Dutch Elm Disease Continues to Devastate Trees Throughout Areas of the Province

By Alexis Warren, Dutch Elm Disease Laboratory Technician, and James Bush, M.Sc., AAg, Manager, Crop Protection Laboratory, Crops & Irrigation Branch, Regina

Dutch Elm Disease (DED) continues to significantly impact native and planted elms trees throughout Saskatchewan, affecting both urban and rural areas. The first confirmed incidence of DED occurring in Saskatchewan was in Regina, in 1981. The one tree was removed and for about a decade, DED was not found. Unfortunately, in 1990 DED was found in Woodlawn Regional Park near Estevan. It was suspected that the disease was brought to the park inadvertently by a camper, who had brought infected elm firewood with them. That winter, about 3,600 trees were removed from the park in an attempt to eradicate the disease and control its spread. By September of 1991, elm trees infected with the disease were found near Carrot River, Rocanville, Grand Coulee and Shaunavon. The initial spread of DED followed river valleys: in the south, the Souris River Valley; in the east, the Qu’Appelle River valley; and in the north-east, the Carrot River valley and Cumberland Delta. To this day, DED is routinely diagnosed and presents a persistent threat to elm populations.

The key symptom of DED is wilting leaves during summer. Flagging leaves can be observed in early July, and as the infection progresses, leaves wilt and turn brown. The fungus travels rapidly throughout the vascular system of the tree and can kill a healthy tree over a single summer. DED is caused by the fungi Ophiostoma ulmi and O. novo-ulmi. O. ulmi is thought to originate from Europe, and O. novo-ulmi, a newer and more virulent species originating in east-Asia, is responsible for most infections in North America. Leaf wilting occurs when fungal hyphae block the tree’s vascular system preventing the flow of water to the leaves, ultimately resulting in tree death.

This fungus is vectored by the native elm bark beetle, European elm bark beetle and banded elm bark beetle. Beetles feed and lay eggs in elm wood, including firewood, and fungal spores stick to beetles and thus travel from tree to tree. The fungus can also spread within nearby trees through root grafts and contaminated pruning equipment.

Human actions can enhance the spread of the disease. Pruning elms during the summer offers attractive feeding sites for bark beetles and provincial regulations under the Forest Resources Management Act prevent pruning from April 1 to August 31. Also, it is believed that the transport of elm firewood is the main way the disease spreads to new communities, and it is illegal to transport elm wood, except to a designated elm disposal site.

Unfortunately, no cure exists for DED, so preventative measures are in place to slow the spread of the disease. In addition to the regulations, the Ministry of Environment, Forest Service monitors DED in rural buffer zones around eight municipalities: Balcarres, Estevan, Indian Head, Melfort, Moose Jaw, Regina, Tisdale and Wolseley. The Forest Service completes annual elm surveys and removals in these buffer zones to slow the spread into the adjacent urban centres.

The Forest Resources Management Act enables communities to appoint municipal DED inspectors to implement the regulations. There are several municipal DED inspectors appointed across Saskatchewan. Preventative fungicides may also be applied on high-value elm trees; however, this is expensive, short-lived and often ineffective.

The Crop Protection Laboratory offers a free DED testing service for any Saskatchewan resident. Samples should be 4-6 pieces, 10-15 cm long and 1-2 cm in diameter, with twigs and leaves trimmed off. Samples must be obtained from symptomatic branches that are not dried out. Once samples are received at the lab, they are first surface sterilized by an ethanol bath, then a ring of bark is removed around each branch using a scalpel to allow for small chips of wood to be cut away. These chips are then plated on potato dextrose agar and stored in an incubator, where warm temperatures favour fungal growth. Positive samples are confirmed through the identification of the coremia on the wood chips using a dissecting microscope. The coremia, which can be seen in Figure 2, is a stalked fungal reproductive structure found in O. ulmi and O. novo-ulmi. Positive samples can be identified within five days, while a negative result is confirmed if no coremia is observed after 15 days in the incubator.

If the tree is positive for DED, it must be removed and disposed of properly. The elm wood is either burned or buried at least 25 cm below the soil surface, with stumps cut to ground level and the option to treat with registered herbicides.

This year, 432 samples have been plated to date with 208 positives (48%). Regina has seen an increased number of positive DED samples compared to previous years. City crews are working hard to cut down infected elms and monitor nearby trees to curb the spread of the disease.

The distribution of DED in the province is illustrated in the 2023 known extent of DED in Saskatchewan map developed by the Ministry of Environment which will be updated in 2025.

Dutch elm disease has established itself as a persistent and pervasive threat to elm trees across Saskatchewan. Since its initial detection in Regina in 1981 and subsequent spread through various regions, the disease has continued to challenge both urban and rural communities. The rapid dissemination of the pathogens O. ulmi and O. novo-ulmi, aided by elm bark beetles and human activities such as the transportation of infected firewood, underscores the complexity of managing this tree-killing fungus. Despite stringent regulations, active monitoring and measures implemented by the Ministry of Environment and local municipal inspectors, the battle against DED remains ongoing. The high incidence of confirmed cases, particularly in Regina, highlights the need for continued vigilance and adherence to prevention protocols. As efforts to combat DED evolve, proactive strategies, public education and ongoing research is critical to mitigate the impact of this devastating disease on Saskatchewan’s elm populations.