Yellow dragon disease

Huánglóngbìng 黃龍病 HLB from the Chinese for yellow dragon disease is a pathogenic disease of plants known as Citrus Greening Disease, yellow shoot disease and that’s-the-end-of-my-farm disease. It is caused by one of several related alpha-proteobacterial species including Liberibacter americanus. Alpha-proteobacteria include

• Rhizobium meliloti [root nodule nitrogen fixer in clover, peas and beans]
• Agrobacterium tumefaciens [T-plasmid genetic engineering for plants]
• Wolbachia [elegant ways of knobbling several human disease vectors: prevs zika; dengue; onchocerca river-blindness]

The disease acquired its exotic chinese name and the associated acronym, because it was first identified as a problem in China wiping out mandarin orange groves. They worked out that it was caused by a bacteria that settles in the phloem channels of many plants where there is a rich and nutritious flux of sugars and nutrients. Liberibacter is called 'fastidious' because it is fussy about where it can / will grow. Having cast its lot to live in a rich soup, it has trimmed its genome to about a quarter the size of, say, E.coli.  This sleek efficient genome has lost a lot of biochemical capability, so microbiologists haven't succeeded in cultivating it reliably on any sort of Petri dish.  If you can't culture it, you cannot easily test to see what will kill it in the lab and so develop a counter-measure and save the citrus groves of the sub-tropical world.

Liberibacter appeared in Florida orange and lemon groves nearly ten years ago and spread to California and Texas in 2012. Whatever effort American agritechnologists are prepared to make to cure damaging diseases in the Third World, they really perk up when American farms are at risk - not least because people are prepared to pay big money to save US livelihoods. One angle is to treat the bacteria with systemic anti-biotics - penicillin will do for a start because there is (as yet) no antibiotic resistance. Although this is really hard to determine because you can't grow it in the lab and apply a variety of antibiotic disks to see which will work. The reduced genome size gives promise that this approach may work: the genes to build resistance may not be there.  I am going to task a final year project student this year to search the Liberibacter genome for anti-biotic resistance genes and transport systems.

It is a bloomin' marvellous wonder of biotech that, although nobody can grow the bacterium in a laboratory, they have nevertheless been able to sequence the whole genome and make that publicly available. This allows us to find out a lot about the capabilities and modus operandi of pretty much any bacterium - unculturable members of your intestinal flora for example or the microbiome of a potato field in Wexford.

How did HLB arrive in Florida? Nobody knows, but it's Lombard Street to a china orange, that some farmer imported a highly productive cultivar of citrus trees from a far-off place and reaped a whirlwind of destruction for himself and his neighbours. One of the messages being given out to the public is not to take slips from citrus trees and don't transport them across county lines. But that's dealing with bigger-than-a-breadbox transporters of contamination and it's having a trivial impact on the spread.

Other creatures have noticed the nutritious qualities of phloem notably hemipteran sucking insects including the Asian citrus psyllid, Diaphorina citri [R getting its mouth-parts wet] which is native to China but has popped up in a wider swathes of US states than those which have know out-breaks of HLB. It seems only a matter of time before D. citri in Alabama or Louisiana take a trip to Texas and bring back a foul holiday souvenir. The USDA has in the past had a tendency to spray crops with insecticides, which I have deprecated as an Almaric Solution. For starters, killing D. citri in this butcher's way will likely kill Brachygastra mellifica, the Mexican honey wasp, which has a taste for psyllids and an uncanny ability to seek and out and consume D. citri in the foliage of citrus groves.

But wait! What about Wolbachia? We've seen that other alpha-proteobacteria being put to good use against insect vectors for Zika and Dengue Fever and river-blindness parasitic nematodes. I am happy to report that one of the prize-winning post-graduate students from The Institute has just graduated and secured a 2 year contract to work in Kentucky for a company that has HLB in its sights.