I always look forward to this time of year when citrus supplies are plentiful. My favorite one has always been the tangelo, but that is changing. It’s not because I like them less, it’s because they are nowhere to be found.
The reason for this is the citrus greening disease, sometimes called huanglongbing disease or HLB. It’s caused by a bacterium spread by a tiny sucking insect called a psyllid. Bacterial diseases are difficult to control in plants.
Bacteria reproduce extremely fast, so host plants must be treated constantly. That’s impractical for large acreages. The bactericides and insecticides also must be systemic and enter the plant tissue to be effective. Chemicals also can’t be toxic to the consumer.
Tangelos are among the most susceptible of all citrus types. Grapefruit, key lime, and clementine are more resistant. If you have been surprised to not find tangelos in the store, there’s good reason. There are none.
Perhaps you’ve noticed more mandarin oranges at grocery stores than in past years. Mandarin types like clementines are somewhat more tolerant to the disease, and in recent years more farmers have planted them.
Once a tree is infected with HLB, there is no cure. The fruit yields in Florida are a fraction of what they were in 2000. Fruit prices have been on the rise to compensate for decreasing yields and everyone has suffered.
Plant pathologists have been working hard for over a decade to develop citrus that is resistant to these bacteria. They’ve also initiated programs to hinder the psyllid insect that carries them.
So, what does citrus greening disease actually do, you ask? Trees infected with the bacteria produce fruit that stays green even after it is ripe. It’s also bitter and poorly shaped. Sugar can’t be transported properly inside the plant, so fruit quality is poor and energy to the entire plant is limited. Infected trees usually die within a few years.
A few decades ago, another bacterial disease called citrus canker ravaged Florida. It’s still a concern. Entire orchards were quarantined and destroyed under government supervision. Many farmers were ruined, but it was minor compared to the present citrus greening problem.
Trees infected with that disease had fruits that had unsightly surface blemishes. They still were acceptable for much of the juice market. Citrus greening is different. Fruit quality is unacceptable for any human consumption. Sugars won’t translocate to the fruit normally.
Controlling both these diseases involves reducing the amount of disease inoculum available. That means destroying infected trees. Backyard gardeners must comply too. If either of these diseases is detected on any tree in a commercial orchard, nursery or private residence it must be destroyed.
Until recently the disease was limited to Florida and Texas in the US. It has now spread to California. Their problem has been less severe, but I suspect part of the reason is that the climate is less ideal for vector and pathogen growth.
Florida’s climate is hot and humid, while citrus producing areas in California are hot but far less humid. Whatever the case, we’re facing several more years of reduced citrus quality and yield. That’s a shame.
Ted Manzer teaches agriculture at Northeastern High School.
