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.

Biomass and dynamics of attached dead branches in the canopy of 450-year-old Douglas-fir trees

The amount, physical characteristics, and spatial distribution of attached dead branches in the canopy of 450-year-old Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco var. menziesii) trees were studied over a 5-year period (1998–2003) to quantify their contribution to the canopy woody detritus pool of an old-growth Douglas-fir – western hemlock (Tsuga heterophylla (Raf.) Sarg.) forest in the western Washington Cascades. We developed a five-class decay rating for attached dead branches. From the size distribution and relative amounts and vertical distribution of attached dead branches in the five decay classes, we inferred that death of large original branches had occurred in the recent past, followed by the production and death of epicormic branches. Tree height was an important variable for predicting branchwood dry mass per tree. We estimated that attached dead branches of Douglas-fir contributed 63.8% of the total canopy woody detritus pool of the stand. During the study period, 0.24%·year–1 of live branchwood mass died attached, and 0.37%·year–1 and 4.34%·year–1, respectively, of live- and dead-branch mass were lost due to branchfall and fragmentation. Because branch death and branchfall are stochastic processes, long-term monitoring in the canopy and on the ground is needed to understand the dynamics of canopy woody detritus.

First Report of Occurrence of Sclerotinia Blight on Petunia in Louisiana

Petunia × hybrida Wave series cultivars were observed with symptoms of tan to brown stem lesions, wilt, and branch death in a demonstration/trial planting at Burden Research Center in Baton Rouge, LA, during January and February 2000. Disease signs included the presence of white, cottony mycelia on infected stems and the presence of black sclerotia. Seventeen of 131 plants were infected on 20 February and included individuals of cvs. Wave Purple, Wave Rose, Wave Misty Lilac, Wave Pink, Tidal Wave Hot Pink, and Tidal Wave Cherry. Isolations were made by placing diseased stem sections on acidified potato-dextrose agar. A fungus that produced white mycelia and black sclerotia was consistently isolated from infected stems and identified as Sclerotinia sclerotiorum. Pathogenicity tests were done by pipetting 1 to 2 ml of blended mycelia and sclerotia (one plate culture blended in 100 ml distilled water) at the base of flowering-age Wave series plants. Plants were held for 3 days in a dew chamber maintained at 22°C and then moved to a greenhouse held at 25°C. Wilt and branch death developed on inoculated plants after 5 days and S. sclerotiorum was reisolated. Uninoculated plants remained healthy. Wave series cultivars have a prostrate growth habit that is conducive to disease development. No plants in the trial planting were killed and infected plants had recovered by 1 May and had uniformly covered the plant bed. Sclerotinia blight was previously reported on Petunia × hybrida from Bermuda (2) and Florida (1). References: (1) D. F. Farr et al. 1989. Fungi on Plants and Plant Products in the United States. The American Phytopathological Society, St. Paul, MN. (2) J. M. Waterston. Dept. Agric. Bermuda Rep., 1947.

First Report of Web Blight on Verbena Caused by Rhizoctonia solani

Web blight was observed on verbena (Verbena × hybrida) during July 1999 in a cultivar trial planting at Burden Research Plantation in Baton Rouge, LA. Foliage blight, stem lesions, and branch death were common symptoms on 12 of 24 cultivars in the trial. Plant death occurred in cvs. Babylon Florena (one of four plants), Purple Princess (two of four plants), and Taylortown Red (two of four plants). Isolations from infected leaves and stems on acidified water agar consistently yielded a fungus with the mycelial and cultural characteristics of Rhizoctonia solani. Pathogenicity tests were carried out by placing 5-day-old fungal mycelial plugs, grown on acidified potato dextrose agar, at the base of healthy verbena stems and holding plants in a dew chamber at 26°C. After 3 days, foliage blight and stem lesions appeared on inoculated plants, and plants were moved to a greenhouse where temperatures ranged from 23 to 32°C. Seven of nine inoculated plants died after 7 days; noninoculated plants remained healthy. The fungal pathogen was reisolated from all inoculated plants. The fungus was identified as R. solani anastomosis group (AG)-1 IB based on multinucleate condition, type of sclerotia produced, and ability to anastomose with R. solani tester isolates of AG-1 IB. This is the first report of web blight on verbena.

Pathogenicity of Bursaphelenchusxylophilus on three species of pine

Three species of 11-year-old pine trees were inoculated with Bursaphelenchusxylophilus in the field. Four branches in single whorls on red, Scots, and jack pine trees were wounded and inoculated with 10 000 nematodes each or with water extracts from Botrytiscinerea cultures. Prior to field inoculations, the pathogenicity of the nematode isolate was confirmed on seedlings in the greenhouse. Fourteen weeks after inoculation, 27 of 80 and 13 of 52 branches were dead or dying on Scots and jack pine trees, respectively. No symptoms were observed on red pine trees inoculated with B. xylophilus or on any controls. Branch death was attributed to the formation of girdling cankers resulting from inoculation. An average of 9.14, 10.39, and 0.02 nematodes were extracted per gram of wood from branch samples collected from Scots, jack, and red pine trees at 14 weeks, respectively, and at 58 weeks an average of 13.82, 1.01, and 0.05 nematodes per gram of wood sampled were recovered. Proportions of branch samples with nematodes declined from 14 to 58 weeks after inoculation. Although limited mortality of branches occurred, the pine wood nematode was not found to cause tree death following inoculation.

Groove formation in the stem of red pine associated with branches

Annual changes in the radial and longitudinal extent of grooves above and below branches on the stem of a 40-year-old red pine (Pinusresinosa Ait.) were determined by examination and measurement of annual rings on cross sections. Grooves appeared above branches when they were in the fourth whorl and maximum vertical extent occurred about the time of branch death. Ring widths within grooves compared with those between grooves decreased slowly at first, then more rapidly, reached a minimum the year after branch death, and then increased. Below branches, groove initiation was later, far less extensive, and minimal within-groove compared with between-groove ring widths occurred the year of branch death. The course of groove development appeared to be associated with the changing vigor of branches and with the nature of the junction between stem and branch. It is suggested that groove development above branches in red pine is the result of localized water deficiency, whereas groove development below branches is related to reduction in photosynthate availability.

A Data Structure Analogue for Modelling Tree Crowns

A rooted-tree computer data structure is used as an analogue of coniferous tree crowns in building a simulation model of a single tree. Submodels of branch death and the allocation of stored reserves are used to illustrate the model. The programming language used is PL/1.