Morphological, genetic, and host range variability in Phomopsis, a pathogen associated with spruce decline in Michigan

Background/Question/Methods

Recently, landscape spruce trees in Michigan have begun expressing multiple disease symptoms, including needlecast, tip blight, and branch death. We are collectively calling this affliction spruce decline. While traditional spruce diseases can cause these symptoms, this recent epidemic is different for the following reasons: it affects both juvenile and mature trees, disease severity is high (up to 50 percent of landscape spruce trees in some locations), and the fungal species frequently associated with disease symptoms are not commonly isolated from landscape spruce. The fungal species most commonly isolated from trees with this emerging disease has been Phomopsis sp. To investigate the role of Phomopsis sp. in spruce decline, the morphological characteristics of 31 isolates collected from spruce trees throughout Michigan were compared. Genetic variability tests were performed on these isolates using internal transcribed spacer (ITS) sequencing. Phomopsis occulta (ATCC 200761) was used as a cross-reference, as earlier reports suggested this species caused disease on nursery spruce trees in Michigan and Wisconsin. Correlations were assessed comparing ITS groups with morphological characteristics such as fungal color, growth rate, geographic location of the isolate, and the tree species from which the isolate was obtained.

Results/Conclusions

A total of five ITS groups were found based on substitutions and insertions at 11 variable positions from the 600 base pair sequences. ITS group 1 represented 19.4% of all isolates, group 2 represented 12.9%, group 3 represented 19.4%, group 4 represented 22.6%, and group 5 represented 25.8% of all isolates. Phomopsis occulta (ATCC 200761) matched ITS group 3 sequences. These results suggest that more than one species of Phomopsis may be associated with spruce decline. No significant correlations were found between ITS sequence and fungal color, growth rate, geographic location, or the tree species from which the isolate was obtained. Low sample size could have played a large role in the results; therefore, future research will include more extensive, stratified sampling throughout Michigan. The five Phomopsis groups are being tested to determine if they differ for host range. Microsatellites are being used to assess population level genetic variability, and these data will be cross-referenced with the ITS sequences obtained in this study.