14 Regulatory Bulletin No. 163 *
All restricted plants imported under the conditions listed above
are limited in size and age to the youngest and smallest which can be
successfully freed from soil about their roots, transported to the
United States and established in this country with a reasonable degree
of success. Certain classes of plants permitted entry under quarantine
37 are required to be grown by the importer under post entry inspection
regulations. Such plants are not released to the trade until such time
as their freedom from plant diseases and insect pests has been estab-
lished. The plants are therefore grown for one or more years in a
place where the state inspector may have access to them for inspection
purposes for such time as appears necessary. When their freedom from
pests and diseases has been established, the plants under quarantine
are released.
OAK WILT _
Oak wilt disease (Endoconidiophora fagacearwm) is now well estab-
lished in the woodlands of Kentucky. The disease is caused by a fungus
organism that can be identified by plant pathologists in one to two
year old vascular tissue from infected trees.
Varieties of the red and black groups seem to become infected with
oak wilt more readily than white and burr oaks, although all species
and varieties of oaks are susceptible to the disease.
The first symptoms in the red and black oaks are shown by the
appearance of leaves on the upper branches. They show dull light green
color and curl upward. Later the leaves may turn yellow or reddish
brown before falling. All leaves may fall within a month after first
symptoms occur. In white and burr oaks the disease develops more
slowly with one or more branches near the top showing disease symptoms
first.
Spread of the disease from diseased to healthy trees within native
stands of oaks can occur through natural root grafts or unions. During
recent years it has been proven that certain insects are capable of
carrying the disease from tree to tree and that even squirrels might
possibly spread the disease. '
As the oak wilt fungus develops under the bark of infected trees,
fungus cushions or mats are formed. These mats enlarge and thicken,
thereby creating sufficient pressure to crack the bark and separate it
from the wood. As soon as the cracks are formed they are invaded by
several species of sap beetles known as Nitidulids. These beetles, as
well as the common fruit flies, are attracted by the characteristic odor
of the fungus. After crawling over the fungus mats and becoming contami-
nated with spores of the fungus, the insects move on to other trees and
wherever there is avnund in the tree the contaminated insect is capable
of bringing the spores of oak wilt into contact with the sap wood of
uninfected oaks, thereby starting`new infections. ·
There seems to be some association between the long distance spread
of oak wilt and the activities and travel of man since so many of the