TREE AND SHRUB HOSTS 
OF VERTICILLIUM ALBO-ATRUM 
VERTICILLIUM WILT, caused by Verticillium albo- 
atrum Reinke and Berth., has become increasingly 
prevalent among plantings of ornamental trees and 
shrubs in Illinois. Persons not familiar with Verti- 
cillium wilt symptoms frequently attribute plant deaths 
resulting from this disease to other causes, such as 
transplanting shock and winter injury. Because this 
fungus affects a wide range of woody ornamental 
plants, it is difficult to estimate the economic loss it 
causes to nurserymen and homeowners. Rankin (1914) 
first reported the disease on maple in the United States 
and identified the fungus as a species of Acrostalagmus. 
Zimm (1918) later proved pathogenicity and identi- 
fied the fungus as a species of Verticillium. A mono- 
graph on the disease by Rudolph (1931) lists the 
known host range up to 1931. Engelhard (1957) pub- 
lished a host index for V. albo-atrum that included 
both herbaceous and woody plants. 
NEWLY IDENTIFIED HOSTS 
Routine isolations have been made at the Illinois 
Natural History Survey from branch samples of dis- 
eased trees and shrubs since 1926. During the last 42 
years V. albo-atrum (including V. dahliae Kleb.) has 
been obtained from several hosts growing in widely 
scattered areas of the state. Table 1 shows the woody 
hosts reported for Illinois by 1956, including 30 tree 
and shrub species and varieties (Carter 1938, 1940, 
1945; Engelhard & Carter 1956). Since the last of 
these reports was published, 26 additional species and 
varieties of woody ornamentals, also listed in Table 1, 
have been identified as hosts in Illinois. Twenty-two 
of the plant species or varieties, identified at the Illi- 
nois Natural History Survey and listed in Table 1, are 
believed to be previously unrecorded hosts in the 
United States, as no records of these hosts were found. 
However, at least 3 of these 22 hosts have been re- 
ported in other countries. A few of the new Verticil- 
lium hosts reported here have been mentioned in re- 
ports which were primarily field survey reports. Where 
there is no indication that the fungus was isolated 
from infected wood and positively identified, the field 
report usually is not cited. 
PATHOGENICITY TESTS 
Since 1957 Verticillium isolates from various hosts 
have been tested at the Illinois Natural History Survey 
for pathogenicity to the respective host plant. Most of 
E. B. Himelick 
the trees and shrubs were available in our research 
arboretum. Occasionally certain species were not 
available and had to be purchased as seedlings. These 
seedlings were established in pots in our greenhouse. 
Pathogenicity was tested by inoculating 5-20 plants 
of each species or variety, and an equal number of 
uninoculated plants was used as a control for each 
species. Each plant was inoculated by placing a drop 
of spore suspension in a ehisel wound made in the 
base of the trunk. An attempt was made to reisolate 
the fungus within 7 days following the initial appear- 
ance of foliage symptoms. External symptoms were 
produced 2 weeks to 3 months after inoculation on 
all species except silver maple. Silver maple did not 
produce typical foliage symptoms, but did have 
scattered faint green streaks in the twigs 4-5 feet 
from the point of inoculation. The fungus was re- 
covered from the discolored area 2 months after inoc- 
ulation. 
Pathogenicity was established for 18 of the 22 plant 
species listed in Table 1 as new hosts in the United 
States. Pathogenicity was established for an addition- 
al four plant species that were newly discovered hosts 
in Illinois and for an additional eight species from 
which V. albo-atrum had been isolated but for which 
proof of pathogenicity had not been established. 
HOSTS OUTSIDE OF ILLINOIS 
Table 2 lists the Verticillium tree and shrub hosts 
identified in addition to the 1957 list of Engelhard and 
those listed for Illinois in Table 1. Most of these hosts 
have been reported since 1957. Engelhard’s list and 
Tables 1 and 2 include most of the world’s known 
Verticillium wilt hosts among trees and shrubs. 
VERTICILLIUM WILT SYMPTOMS 
Some tree and shrub species tend first to show 
foliage symptoms during a limited period of the grow- 
ing season; other species have symptoms appearing 
at any time throughout the growing season. The times 
when plant growth begins and ceases are variable in 
Illinois, since the state has three climatic zones 
(Rehder 1940:ii,xii). In Illinois, Verticillium wilt ap- 
pears to be most prevalent in trees and shrubs in the 
area that demarks the line between climatic zones IV 
and V and appears progressively less prevalent on 
both sides of this imaginary line. Both temperature 
and moisture may be factors that contribute to the 
