Diaporthe canker of Douglas-fir

The fungus Diaporthe lokoyae is an ascomycete pathogen responsible for causing stem and branch cankers on various conifer species. It is a parasitic organism that invades the bark and cambium layers of the host, leading to cell death and the formation of characteristic sunken lesions. Under suitable conditions, the fungus produces pycnidia that release infectious spores.

The primary host for Diaporthe lokoyae is the Douglas-fir (Pseudotsuga menziesii). While it can affect trees of various ages, young trees, particularly those in nurseries or newly established forest plantations, are the most susceptible. The disease can cause significant economic losses by reducing the quality of the timber and causing mortality among seedlings.

Key symptoms include the development of necrotic cankers on stems and branches. These areas are often marked by resin flow (pitching), which is the tree's natural defense mechanism. The foliage beyond the canker usually undergoes a color change, turning yellow and then brown as the branch is girdled and the water supply is cut off, ultimately leading to dieback.

Development and spread of the fungus are favored by moist environments and moderate temperatures. The spores are typically dispersed by rain splash or wind, entering the host through wounds, such as those caused by insects, pruning, or weather-related damage. Trees experiencing physiological stress, such as prolonged drought, are significantly more vulnerable to infection.

Effective management requires a combination of silvicultural practices and proactive monitoring. Prevention is focused on maintaining tree vigor through appropriate site selection and thinning to reduce moisture pockets. Infected branches should be pruned and removed from the site to lower the inoculum pressure. In high-value nursery stocks, chemical fungicides may be employed as part of an integrated pest management strategy.