Distribution of a Foliage Disease Fungus Within Canopies of Mature Douglas-Fir in Western Oregon

Abstract

Nothophaeocryptopus gaeumannii is a common native, endophytic fungus of Douglas-fir foliage, which causes Swiss needle cast, an important foliage disease that is considered a threat to Douglas-fir plantations in Oregon. Disease expression is influenced by fungal fruiting bodies (pseudothecia), which plug the stomata and inhibit gas exchange. Trees are impacted when pseudothecia plug stomates on 1-year-old and older needles resulting in early needle abscission. Mature (100 years+) trees appear to be less impacted from disease, and we hypothesize this is due to the greater emergence of pseudothecia on older than younger needles, which allows for more needle retention. We measured the density of pseudothecia occluding stomates across 2- to 5-year-old needles from upper, middle, and lower canopy positions of mature trees at three sites in the Oregon Coast Range and two sites in the western Oregon Cascade Mountains. Binomial generalized linear mixed model (GLMM) was used to test for the effects of canopy position (upper, middle, and lower), sites, needle age (2-5 years old), and years (2016 and 2017), and their interactions on the pseudothecia density. Pseudothecia density varied annually depending on sites, needle age and canopy positions. Pseudothecia density peaked on 3-, and 4-year-old needles, however, needles emerging from the same year, like 2-year-old needles in 2016 and 3-year-old needles in 2017 both emerged in 2014, had consistently similar patterns of pseudothecia density for both years, across site and canopy positions. Canopy position was important for 3-, and 4-year-old needles, showing less pseudothecia in the lower canopy. This research confirms that N. gaeumannii pseudothecia density is greatest in 3- and 4-year old needles in mature trees in contrast to plantations where pseudothecia density usually peaks on 2-year-old needles, and that pseudothecia density (disease severity) is generally lower in mature trees. Something about mature forest canopies and foliage appears to increase the time it takes for pseudothecia to emerge from the needles, in contrast to younger plantations, thus allowing the mature trees to have greater needle retention.

Keywords: Douglas-fir; Nothophaeocryptopus gaeumannii; Swiss needle cast; foliar pathogen; fungal disease; tree canopy.

FIGURE 1 |

Photo of pseudothecia blocking stomates on the underside of a Douglas-fir needle.

FIGURE 2 |

Map of study sites in the Oregon Coast Range and western Cascade Mountains of western Oregon, United States. Cascade Head, Falls Creek, and Moose Mountain are in the long-term ecological monitoring plot (LTEM) system, and Klickitat Mountain and Woods Creek are not part of the LTEM system. Cascade Head, Klickitat Mountain, and Woods Creek are in the Siuslaw National Forest. Falls Creek and Moose Mountain are in the Willamette National Forest.

FIGURE 3 |

Average winter (December, January, and February) mean temperature (lines) and average summer (May, June, and July) precipitation (bars) of each study site from 2010 to 2017.

FIGURE 4 |

Swiss needle cast average disease severity (incidence × density) for 2-year-old needles in young nearby plantations, and for 2, 3, 4, and 5-year old needles in mature stands. Two years and three canopy positions were averaged at each site.

FIGURE 5 |

Pseudothecia density by site for 2- to 5-year old needles in 2016 and 2017 from mature Douglas-fir trees in western Oregon. Sites listed on the X-axis are from coast (left) to inland (right). Different letters represent that the two groups are statistically different.

FIGURE 6 |

Pseudothecia density by canopy position for 2- to 5-year old needles in 2016 and 2017 from mature Douglas-fir trees in western Oregon. Different letters represent that the two groups are statistically different.