FUNGAL DISEASES
The past few springs in Michigan have been particularly wet and cool. This has promoted all the fungal foliar diseases that affect trees and evergreens. Even trees that have been sprayed with fungicides may be affected, although less severely than if they had not been sprayed. Trees that had been injected with fungicides were also infected, although less than if they had not been injected. Trees which were infected last year but not treated may show worsening this year.
ANTHRACNOSE causes defoliation of sycamores and ashes. White oaks (especially burr oaks) often have lower leaves darkening, curling downward and falling. For most trees this is a nuisance problem with falling leaves, and a cosmetic problem with defoliation and brown dry leaves, but the trees will re-foliate by mid-summer. Fertilization or nutritional injections may help these trees with the stress of defoliation. Fungicidal injections or sprays may help to lessen the problem in the future, but do not totally prevent the problems during a wet spring season.
NEEDLECAST AND TIP BLIGHT in evergreens were more severe and more difficult to control last year. Although fungicide sprays do not completely prevent the problems, trees that have been sprayed should fare much better than trees that have had no sprays or sprays with the wrong fungicides. We have been seeing alot of phomopsis tip blight in junipers in the past few springs, a problem we usually do not see in dry springs. Fungicide applications are made when IPM inspections show any of these fungal diseases in evergreens. NEEDLECAST in blue spruces and pines requires two to three fungicide sprays, starting in May. TIP BLIGHT in pines requires four fungicide sprays beginning in April. We will be rotating fungicides to deal with possible fungicide resistance.
SCAB AND LEAF SPOT DISEASES in flowering trees, especially flowering crabs were severe last year because of the wet spring.
FUNGICIDE SPRAYS can only be successful if applied enough times, at the right times and with fungicides specifically labelled for the disease being treated for. Although many landscape and lawn care companies add fungicide to some sprays, they do not apply the specific fungicides needed nor do they apply them at the specific times necessary for control. Most fungal diseases require 3 to 4 sprays during a 6 to 7 week interval from late April to mid-June.
INSECT PESTS
COTTONY MAPLE SCALE (CMS) and LECANIUM SCALE. These scales will be severe again this year in the Tri-Cities. Please refer to the specific page dealing with scale treatment.
MAPLE PETIOLE BORER is causing many sugar maple leaves to fall this spring. Again, this is a nuisance but not a serious problem for the health of the tree. It is difficult to control and usually does not need control. Once leaves have started falling, there is no reason to spray or otherwise treat the tree. A trunk injection of imidacloprid in the late summer may help prevent the problem the next spring.
TWIG GALLS ON OAKS. Each year we get many calls about gall formations on oak trees. There are literally dozens of different types of galls which form on leaves, petioles (leaf stems), and twigs of oak trees. Most are caused by some type of insect. Perhaps the most persistent and most common cause for customer calls are the twig galls. Twig galls are hardened woody swellings on the twigs of oak trees, seen best when the leaves are not present. They vary in size, with the average being about the size of a golf ball.
TYPES. The two common types seen in Michigan are Gouty Oak Galls and Horned Oak Galls. Both are caused by a type of wasp. The wasp deposits its eggs in the twig and as the larva develops, a swelling of the woody twig tissue develops. Each of these “galls” contains several developing larvae. In the spring, the adult wasp emerges and feeds on the leaves for a time, then mates and lays eggs in twigs and the cycle begins again. The cycle may be two or three years in duration.
NATURAL HISTORY. The old galls never shrink away, but remain present for as long as that branch or twig remains on the tree. Sometimes the twig is killed beyond the gall, but that is uncommon. The galls seldom cause large branch dieback, even when present in large numbers. They are primarily a cosmetic problem which is much less obvious during the summer when leaves are present.
MANAGEMENT. BAD NEWS AND GOOD NEWS. The bad news is that there is no proven way to really control this pest. Sprays could be applied during the adult phase, but would involve spraying large heavily foliated trees with insecticides several times without good evidence that it does much. Trunk injection of insecticides may hold some promise, but would be an expensive way to treat a pest which does not kill or seriously harm the tree. That introduces the good news, that the pest is primarily cosmetic and does not kill the tree even when the tree is heavily infested.
RECOMMENDATIONS. We recommend deep-root fertilization of trees once every two to three years to help maintain vigor and healthy foliage. The galls are far less noticeable in the summer months, especially if the trees have healthy, thick foliage. If branches die, they should be pruned during the months of September through March, avoiding the warm months when the beetle vector of oak wilt is active.
TRUNK INJECTION. Trunk injection may give control of this pest. If a tree has soft scale infestation (lecanium scale), injection will control both the scale and the galls. It is difficult to document and quantify the results on the galls, but it will be easy to document success with the scales.
