Leucostoma Kunzei - Disease Cycle

Disease Cycle

Infection is initiated by the pathogen entering into the recent wounds of a susceptible host tree. These wounds can be created by mechanical means such as tools, via insect entry, or in natural wounds due to environmental stress factors such as snow or ice. Generally, most infection is thought to occur in the early spring, although spring my also renew latent infections from the previous season, once environmental conditions are advantageous to the fungus. Typically, unfavorable environmental conditions such as drought, high temperatures, or site specific stresses favor development and infection of this disease. This pathogen can be found in outer bark of what appear to be healthy branches, which indicates latent infection may occur prior to lesion development. Both the conidia and ascospores of this fungus are infectious.

Conidia are released during wet weather conditions of spring, but also throughout summer and autumn. These conidia can withstand freezing temperatures, which aids in survival of this fungus, and conidia germinate at approximately 20-33°C. The optimal temperature for conidial germination and initial growth of the fungus is around 27°C. The ascospores are also released in spring, as well as, early summer. Dispersal of conidiospores and ascospores via running or splashing water provides evidence for the year-to-year intensification and increasing progression of symptoms. Both spore types have also been found dispersed on the wind in the vicinity of diseased trees, yet the means of becoming airborne is still uncertain. There is speculation that this dispersal by air may in fact be due to spore release via rain droplets which become free in air as the droplets evaporate. The conidia are found to be more abundantly dispersed in water and air, in contrast to ascospores. These airborne spores, as well as, insects provide an explanation for the spread of this disease from tree to tree. Once a branch or stem of a susceptible host has been girdled, the pathogen will rapidly colonize large areas of bark beyond the site of girdling. Following this, the pathogen will produce numerous pycnidial and later perithecial stromata. The probable duration of this disease cycle is 1 year, due to pycnidial stromata which form within the first year of lesion enlargement.