Spruce Decline andDiaporthe: Incidence, Taxonomy, Virulence, and Tree Susceptibility in Michigan

In the early 2000s, spruce trees in Michigan began displaying basal needle drop and branch death that slowly progressed upward, symptoms of what we call spruce decline. A survey in 2013 revealed that spruce decline was common throughout Michigan’s Lower Peninsula, and Diaporthe was the most likely pathogen causing the cankers associated with these symptoms. Greenhouse inoculation studies completed Koch’s postulates, confirming that Diaporthe could cause cankers that cause needle loss and branch death. The five different Diaporthe haplotypes isolated from symptomatic branches during the survey differed in virulence. Haplotypes 2, 4, and 5 were more virulent, and differed from each other by only one or two base pairs using the internal transcribed spacer (ITS) region and did not differ using the β-tubulin (TUB) gene. These haplotypes were unresolved phylogenetically. Haplotypes 1 and 3 were weakly virulent to avirulent on multiple spruce taxa, and fell into a resolved Diaporthe eres clade. Spruce taxa varied in susceptibility, with Colorado blue spruce (Picea pungens) the most susceptible, followed by Norway (P. abies), then white spruce (P. glauca). Spruce taxa that were much less susceptible were Black Hills (P. glauca var. densata), Serbian (P. omorika), and Meyer spruce (P. meyeri). We demonstrate that one or more Diaporthe species is causing cankers on declining spruce in Michigan, and these cankers elicit symptoms similar to the branch death expressed by declining spruce in the landscape. Future work will focus on further characterizing Diaporthe to species, and determining biotic and abiotic stressors that may predispose spruce trees to express decline symptoms.

Einfluss von Klima und Baumvitalität auf den Befall von Waldföhren durch rindenbrütende Insekten

Effects of climate and tree vitality on the infestation of Scots pines by bark-dwelling insects The development of insects depends not only on temperature but also on the chemical composition of their host plant. On the one hand, the nutritional quality and in particular the nitrogen content affects insect propagation, and on the other hand, toxic defense compounds impact the colonization by and the survival of insects. The production of such plant defense compounds is restricted under dry conditions. In the Valais, Switzerland, studies on the influence of temperature and drought on bark-dwelling insects were carried out during the last 15 years. The buprestid Phaenops cyanea, the bark beetle Ips acuminatus, and to a lesser extent the bark beetle Tomicus minor showed the greatest potential for damage. They were able to colonize Scots pines within a large range of vitality levels, measured as crown needle loss, but with large regional differences. Infestation levels by these and other bark-dwelling beetles increased with increasing needle loss. The same pattern was found for predatory insects. The intensity of maturation feeding of Tomicus bark beetles depended on crown transparency interacting with tree water supply. The temporal development of infestations by aggressive bark-dwelling insects followed the course of an index of tree drought stress quite closely. In general, infestation levels showed large regional differences. Breeding density of aggressive bark-dwelling insects increased with elevation which can be attributed to decreasing numbers of natural enemies and to reduced resin flow at lower temperatures.

Impact of hemlock woolly adelgid (Adelges tsugae) infestation on xylem structure and function and leaf physiology in eastern hemlock (Tsuga canadensis)

Hemlock woolly adelgid (Adelges tsugae Annand) (HWA) is an invasive insect that feeds upon the foliage of eastern hemlock (Tsuga canadensis (L.) Carrière) trees, leading to a decline in health and often mortality. The exact mechanism leading to the demise of eastern hemlocks remains uncertain because little is known about how HWA infestation directly alters the host’s physiology. To evaluate the physiological responses of eastern hemlock during early infestation of HWA, we measured needle loss, xylem hydraulic conductivity, vulnerability to cavitation, tracheid anatomy, leaf-level gas exchange, leaf water potential and foliar cation and nutrient levels on HWA-infested and noninfested even-aged trees in an experimental garden. HWA infestation resulted in higher xylem hydraulic conductivity correlated with an increase in average tracheid lumen area and no difference in vulnerability to cavitation, indicating that needle loss associated with HWA infestation could not be attributed to reduced xylem transport capacity. HWA-infested trees exhibited higher rates of net photosynthesis and significant changes in foliar nutrient partitioning, but showed no differences in branch increment growth rates compared with noninfested trees. This study suggests that HWA-induced decline in the health of eastern hemlock trees is not initially caused by compromised water relations or needle loss.