food and Agricultural Sciences said in a UF press release.

Modern methods of cultivating citrus trees involve grafting, a process that binds the fruit bearing part of one tree to the root system of another. While this technique speeds nature along, producing more consistent fruit in a shorter time frame, it also means that if one tree is susceptible to a disease or parasite, they probably all are.

"What we've found is that we're planting very similar trees to what were planted 4,000 years ago. These same genotypes do not provide much diversity to resist diseases, and there is very little wild citrus that can be used to easily add diversity to modern citrus," Jeremy Schmutz, one of the lead investigators, stated in a Bioscience Technology news release.

The parasite currently taking citrus trees hostage is the leaf-munching insect called the Asian citrus psyllid. The tiny bug is devastating large swaths of trees in North America, but particularly Florida (and its $9 billion orange juice industry), leaving behind the bacteria that causes greening.

"Current control methods include removing and destroying infected trees, controlling the psyllid, and providing additional nutrition in an attempt to keep infected trees productive," the UF press release writes.

If a cure cannot be found soon, citrus greening could wipe out the entire industry in the next decade.

This study underscores how understanding genetics can facilitate introducing or restoring diversity to citrus and other crops to help growth and resistance to such diseases.

The findings were published in the journal Nature Biotechnology.