Worried about a sad-looking tree in your yard?
Climate change, invasive species and even international trade are taking a serious toll on California trees. An estimated 150 million trees died during the drought that started in December 2011, according to Smithsonian Magazine, and the stressed trees that survived became more vulnerable to attack by a host of newcomer pests, said Philippe Rolshausen, subtropical tree specialist for the Cooperative Extension office at UC Riverside.
“There are lots of invasive pests everywhere because of global warming and the movement of plant materials in general,” he said.
The trees that shade, cool and feed people from Ventura County to the Mexican border are dying so fast that within a few years it’s possible the region will look, feel, sound and smell much less pleasant than it does now.
Identifying specific tree diseases or pests usually requires an expert, but Rolshausen said three indicators suggest your tree needs help: yellowing leaves, a thinned-out canopy and branch die-back.
If you’re willing to wait, researchers or master gardeners in the state’s county Cooperative Extension offices can help you diagnose a sick tree for free, Rolshausen said.
Professional consulting arborists usually can respond more quickly but charge $200 to $400 for a consultation, said Darren Butler, a Los Angeles-based consulting arborist, horticulturist, landscape designer and cocreator of the GardenZeus.com. When you consider how healthy, mature trees boost property values, that’s a relatively small fee to pay, he said, but people often wait until it’s too late to ask for help. Search for trained arborists through the American Society of Consulting Arborists or the International Society of Arboriculture.
A few yellow leaves aren’t an issue, but if whole parts of your tree have brown or yellow leaves, that’s a sign it needs more nutrients, Rolshausen said. If you don’t fertilize your tree every year or two, it will eventually run out of nutrients, but a sudden big jolt of fertilizer could stress the tree even more, he warned. The best approach is gradual, adding compost and mulch around the drip line of the tree — that is, the place where water drips off the outer branches — so the trees can absorb the nutrients slowly.
Homeowners unwittingly remove the best fertilizer and mulch for trees — their own fallen leaves, Butler said.
“The tree spent so much effort gathering nutrients for those leaves, to remove that leaf litter is devastating to the tree,” Butler said. “Mulch is very, very important for trees, as long as it’s not right up against the trunk. You should have bare soil around the tree, for a distance about three times the diameter of the trunk, so the roots can breathe.”
Mineral deficiencies or pollutants such as salt or even dog urine also can cause problems, especially if your dogs have been peeing around the same tree for years, Butler said.
One option is to send samples of your soil and tree tissue to a diagnostic lab to determine what if any deficiencies they have, Rolshausen said. UC Riverside plant pathologist Marcella Grebus created a list of diagnostic labs around the state, but be sure to call them first to learn how they want samples collected and packaged.
Thinned-out canopies and branch die-back
Healthy trees grow lots of branches and leaves — their canopy — as thickly as they can, to gather all the sunlight they need, Butler said. “Trees feed themselves through their leaves, so when you’re standing under a tree, you shouldn’t see a lot of blue sky. If you see a lot of blue sky, that’s an indication of a problem,” he said.
Another sign of trouble: bare branches sticking out of the canopy,
More likely than not, Rolshausen said, thinning canopy is the result of a soil-born disease such as phytophthora, aka root rot, that’s caused by excessive water.
“Homeowners have a tendency to over-irrigate a tree that’s not doing well, but soil-borne diseases actually thrive in wet soils, so that’s making things even worse,” he said. “Trees don’t like standing water on their root systems because they can’t breathe.”
Sprinklers that constantly hit tree trunks are another contributing factor, Butler said. People tend to plant lawns around trees, then water everything the same way. That’s detrimental to the trees, which need sporadic deep watering instead of the frequent shallow watering most people give their lawns.
“If you see green moss or algae on a tree in Southern California, it’s usually a sign that the tree is being hit by irrigation water,” Butler said. “Trees are not equipped to deal with sprinklers hitting their trunks all the time and it will probably make the tree sick.”
Sprinkler sprays can actually spread the disease to other trees, Rolshausen said; drip irrigation is better for trees. Water slowly and deeply along the drip line of the tree, then make sure the soil dries completely before you water again.
Huanglongbing, aka HLB or citrus greening
One of the biggest threats to orange, lemon and other citrus trees is the Asian citrus psyllid — tiny sap-sucking insects that are spreading a bacteria deadly to citrus trees known as huanglongbing (“yellow dragon disease” in Chinese), which takes about five years to kill infected citrus trees and has no known cure.
Infected trees send up shoots of bright yellow leaves — the “yellow dragon,” Rolshausen said. Eventually new leaves get twisted and mottled and the fruit stops ripening, becoming misshapen and bitter, leading to the other name, citrus greening disease.
Huanglongbing (pronounced wong-long-bing), or HLB, made its first appearance in the United States in Florida, where it has infected more than 80% of the trees and devastated the state’s citrus industry, according to Science Daily. The disease was first spotted in Southern California in the late 1990s and has since been detected in Los Angeles, Orange, San Bernardino and Riverside counties, according to a map prepared by the University of California Division of Agriculture and Natural Resources.
It’s hard to spot HLB in the early stages, because the psyllid that spread the bacteria are so small, Rolshausen said. Mottled, yellowing leaves indicate other problems, he said, so it’s not a reliable way to diagnose an infestation. The University of California Integrated Pest Management Program has tips for spotting the pest. Lots of ants running up and down your tree are one such sign; ants tend to protect the psyllid so they can “harvest” the white honeydew the pests produce. Use ant traps to try to keep them away.
Rolshausen said he uses a slow-release systemic insecticide-fertilizer, such as Bayer Advanced or BioAdvanced Tree and Shrub Protect and Feed, to try to discourage the psyllid. It’s not 100% percent effective, he said, and the brands available to homeowners are lower strength than commercial varieties. An organic option is using a spray of kaolin clay, such as Surround WP, which will give your trees and even vegetables a ghostly appearance but protects against sunburn and deters insects from settling on the plant.