JUNE 1, 2011
This has definitely been an unusually cold and wet spring. Because of that we are seeing and will see issues and problems somewhat different from last year, an unusually warm spring. We have observed problems and have fielded calls typical of this kind of weather conditions.
FUNGAL LEAF DISEASES- ANTHRACNOSE, TIP BLIGHT, NEEDLECAST, CRABAPPLE SCAB. Fungal foliar diseases of both deciduous trees and evergreens are more severe in cool wet springs. We see the immediate effects of anthracnose and will see the effects of leaf spot and scab diseases later in the year, even if the trees have been sprayed for those problems. Later in the year and next year we will see more needlecast and tip blight diseases of conifers, even if the trees have been sprayed. Certainly the sprays will lessen the extent and severity, but when weather conditions are cool and wet for an extended period, the fungi can get the upper hand in spite of our efforts. Some specific conditions are discussed below:
Ash Anthracnose. Every year we see some ash anthracnose, characterized by ash leaves falling in May and early June, many of which have blackened areas, twisting and distortion. Defoliation is usually less than 5%. This year we may see 10 to 20% defoliation. This defoliation is mainly in the lower canopy of the tree, whereas defoliation due to ash borers is more severe at the top. There is no treatment available or necessary for this condition.
Anthracnose of sycamore, maple, oaks. Sycamore and oak anthracnose may be severe this year, with many browned and distorted leaves falling and leaving the trees severely defoliated. If this happens year after year, it can cause branch dieback, witches brooming and cankers. We do have trunk injected fungicides that can be useful in preventing the nuisance of spring leaf cleanup and the long term consequences of repeated defoliation, cankering, etc. Maples have significant anthracnose only in severely cool and wet springs, and treatment is rarely necessary.
Tip blight and needlecast of conifers. These fungal diseases can be lessened with timely sprays of specifically labelled fungicides. It is too late to get the early sprays of systemic fungicides needed for tip blight, but needlecast sprays can be effective if done now (early June) and again in late June or early July. Most years the timing for needlecast sprays is mid-May and mid-June in Michigan, but the cold spring delayed the progression of the new growth, so that in the tri-city area, many spruces are just now ready for the first spray.
Scab and leaf spot. Scab of crabapple will probably be more severe this year. If at least two early sprays were applied before the blossom, the severity should be lessened, but even trees that are sprayed may have significant scab this year. Other leaf spot fungi may be more severe, but those usually cause minimal defoliation and are usually only cosmetic problems (e.g., tar spot of Norway and silver maples).