Summer heat and an epidemic of cytospora canker of Alnus

Abstract

An epidemic of cytospora canker [Valsa melanodiscus, anamorph Cytospora umbrina] is associated with dieback and mortality of Alnus incana ssp. tenuifolia in the Southern Rocky Mountains and had begun by the late 1980s. Isolations showed that inoculum was often abundant on bark and bud surfaces even in winter, especially near diseased stems. The pathogen was occasionally isolated from internal tissues of dormant buds and asymptomatic wood and phloem. In infected stems, the pathogen was detected up to 5 cm beyond visible canker margins. The data suggest that the pathogen can cause latent infections, but the infection court remains unclear. Inoculations of healthy stems in the field did not induce canker formation. Fruiting of the anamorph was primarily in late winter and spring, and sexual maturation occurred in late summer and fall. Stem age and growth data support earlier conclusions that the mortality does not represent a steady-state condition with normal attrition of older stems. Canker expansion and killing of branches and stems occurred almost exclusively in the warmest part of summer. We present a hypothesis and supporting evidence suggesting that warm summer temperatures are conducive to the disease. Locally, summer temperatures, and especially maxima, have varied with a dominant oscillation period of approximately 21 years. We speculate that alder populations expand during cool climate phases and shrink during warm phases due to epidemics of cytospora canker. More recently, the oscillation has dampened as an increasing trend of temperature has become dominant, locally and globally. If the trend continues, this already severe epidemic may become more damaging, without intervening opportunities for alder populations to recover.