Knowledge about micronutrient deficiency symptoms in citrus trees is extremely important to growers and pest control advisers in determining if a malady found in the field could be the lack of an important element or the dreaded Huanglongbing disease.
HLB is considered the worst citrus disease in the world.
University of California citrus farm advisor Neil O’Connell of Tulare County says deficiencies of zinc, iron, and manganese can resemble leaf symptoms found in a HLB-infected citrus tree.
“Some deficiencies have fairly similar symptoms,” said O’Connell, a 31-year California citrus industry veteran. “If you are very familiar with the deficiency patterns in these elements then it is much easier to separate this out. You can recognize whether the problem is zinc, iron, manganese, or another deficiency while possibly ruling out HLB.”
Zinc deficiency symptoms in the leaf most closely resemble symptoms from a HLB-infected citrus tree, O’Connell says. Suspected deficiencies of iron and manganese symptoms should be examined closely, but are not as similar to HLB leaf symptoms.
A normal citrus leaf has a uniform green color. An HLB-infected leaf has a blotchy appearance, yellowing of the leaf blade, and a yellowing or clearing (cream color) of the mid rib, the center portion of the leaf.
A zinc deficiency results in a blotchy leaf appearance with irregular-shaped yellow spots (leaf yellowing) scattered across the leaf blade. Leaf yellowing is usually confined between the small veins.
“In an HLB-infected leaf, the distinguishing characteristic is that the yellow area crosses from one interveinal area to another,” O’Connell said as he checked out a citrus orchard in northern Tulare County in mid May.
HLB, also called citrus greening, is a deadly malady which has devastated commercial citrus orchards around the world including in Florida.
A single case of HLB was found in California in March in a backyard pummelo-lemon citrus tree in the Hacienda Heights neighborhood in southeastern Los Angeles County. There are no confirmed HLB cases in California commercial citrus. The first signs of infection typically appear several years after initial tree infection.
The disease has not been found in Arizona.
Commercial citrus is a $2.1 billion industry in California (2010 figure) and a $37 million dollar Arizona crop (2009/2010 crop year).
Iron deficiency in the tree also can resemble HLB symptoms with one key exception. Instead of having a blotchy, mottle appearance, the leaf blade has an overall pale look while the small leaf veins appear as a green-colored, connected network.
As the iron deficiency progresses, the leaf takes on more of an overall yellow appearance. The entire leaf blade turns yellow in advanced stages.
A manganese deficiency can be confused with zinc deficiency. A leaf with reduced manganese has an overall background pale-green color to the leaf blade. The dark-green veins are more pronounced against the pale-green leaf blade.
Every HLB-infected citrus tree eventually dies. The disease disrupts the tree’s phloem tissue; shutting down the tree’s ability to deliver nutrients throughout the tree. Overtime the tree collapses.
HLB-infected fruit can be misshapen and resemble the symptoms of citrus stubborn disease, O’Connell says, with “lopsided” fruit or flattened fruit on the blossom end. With HLB, the fruit eventually develops a sour taste and becomes unmarketable.
O’Connell says many excellent resources are available to growers, PCAs, and others on HLB. The University of California, Davis ANR Publication 8218, “Citrus Bacterial Canker Disease and Huanglongbing (Citrus Greening),” reviews macronutrient and micronutrient deficiencies compared to HLB-related symptoms.
The publication is available online by clicking on this link - http://anrcatalog.ucdavis.edu/pdf/8218.pdf.
The publication also includes photo comparisons; for example photos of a normal citrus leaf side-by-side with an iron-deficient leaf and a HLB-infected leaf. It also includes photos of misshapen and off-colored fruit from a HLB-infected tree.
Publication 8218 is authored by MaryLou Polek of the California Citrus Research Board, Georgios Vidalakis of UC Riverside, and Kris Godfrey of the California Department of Food and Agriculture (CDFA).
Another good UCD-ANR resource is Publication 8205 available online by clicking on this link - http://anrcatalog.ucdavis.edu/pdf/8205.pdf. The publication explains the Asian citrus psyllid (ACP) insect; the bug which carries the bacteria Bacterium liberibacter to citrus trees to cause an HLB infection.
The authors - Beth Grafton-Cardwell of UC Riverside, Kris Godfrey, and others – cover the insect’s life cycle and the damage from the ACP feeding on citrus trees.
In the spring and early summer, growers and PCAs often find atypical leaf patterns during orchard walkthroughs. O’Connell says this is the perfect opportunity to utilize these and other pest and disease resources.
He encourages those involved in citrus at the field level to carry printed or electronic copies of publications in vehicles as hands-on reference materials.
“I hope growers and PCAs will become very familiar with detailed nutritional deficiency patterns so they can more easily distinguish and identify symptoms in the orchard,” O’Connell said. “Patterns can change over time so a good understanding is critical to discern between an element deficiency and HLB.”
O’Connell recommends that citrus packinghouse field staff also be well versed on these issues since they are in the field daily during the citrus harvest.
“The potential for devastation from HLB in the western citrus industry is very real,” the citrus veteran said.
The industry is conducting a major educational campaign to inform residential and commercial citrus tree owners about the pest, the bacterium, and the disease.
In California, anyone who suspects HLB in a citrus tree should immediately contact the County Ag Commissioner’s Office. Another option is to call the CDFA Pest Hotline at (800) 491-1899.
The phone number for Arizonans is (602) 542-0955.