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Manitoba Agriculture, Food and Rural Initiatives

Bronze Leaf Disease of Poplar

Bronze Leaf Disease (BLD) of poplar is caused by the fungus Apioplagiostoma populi. Infection begins through the leaves and becomes systemic, eventually resulting in dieback of the infected tree. Degree of damage that can be expected is difficult to predict since there does seem to be a difference in the degree of susceptibility depending on the species or hybrid of poplar affected. Many native poplar species are susceptible and many poplar hybrids are also known to be susceptible. Some poplar clones such as Tower poplar and Swedish aspen are known to be highly susceptible to the disease.

Symptoms

Symptoms of this disease first appear in midsummer. Areas of yellowish, orange-brown or reddish brown discoloration begin near the leaf margins. The discolored areas expand and the discoloration moves toward the leaf base.  eins may appear to remain green for a time and the midrib and petiole may remain green. All the leaves on a shoot may have symptoms. As the infection and the season progress the affected leaves become bronze in color or a dark reddish brown color and this symptom gives the disease its name. One reference also indicates a chocolate brown color of leaves as symptoms develop. 

Bronze Leaf Disease of Poplar
Reddish brown leaves, among healthy leaves
 

Reddish brown/bronze leaves
Reddish brown/bronze leaves
 

Poplar species may influence symptoms to some degree.  Symptomatic leaves could be scattered throughout the crown of the tree or confined to a few branches, especially in the first year of infection.  Symptomatic leaves tend to remain attached to the tree throughout the winter.  The fungus moves into the stem and moves systemically in the infected stem resulting in symptom development in other leaves and new developing leaves of growing shoots.  New leaves on infected stems may remain underdeveloped.  Branch dieback occurs and clones or hybrids that are highly susceptible to the disease can decline and die after several seasons of infection.  Trees weakened by BLD will also be more susceptible to other diseases.
Diagnosis of the disease is based on observation of symptoms as well as examination of symptomatic leaves in late summer and again the following spring to look for the spore producing structures of the fungus. In late summer and fall, fruiting structures called acervuli develop on some infected leaves and in the following spring, fruiting structures called perithecia develop on the overwintered leaves. Microscopic evaluation of overwintered leaves would be necessary to confirm the presence of the disease.

Perithecia (pimples) on Tower poplar leaf
 Perithecia (pimples) on Tower poplar leaf
 

Disease Cycle

Although the disease cycle has not been monitored in Manitoba, development of the perithecia in other areas is known to be completed at about the time of bud break. Ascospore dispersal begins when the perithecia are mature. 

Most spore dispersal is believed to occur when daily temperatures are averaging 18ºC. Some moisture in the form of rainfall is necessary for the final development of the fruiting bodies and release of spores to occur. Spore dispersal could begin as early as April and continue through May or June depending on environmental conditions.  In a two year study in Minnesota, more than 60% of the spores were thought to be released during the last week of April and the first two weeks of May. The exact timing of spore release and symptom development is likely to be variable from season to season due to the influences of moisture and temperature.

Because the disease is now known to occur systemically in infected specimens, it is possible that the disease could be spread through propagation procedures in tree nurseries.

 

Disease Cycle

Prevention and Control

Avoid planting shelterbelts or ornamental specimens of highly susceptible cultivars in areas where the disease has been identified. The disease has been identified in the Carman and Graysville area and is likely present in other parts of the Central region of the province. Distribution beyond this area is not yet known. 

Poplar clones Populus x canescens Tower (Tower poplar) and P. tremula Erecta (Swedish columnar aspen) are known to be highly susceptible. 

Hybrids between trembling aspen and bigtooth aspen and between bigtooth aspen and white poplar are known to be highly susceptible. The tree supplier should be able to provide information on the parentage of the type of poplar you are considering. See the box on this page for a list of poplar species known to be susceptible. Degree of susceptibility for many of these species is not known at this time.

There are no registered chemical controls for BLD. Because the disease cycle for this disease has only recently been studied, no strategies for reducing the damage have been developed.

For most foliar diseases, sanitation in the form of removal and destruction of fallen leaves is recommended to reduce the source of inoculum. With this disease, sanitation is likely to be of limited value due to the tendency of infected leaves to remain attached to the tree over the winter and into the following spring. 

General tree maintenance to keep trees vigourous is recommended.  Dead branches should be pruned out regularly.  Pruning to thin the tree canopy where shoots are dense would help to improve air circulation in the canopy and thus reduce moisture and reduce the tendency of dead leaves to collect in the canopy.  Avoid dense planting.  Space trees to allow air flow and reduce accumulation of leaf debris.

Poplars
 

Populus Species Known to be Susceptible


References:

Northover, P.R. and  Desjardins, M. 2003. First report of bronze leaf disease on Hybrid Poplar (Populus x canescens ‘Tower’) caused by Apioplagiostoma populi in Manitoba, Canada. Plant Dis. 87:1538.

Sinclair, W.A., Lyon, H.H., and Johnson, W.T. 1987.  Diseases of Trees and Shrubs. Ithaca: Cornell University Press, 122.

Smith, J.A., Blanchette, R.A. Ostry, M.E., and Anderson, N.A. 2002. Etiology of bronze leaf disease of Populus. Plant Dis. 86:462-469.

For further information, contact your GO representative.