How to protect palm trees from disease, pests
A new batch of problems are out to get your palms, but you can fight back with prevention and care.
By Yvonne Swanson, Special to the Times
Published February 16, 2008
Palms around Tampa Bay are under attack by new and lethal diseases, but homeowners can fight back with this five-point survival strategy that will help improve the health of palms. A healthier, more robust plant will be better protected when airborne fungi and pests wage their war.
A variety of factors have contributed to new and greater numbers of palm-prone diseases and pests, among them warmer temperatures in our growing zone and improper pruning that puts palms at greater risk of disease. Numerous palms are the targets, including Queen, Washingtonian, Phoenix species Canary Island date, Coconut, Adonidia (Christmas tree palm) - most not native to Florida.
Fusarium wilt, caused by a fungus, is threatening Queen and Washingtonian palms, although other palms are at risk, says Jane Morse, commercial horticulture agent at the Pinellas County Extension in Largo.
There's no cure for the disease and it's easily spread in the wind and by unclean pruning tools, Morse says. (See box for disease symptoms.)
Another new palm disease, Texas Phoenix Palm Decline, attacks Queen palm and the Phoenix species, typically turning the plants yellow before killing them.
Those killers join the list of more familiar palm diseases, such as Lethal Yellowing, Ganoderma Butt Rot and False Smut. (See box for details.)
Early identification, diagnosis and removal of sick palms will prevent the spread of diseases to others. But that's easier said than done.
The challenge is making the diagnosis. Many palms already suffer from nutritional deficiencies, which result in discolored yellow and brown fronds. That makes it difficult for most homeowners to discern whether lackluster palms are diseased. Or could they simply be thirsty, cold, starved for nutrients or over-fertilized?
If you follow these five strategies, you'll boost the health of your palms and their ability to fight disease. With their healthy green fronds, it will also be easier for you to catch problems early.
Fertilize palms this month. Don't use just any fertilizer. Experts recommend a special, time-release 8-2-12-4 (phosphorous-potassium-nitrogen-magnesium) formula created by the University of Florida after several years of field testing. Sold under the "Florida's Finest" label, the fertilizer is unique because it includes major and minor elements and all are coated for slow release, so you can't overfertilize. In contrast, most palm fertilizers coat only the nitrogen element for slow release.
"Palms aren't crazy about nitrogen anyway. They like all those minor elements. That's what keeps them dark green," says Rick Tetrault, general manager at Garden and Hardware Distributors in Pinellas Park, which manufactures the palm fertilizer and sells it through independent garden retailers.
Because it's entirely slow-release, the fertilizer is applied just three times a year (February, May and September). Other fertilizers require four feedings yearly.
Fifty-pound bags of Florida's Finest palm food retail for about $35 and are available locally at Jene's Tropicals, Willow Tree Nursery, Dolan's Garden Center, Wilcox Nursery and The Garden Room (all in Pinellas County) and Earl's Garden Shop in Hillsborough.
Don't prune green fronds (unless they pose a danger). It's not true that removing green fronds will encourage growth or shape up a palm, despite the advice of some tree trimmers who promote this practice. Over-pruning increases the palm's susceptibility to diseases and worsens any nutritional deficiencies. Only dead or nearly dead fronds should be pruned.
"With all the pruning that is going on with these palms, it just makes me want to scream," Morse says. "People think this is the way that palms are supposed to look. These poor things (palms) have nutritional deficiencies, so they cut the fronds off. You can't see the symptoms" of nutritional deficiencies and disease "because they have cut off the fronds."
If you do prune, always use sanitized tools. Diseases can spread from one palm to another on tool surfaces. Clean blades with a diluted bleach solution (one part bleach to one part water), and if you employ a tree service, insist that equipment is cleaned before use on your property and between individual palms.
Diagnose palm problems early. Consult your county's extension service to determine if your palm is diseased or has a nutritional deficiency. Pinellas County operates a free hotline staffed by horticulturists from 9 a.m. to noon and 1 to 4 p.m. Monday through Friday at (727) 582-2110. Even better, bring a frond to the office (12520 Ulmerton Road in Largo) for a visual inspection, or e-mail a digital photo of your palm to firstname.lastname@example.org.
Morse offers this tip for judging your palm's health: If a frond turns brown and drops quickly (within a few days), that's normal. However, if a yellowing frond hangs on the palm for more than three days, your palm needs to be fed.
Plant native palms. They're resistant to most diseases and are low maintenance. Good choices for our area include cabbage palm and saw palmetto.
Yvonne Swanson is a freelance writer in St. Petersburg and a master gardener for Pinellas County.
How to identify palm diseases
There are many palm disorders and their incidence is growing. Most are lethal, but a few can be treated if caught early. Contact your county extension service for recommended treatments. The following affect a variety of palms.
Fusarium Wilt (fungus)
Symptoms: Begins on lowest leaves and moves up through canopy; brown stripes on stems (petiole) extend into shaft (rachis) of leaf; leaflets on one side of frond die while those on other side remain green.
Treatment: Remove immediately; no cure.
Texas Phoenix Palm Decline
Symptoms: Death of flowers; premature fruit drop; discoloration of lowest or oldest leaves beginning at the leaf tips.
Treatment: If caught early, treat with antibiotic.
Red Palm Mite
Symptoms: Scattered yellow spots on both leaflet surfaces; yellowing of entire frond, especially lower region of plant; premature drop of flowers and fruit
Symptoms: Premature fruit/nut drop; blackening of new flower stalks; yellowing fronds, beginning with oldest.
Treatment: If caught early, treat with antibiotic.
Ganoderma Butt Rot (fungus)
Symptoms: Older fronds droop; new growth is pale green or yellow; shelf-like "conk" fungus protrudes from lower trunk. Fungus degrades or rots the lower 4 to 5 feet of the trunk.
Treatment: Remove immediately; no cure. Fungus remains in soil (don't plant new palm in same spot).
Trunk Rot (fungus)
Symptoms: Scorched black new fronds; oldest leaves wilt and brown, followed by others; trunk weeps black, gummy substance; soft, sunken, black spots on trunk
Treatment: If caught early, there are treatments, but usually disease is fatal.
Palms at risk
Palms typically suffer from one or more of the three leading nutrient deficiencies:
Manganese deficiency affects new growth, causing yellowing and smaller leaves. New leaves can wither and have a "frizzled" look. All palms are susceptible, and the condition can be fatal.
Magnesium deficiency affects older leaves, causing light yellowing at the edges while the center band stays green. Most palms are at risk, but it's rarely fatal.
Potassium deficiency affects older leaves first and then new leaves as the deficiency becomes more severe. You'll see translucent yellow or orange spots on the leaflets. It's the most widespread and serious disorder in Florida palms.
Symptoms can overlap, and treatment can take years to rejuvenate starving palms. Once foliage is damaged, it must be replaced by new, healthy growth. With slow-growing palms, that can mean just one new leaf per month in the warm season and even less in winter.
There are products available to treat the individual deficiencies, but experts recommend regular use of a palm fertilizer specially formulated to provide major and minor nutrients.