I promised you more about that virus that has been infecting oranges trees…..
It’s actually not a virus at all. It’s a bacteria: Liberibacter. The disease it causes is Huanglongbing, also known as HLB or citrus greening disease. It was initially thought to be caused by a virus, but it’s actually a bacteria that’s just really difficult to grow in culture.
It is called “greening disease” because infected trees produce fruit that are green, misshapen, and bitter. The infected trees eventually die. According to a 2019 news article, HLB decreased the production of oranges for juice and other products by 72% in the last decade in the US.
HLB is transmitted by a bug called a psyllid (similar to a whitefly or aphid), which is referred to as the “Asian citrus psyllid” or ACP. HLB was initially found in Asia and Africa, where it’s been known about since the first half of the 1900s. It wasn’t identified in Florida until 2005, but it spread quickly. It also popped up in Southern California in 2012. So far, the Bay Area and Northern California seems okay, but it’s only a matter of time.
Right now, there is a citrus quarantine in California that prohibits the movement of plant or plant parts outside of six quarantine zones. (Commercially cleaned and packed citrus are excluded from the quarantine.)
There is no cure for HLB at this point. If a tree gets infected, it needs to be removed. Antibiotics have been tried, but antibiotics are not heat stable, which makes them less than ideal for use in a hot citrus grove. Also, antibiotics kill both good and bad bacteria and can lead to resistance, which is bad for everyone.
When I was at the orchard harvest, I overheard someone saying that there is now a “vaccine” for the disease. Well….
In late 2020, an article in PNAS reported the identification or development of a “stable antimicrobial peptide” (SAMP) the treats HLB and also prevents infection, much like a vaccine. I don’t totally understand why this SAMP is different from an antibiotic, but according to the article, it kills bacteria faster than antibiotics do, and it also works through “nonspecific mechanisms.” These features both make development of resistance less likely. Also, the SAMP is heat stable unlike antibiotics.
The SAMP is sprayed on the plants, is taken up by the leaves, and moves through the plants vascular system for up to a week.
With any luck, our orange trees can be saved. But in the meantime, eat all the citrus while you still can! Perhaps I should freeze or can the oranges that I have, in case the SAMP doesn’t work out. Twenty years from now, people might not remember what oranges tasted like…. what a sad world.