The root pathogen, Phytophthora lateralis Tucker and Milbrath,
continues its destructive spread among Port-Orford Cedars (Chamaecyparis
lawsoniana (Murr.) Parl.) of southwestern Oregon. This
thesis concerns spread of the fungus between root systems of
individual trees in a 49-year-old stand predominantly of Port-Orford-
Cedar. The contribution of root grafts to spread is given special
Root systems of five groups of trees, 36 in all, were hydraulically
excavated and examined for the frequency and character of root
grafting and for the part played by the grafts in disease spread.
Eighty three percent of the trees studied had at least one root
graft and frequently trees were grafted into complexes of as many as
nine trees, functionally uniting widely spaced (39 feet) individuals.
The frequency of root grafting was found to be a linear function
of both horizontal and vertical distances between trees of a pair. If
either distance is increased the frequency of root grafting decreases.
Root grafting is therefore most prevalent in dense stands on level
ground and least prevalent in sparse stands on slopes.
Disease may spread between trees by fungus sporulation on
infected roots followed by reinfection (discontinuous spread) and/or by
vegetative spread between root grafted trees (continuous spread).
The two forms were compared with respect to rate of spread.
Approximately 91 percent of the disease spread in the study trees
resulted from discontinuous spread, while only 9 percent resulted
from continuous spread through root grafts.
An elementary simulation model was formulated to determine
the degree of association of roots of trees separated by given distances.
The model is used to explain disease spread between trees
in the absence of root grafts. Here, an hypothesis, "wash back," is
introduced to explain disease spread upslope by the overlap of infected
and healthy roots.
The following conclusions concerning disease spread emerge
from examination of root systems: (1) disease spread through root
grafts is of minor importance over time as compared with spread by
sporulation and reinfection; (2) root grafts are important in disease
spread in the absence of sporulation and reinfection (vegetative fungus
spread through grafted complexes will result in continued tree
mortality), and (3) disease spread upslope through root grafted trees
becomes less likely as slope steepens. Disease spread between overlapping
root systems of healthy and infected trees can account for
disease spread upslope in the absence of root grafts.