Foliar pathogens

There are numerous fungi that cause foliar diseases on both deciduous and coniferous trees in British Columbia. Foliar pathogens can defoliate trees which results in reduced photosynthetic capacity and growth loss. In some cases these pathogens also can damage trees by through branch and stem deformation.

Conifer foliar pathogens

Conifer foliar pathogens can be damaging when trees are found outside of their native range (off-site), grow in dense pure stands, and when favourable weather patterns (cool/warm and moist) occur during certain stages of the disease cycle.

Several conifer foliar pathogens have been termed ‘needle casts’ or ‘needle blights’. The two are differentiated when  a symptom or a visible sign of the pathogen is emphasized:

• Needle cast refers to foliar diseases whose noticeable characteristic include premature needle drop (this is a disease symptom)
• Needle blight refers to diseases where killed needles remain attached with signs of the pathogen visible on the needle (this is a disease sign)

Needle cast or needle blight, either complete or partial, can cause serious problems for conifers as the growth of conifers is dependent on the photosynthetic activity of several years of needle growth. Unlike deciduous trees, any appreciable loss of needles is not followed with a complete replacement of new needles the following year. Any needle loss will have an impact on tree growth and extreme conifer defoliation is typically fatal.

Common conifer foliar pathogens impacting commercial forests include:

• Cedar leaf blight (Didymascella thujina)
• Dothistroma needle blight (Dothistroma septospora)
• Elytroderma needle disease (Elytroderma deformans)
• Pine needle cast (Lophodermella concolor)
• Lophodermium needle cast (Lophodermium spp)
• Larch needle cast (Meria laricis)
• Swiss needle cast (Phaeocryptopus gaeumannii)
• Rhabdocline needle cast (Rhabdocline pseudotsugae)

Deciduous foliar pathogens

Foliar pathogens of deciduous trees are of relatively minor importance when they cause partial defoliation. Complete defoliation is rare and only has great significance when it occurs early in the growing season and over consecutive years. Although defoliation may not result in mortality, it does result in both  reduced growth and reduced resistance to other stress factors.

Common foliar pathogens of deciduous trees include:

• Cottonwood leaf rust (Melampsora occidentalis) 
• Poplar shoot and leaf blight (Septoria musiva)
• Aspen shoot and leaf blight (Venturia macularis)