MSU Extension

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

This year we are experiencing severe Dothistroma needle blight in several regions of the state. My experience is that such damage is seen only every 10 years on average. Austrian pine is the primary species in Michigan where we see this disease which usually turns the foliage of the lower half of the tree brown. Ponderosa and limber pines are also susceptible as is the rare Chinese pine P. bungiana. The latter three trees are mentioned here because we can see damage on the campus trees that has surprised our arborists. A severely damaged Pinus bungeana stands out prominently near the north-northeast corner of the Spartan Stadium.

Characteristic symptoms of Dothistroma infection are presence of needles showing a brown upper (distal) half, green basal half and a single protruding spherical black fruiting body embedded at the junction of the color change. The pathogen forms a toxin, Dothistromin, which diffuses killing tissue ahead of the infection.

Severely damaged Austrian pines have been seen along Michigan’s highway plantings this spring by many in the public. Dr. David Smitley and I have traveled with the Michigan Department of Transportation staff to evaluate the health of these trees. The pines along I-69 southwest of Lansing and Mason exhibited a complex array of interacting problems. Among the damaged Austrian pines, Scots pines showed no damage, drought stress, or other severe malady. The Austrian pines varied greatly in the amount of damage evident, with some trees healthy side-by-side to severely diseased trees. In regards to Dothistroma needle blight, resistance to the disease appears to be present and highly variable within the sources of germplasm planted along the highways within individual groves. Some Austrian pine were healthy, others showed mild spotting and browning, still others had two years of needles infected while still others had dropped one set of needles already and the single remaining set was severely damaged. Several trees had no foliage left on the branches of the windward side. These latter trees, we believed, had received excessive deicing-salt from drifting mists generated by the winter traffic. However, repeated years of Dothistroma infection can permanently denude branches, also.

Other contributing factors included some severe Diplodia tip blight damage on particular trees. Surprisingly we also found that some trees had suffered a recent episode of hail damage which had created multiple resinous wounds on the top surface of branches. Beneath these wounds were cankers of 1-2 inches long that yielded Diplodia cultures. Endophytic, latent infections of Diplodia are known to turn virulent following hail damage, and result in the death of branches.

The Austrian pines also varied in the amount of pitch masses at branch whorls caused by active feeding of the Zimmerman pine moth larvae. David reminded us that Zimmerman generally select trees that are stressed as hosts; and certainly the most damaged Austrian pines appeared to have more pitch masses.

Research knowledge on control of Dothistroma needle blight is obsolete due to decades without funding. Therefore, recommendations for control of Dothistroma still refer to the fixed copper fungicides as being the only effective treatments. The Dothistroma pathogen can infect both old needles and new needles on a pine, which is one reason the needleblight can be so severe and occasionally kill trees. Older needles are generally infected during mid-May and new needles generally in mid-June so applications of fungicides are timed to cover these periods.

Michigan Christmas tree growers did contribute to the newest scientific discoveries concerning Dothistroma needle blight. I collected infected needles from various Austrian pine plantations in Michigan with Jill O’Donnell back in 1999 and took them to South Africa. There, a talented graduate student, now Dr. Barnes, discovered that the world had been wrong in considering the north central states pathogen the same species as that which occurred on Monterey pines in New Zealand and worldwide (our fungicide recommendations come from New Zealand research, sad but true). So recently our pathogen has been given a different name than the more widely distributed species.

More surprising is that Dothistroma needle blight in British Columbia has recently been reported as the first plant disease to become super severe as a consequence of climate change.

Dr. Adams's work is funded in part by MSU's AgBioResearch.