Stigmina Leaf Spot. Photo by Joseph OBrien, USDA Forest Service, Bugwood.org
I wanted to discuss one that I have seen increase in my area of southcentral Pennsylvania, and that my colleagues have seen in other parts of the state. That is the fungal pathogen Stigmina needle cast of spruce. Rhizosphaera needle cast is also very common, but Stigmina has really increased over the past few years. Colorado blue spruce is the primary host in southcentral Pennsylvania. Like so many of our foliar pathogens, spores are released in the spring when there is new growth. Infection takes place and over time the needles turn a brownish purple color and fall off the tree. This causes thinning of the tree canopy and then death of twigs and branches. If there is no intervention, and spring conditions are conducive in consecutive years for spore production and infection, the tree dies. Fungicides that contain the active ingredient cholorthalonil can be applied in the spring to the new flush of growth. Two applications at minimum are necessary. The first one is when the needles are at 50% elongation. The next one is two to three weeks later. In other words, now is not the time to treat. Stigmina is persistent so treatments are going to be necessary for three consecutive years. (There is limited information on chemical management of Stigmina. You are basically following a Rhizosphaera program, but for a longer time period.) There are other issues that can cause needle drop and branch dieback of spruce - heavy spruce spider mite populations, the fungal canker Cytospora, improper initial planting, inappropriate growing site - but Stigmina can be confirmed by using magnification to check the underside of the needles. Stigmina produces tiny, black spheres with hair-like projections. These are visible in the spring and into summer. Rhizospaera looks very similar, but the spheres lack the "hairs."
The other issue that I have seen more of over the past few years is decline of oaks. I have looked at numerous samples and sent others to the Penn State Plant Disease Clinic, and yet, have not been able to determine an exact cause for many of them. I am inclined to think that one issue is bacterial leaf scorch. This pathogen is moved by immature and adult spittlebugs, tree hoppers and leaf hoppers. Leaf edges turn brown as the bacteria block the vascular system. This makes them look like they are drought stressed. Eventually, twigs, branches and the tree itself can die. There is no cure. One strategy is the use of the product Cambistat (active ingredient is paclobutrazol), which is a plant growth regulator. Use of this material slows vegetative growth and in theory allows the tree to invest this energy into defense chemicals and increased root production, which makes the tree healthier. The other thought that I have had on this "oak decline" is that there is a root pathogen(s) at work. However, I have not had a situation yet where I could sample the tree's roots and send them off for analysis.
by Tim Abbey, Commercial Horticulture Educator, Penn State Extension.