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Some of our Street Trees that are Really in Trouble
Nancy Garrison, Urban Horticulture & MG Program Coordinator
Problematic Trees
Tuliptree: Scale and aphids have been raising havoc on tuliptrees throughout Willow Glen in San Jose and beyond for a number of years. The visual cue is the dripping sticky honeydew all over sidewalks and minor to major limb dieback on affected tree. Entomologists from the University of California are currently conducting research to introduce a beneficial insect a parasitic wasp called Metaphycus flavus to help control the scale. It may have some potential, but the results are not yet conclusive. I reported on this in our August 98 MG newsletter. In the meantime, it may be necessary to join together with neighbors and jointly hire a spray company to control the pest where trees are severely affected trees and they're survival is threatened. I can supply the wasp release sites upon request so spraying can be avoided in the release areas.
Sycamore: The sycamores planted as street trees and in home landscapes up until 15 or 20 years ago were almost always seedling Platanus acerifolia (the London Plane tree) rather than one of the two common selected cultivars 'Yarwood' or 'Bloodgood' that have been widely planted since that time. The sycamores get three highly visible and troublesome pests -- Anthracnose, powdery mildew and sycamore scale. The practical solution in most situations is to live with the problem or replant with the most insect and disease resistant cultivar 'Yarwood'.
To really control the powdery mildew found on the mature species sycamores or on the 'Bloodgood' cultivar, you'd have to hire someone with a high powered sprayer to apply fungicides multiple times throughout the spring and summer. Typically, powdery mildew on sycamore appears with the curling of new leaves, which often have a powdery appearance and most troubling, the leaves drop prematurely and continually from late spring through fall. As you've heard before, free moisture on leaves of susceptible plants does not cause powdery mildew, but rather our warm days and cool nights exacerbate the problem. Granted, poor air circulation and shade also make a susceptible plant more vulnerable.
The sycamore scale, again, almost exclusively found on the species and on the California sycamore Platanus racemosa, not on the improved cultivars, causes premature leaf drop for much of the spring and summer. A carefully timed application of a 1 % narrow range horticultural oil at the delayed dormant season (exactly at bud break but before buds open) has proven to be quite effective in suppressing the scale population to tolerable levels. Again, a very high-pressure sprayer is required to do the job correctly, which means a commercial pest control applicator will need to be hired.
If when walking or riding around you see sycamores that look healthy with very big, dark green leaves, you're probably looking at the 'Yarwood' cultivar. To check them out you can see 30 of them planted on my street, Settle Ave. between Willow and Minnesota in Willow Glen. To see the 'Bloodgood' cultivar, drive down Lincoln Ave. in the downtown area of Willow Glen between Willow and Minnesota. 'Bloodgoods' are resistant to Anthracnose but susceptible to powdery mildew.
Eucalyptus: There is a very new lerp-forming Redgum psyllid, Glycaspis brimblecombei, which has been found on a number of eucalyptus species. The psyllid forms a "lerp" which is a conical secretionary structure produced by the nymph as a protective cover, which looks like a scale. It will reach a size of approximately 3mm in diameter and 2mm in height. The nymphs are yellow, or yellow and brown in color.
Currently the psyllids are causing production of a lot of honeydew, which is staining the ground underneath infested eucalyptus trees and causing very severe leaf drop with new growth immediately being attacked. Heavy populations and leaf drop on the red gums is forcing the psyllids off onto other species of eucalyptus that are not usually attacked (according to published literature).
The possibility exists that the trees will be weakened enough through the psyllid attack to invite infestation by the eucalyptus longhorn beetles (Phoracantha). There are, however, a number of ladybird beetles that have been found feeding on this psyllid including Harmonia axyridis, Chilochorus bipunctulatus, and Hippodamia convergens, but the resident populations are not providing adequate control. At this point the introduction of parasites is probably warranted. Dr. Don Dahlston at U.C. Berkeley has been apprised of the situation and may pursue the parasite exploration phase as he has done with other exotic Australian psyllids including the Eugenia psyllid and Bluegum psyllid.
Manmade conditions contributing to street tree problems
Most often, when sidewalks, streets and curb repairs are made, tree roots are cut and often quite severely. Unless great care is taken during these processes, to minimize the cuts and root disruption, trees may experience branch die back and/or overall poor growth. When I see repeated sidewalk repairs near large street trees I often observe subsequent tree decline within three years to the point where the tree becomes a hazard and must be removed.
Carefully thinning of the tree canopy to compensate for damaged root systems can enable the tree to better recuperate. The recommended pruning method uses a thinning cut which removes the limb at it's point of attachment rather than a heading cut, which indiscriminately chops off a branch midway along it's length.
A certified arborist, who understand the difference between the two types of cuts and is able to maintain the form of the tree so that you can hardly tell it's been pruned is highly recommended for any important tree pruning. Always ask for local references and check the work they've done to see if it shows the level of skill you desire.
A well pruned tree will have far fewer problems in subsequent years and pay off by requiring less repeat work and reconstructive surgery. Refer to our certified arborist list to consult with an expert.
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