Bulletin 229, Diseases of Citrus in Florida


tative part of the fungus, varies from thin filmy white wefts
with a radiating marginal growth to whitish, creamy, or old
chamois-colored papery layers of extensive development between
the bark and the wood and also permeating the bark. Freshly
dug roots in which the mycelium of the root rot fungus is still
active have a pronounced mushroom or fungous odor. The fungus
frequently causes a whitish decay of the old attacked roots. In
a few instances the lower divisions of the taproots were still alive
and in several instances the distal portions of many of the more
recently attacked lateral roots still remained alive. The majority
of the root infections quite obviously started at points adjacent
to the base of the tree, the disease spreading quickly to other
roots. When the roots become girdled the ends or distal por-
tions soon die regardless of whether they are invaded by the
fungus. In several cases narrow areas of dead bark varying
from a few inches to a foot or more in length extended along the
under side or other portion of the main lateral roots. In some
instances such areas of bark infection had become delimited
by the formation of a callus around the periphery, in which case
the mycelium of the fungus appears to die and the dead bark
disintegrates and sloughs off.
 By the time the root rot fungus has invaded a portion of the
 root crown and sufficient of the roots to interfere seriously with
 the support of the top of the tree, its decline follows rapidly.
 The foliage on the most severely affected branches becomes pallid
 and small and the leaves bleach out along the midribs as is in-
 variably the case with acute starvation induced by any form of
 girdling of citrus trees. The most seriously affected branches
 die back rapidly and the fruit produced on these devitalized
 branches is usually small and frequently drops before reaching
 maturity, due to the rapid decline of the top of the tree. By the
 time the top of the tree has declined considerably basal lesions
 of dead bark, similar to those developed in foot rot, usually de-
 velop at the base and the bark may crack and gum more or less
 freely. Dead lateral roots may be evident in many cases, even
 without scraping away the soil to any particular extent. Such
 was the appearance of the disease in a number of the trees at-
 tacked by mushroom root rot in the grove on Merritt's Island,
 where a few trees declined rapidly. No infected oak or other
 foreign roots were found under the 8 trees treated in this grove,
 although a couple of well rotted pieces of oak root were found