Sphaeropsis collar rot

image of infected red pine seedling

DNR Forestry has experienced two consecutive years of poor red pine plantation establishment due, in part, to a disease caused by Sphaeropsis sapinea. You may already be familiar with this fungus, it was formerly known as Diplodia pinea. It is best known for causing shoot blight on young red pines planted under older, overstory pines in the Lake States. In other parts of the world, Sphaeropsis causes other types of diseases, notably, collar rot. In August of 2002, we found that collar rot was a problem in our red pine seedlings produced in our state Nurseries.

Collar rot is an infection of the bark, cortical and wood tissues of the root collar. Collar rot infections girdle the seedlings which results in the sudden death of the seedlings during the summer. It is common for infected seedlings not to candle out after they are planted. Symptoms at the root collar include: loose bark, blackened cortical tissues, black/dark blue staining of woody tissues, presence of dark resins and presence of fruiting bodies in bark above the root collar.

Red pine seedlings can become infected as 1-0, 2-0 or 3-0 seedlings while still in the nursery. The main sources of infection in the nursery are pine windbreaks. (Windbreak trees become infected as insects carry spores from nearby pines.) Sphaeropsis spores are produced in infected cones and on blighted shoots. Then, wind-driven raindrops carry the spores down onto the bed of seedlings. Shoot blight data from the 1980's show that seedlings more than 600 feet away from a windbreak can become infected. Within the seedbed, dead or blighted seedlings produce spores that can be water-splashed onto adjacent seedlings. This is the main source of infection for a 2-0 and 3-0 seedlings. US Forest Service researchers, Palmer and Nichols, showed that nearby dead seedlings supplied 10,000 times more spores that nearby windbreak pines.

Dr. Glen Stanosz, Univ. of Wisconsin-Madison, has been working on Sphaeropsis diseases of pines since 1991. He recently found that he fungus can persist in live seedlings without producing any disease symptoms. In other words, Sphaeropsis is a latent pathogen. The fungus passively exists in the bark of the root collar. As long as the seedling is vigorous, the fungus cannot produce the disease of disease symptoms. Only when the seedlings are stressed, by having an internal water deficit, does the fungus become activated. Internal water deficits could be instigated by poor stock handling or storage, transplant shock, J-rooting, drought, hail damage or deer browse.

image of tree to seedbed progression in collar rot image of spores spreading within seedbed

Before the DNR ships seedlings, they are inspected and sorted. Only live, healthy-looking seedlings make it out of the Nurseries. Unfortunately, latent infections cause no symptoms so they cannot be detected and discarded prior to shipment.

Both 2002 and 2003 were droughty years, providing more than adequate drought stress which activated the latent Sphaeropsis infections in red pine seedlings. In new plantations, the effect of Sphaeropsis can be distinguished from the effect of drought when red pine seedlings are planted side by side with other conifer seedlings. In 2002, average losses in red pine were 67% ,while in other conifers, losses averaged 12%. Sphaeropsis caused 55% of the losses. The data is not in for the plantation losses during 2003, but results seem to look similar to last year. The DNR is currently assaying the red pine seedlings that would be shippable in 2004 for the incidence of latent infections. DNR Nurseries may purchase red pine seedlings from other nurseries is studies show that the stock has more than 5% latent infections.

To control Sphaeropsis shoot blight and collar rot, seedling infection needs to be prevented in the nursery and seedling vigor needs to be maintained during handling and planting. The best course of action is to completely remove overstory pines in the nursery thus eliminating the possibility of disease spread from infected overstory pines down to seedlings. That is exactly what the DNR Nurseries did last winter. Additional courses of action to deal with existing young stock are: use systemic fungicides to prevent infection, decreased seedbed density, rogue dead and blighted seedlings, and, avoid heavy nitrogen fertilization of seedbeds. When lifting seedlings, nurseries cull all dead, blighted or wounded seedlings then cool down spring-lifted seedlings prior to shipping. All these management actions are being done in our state Nurseries. At the consumer end, seedlings need to be properly transported, stored and handled. Land managers should avoid drought-prone sites for red pine plantations and reduce weed and grass competition. They should also supervise planting crews so that stock is handled to minimize root drying and J-rooting.

Why now? Why Minnesota? Actually, North Dakota, Wisconsin and Michigan state nurseries are currently struggling with this problem. One theory has it that our native "Lake States" Sphaeropsis strain had been ecologically replaced by the more aggressive (and deadly) "World Wide" Sphaeropsis strain. A little forensic pathology work on older plantation survival and lots of research on Sphaeropsis strains and ecology may shed some light on this epidemic.