Is It Really Rhizospaera? and more U of G research...

January-February 2009

EACH YEAR, I CONSULT with the nursery-landscape industry about their pest management problems and “needlecast diseases of conifers” always comes up as a major pest priority for growers and landscape professionals across Canada. We take these priorities to the annual Minor Use Workshops in Ottawa where we ask the Pest Management Centre (PMC) to conduct trials to help find effective, lower risk pesticide solutions and generate data for the registration of new products.

Last year, the PMC agreed to fund a screening trial for needlecast diseases, and we quickly assembled a cross-Canada team (ON and NS) to do the trials. We found a site that had a good population of Rhizosphaera needlecast on Colorado spruce and we marked out our plots. This project involved several people but in Ontario, Dr. Tom Hsiang (“shung”) from the University of Guelph has been a pivotal resource. Not too long into the field season, Dr. Hsiang and his research team discovered the presence of a fairly new fungus on the symptomatic foliage, Stigmina. Stigmina has also been detected recently in the northeastern United States. Much like Rhizosphaera, Stigmina also produces black fruiting structures in the stomata. But where Rhizosphaera fruiting structures are smooth and spherical, Stigmina fruiting structures are black with tiny little appendages growing out of them (you’ll need at least 20x magnification to see this). Quite often the needles infected with Stigmina are still green and attached to the twig, which poses the question: is Stigmina a pathogen? Well, it is on some other crops but as for spruce we don’t know yet.

When you examine the literature on Rhizosphaera, you’ll question whether the sporulation period has been well documented – “from spring to fall” is what you’ll find. One of the components of the needlecast screening trial was monitoring for sporulation of any fungi. We took several samples of symptomatic foliage throughout the growing season and Dr. Hsiang’s lab meticulously dissected the samples and examined them for disease identification. They found that Rhizosphaera kalkhoffii started producing spores in July, and spores were still present in November. Stigmina spores were present at first sampling in early May and spores were still present in November. These long spore dispersal periods make such pathogens more difficult to manage, because multiple chemical applications may be required to control the diseases.

One would think that newly-emerging growth in the spring would be most sensitive to fungal attack. Which is why it is recommended to use protectant fungicides to reduce foliar diseases during leaf emergence. However, if pathogens are sporulating long after new growth hardens off, chances are they are doing it for a reason. In Sinclair and Lyon’s Diseases of Trees and Shrubs, they state that released spores can survive for weeks on spruce foliage and wait out periods that are not favourable for infection.

Is it possible that Rhizosphaera is able to infect various year’s needles, not just the current season’s growth? One thing we do know is that it takes at least 12 months before symptoms become noticeable on the infected needles. Typically, we see symptoms on growth from two and three years ago but not on last year’s or the current season’s foliage. What does this mean for the management of Rhizosphaera and other needlecast diseases? They could be more difficult to manage than we previously thought.

Tar Spot & Diplodia Tip Blight Research
The University of Guelph research team of Dr. Tom Hsiang, Lynn Tian and Angie Darbyson continued their research on tar spot of maple in 2008 with renewed funding from Landscape Ontario. They made an exciting discovery that tar spot on Manitoba maple is caused by the European species of the fungus, as well as reconfirming the finding from last year that tar spot on sugar maple is also caused by the European species. This is potentially very serious news, since it seems that the European tar spot commonly found on Norway, field and sycamore maples has adapted to native maple species. If this adaptation continues to progress, and the European tar spot becomes as virulent on these native maples as it is on the European species, then industries dependent on native maples may face some new challenges.

Dr. Hsiang’s research group also initiated a study on Diplodia blight of pine with funding from the Canadian TREE Fund and found that the fungus produces spores throughout most of the year. This might help explain why we keep losing the battle against Diplodia tip blight in the landscape. Unfortunately, it also poses a major hurdle for research into the solutions for disease control, since inoculum may be available to start infections almost year round. It seems as though basic tree health and root zone aeration may be our biggest ally in the battle against the disease.

Contact Information

This column is written by Jen Llewellyn, Ontario Ministry of Agriculture and Food and Rural Affairs (OMAFRA) Nursery Crops Specialist

Email any questions you have directly to Jen and we'll publish her response.

P: 519-824-4120 ext. 52671 • F: 519-767-0755

OMAF website:

Nursery Landscape Agriphone: 1-888-290-4441

Our mission is to enhance and promote the care and benefit of trees for present and future generations in Ontario through education, research and awareness.