6 
variability in the prevalence of the disease, along with 
the presence of a large number of tree species growing 
in this area. Because of the loss of elms, a greater 
variety of tree species is being planted than previously. 
Our plant disease clinic records indicate that 34 
percent of the confirmed V. albo-atrum isolations oc- 
curred in the month of July, 20 percent in June, 20 
percent in August, 10 percent in September, 8 percent 
in May, 6 percent in October, and only 2 percent in 
March, April, and November. 
For the past several years observations have been 
made of Verticillium wilt symptoms on various tree 
species (Table 3). Variation in symptom expression 
and in extent and speed of decline often occurs among 
the different species. For simplification, the table in- 
cludes the more commonly observed symptoms for 
each of the host species listed. 
SUSCEPTIBILITY OF HOSTS 
A summary of our laboratory records for the past 
42 years indicates that nine tree and three shrub 
genera are most commonly affected by Verticillium 
wilt. V. albo-atrum was recovered most frequently 
from branch specimens of ash, barberry, catalpa, elm, 
magnolia, maple, redbud, Russian olive, smoke tree, 
sumac, tulip tree, and viburnum. 
In recent years strains of V. albo-atrum have been 
isolated that are quite virulent to numerous tree and 
shrub species. One strain isolated from a naturally 
infected pin oak tree (Quercus palustris Muenchh.) 
and inoculated into both pin oak and red oak (Q. 
rubra L.) produced wilt symptoms and internal dis. 
coloration similar to those of the oak wilt disease. The 
pin oak strain appears to be more virulent than maple 
strains in maple, ash, magnolia, and Russian olive. 
However, inoculation tests indicated that the pin oak 
strain was not pathogenic on white oak (Q. alba L.) or 
bur oak (Q. macrocarpa Michx.). In addition, a strain 
isolated from ash and inoculated into vigorous, grow- 
ing trees, such as maple, redbud, and tulip tree, did 
not produce wilt symptoms. However, when this 
strain was placed in Russian olive, typical wilt symp- 
toms were produced in 2-3 weeks. 
Trees susceptible to Verticillium wilt occasionally 
show symptoms of this disease while they are still in 
a weakened condition following transplanting. Since 
there is evidence that V. albo-atrum isolates differ in 
TABLE 2.—Tree and shrub hosts of Verticillium albo-atrum outside of Illinois. 
Host Plant 
Reported as a Host 
United States Other Countries 
Apricot (Prunus armeniaca L.) 
Azalea (Rhododendron spp.) 
Boxwood (Buxus spp.) 
(B. sempervirens L.) 
Cherry (Prunus avium L.) 
(P. cerasus L.) 
Chestnut, Spanish (Castanea sativa Mill.) 
Dogwood (Cornus florida L.) 
Elm, Scotch (Ulmus campestris L.) 
Holly olive (Osmanthus ilicifolius (Hassk.) Mouillef.) 
Horse chestnut (Aesculus hippocastanum L.) 
Judas-tree (Cercis siliquastrum L.) 
Magnolia (Magnolia grandiflora L.) 
Maple, Japanese (Acer palmatum Thunb. ) 
Oregon (A. macrophyllum Pursh. ) 
Painted (A. mono Maxim.) 
Striped (A. pennsylvanicum L.) 
Sycamore (A. pseudoplatanus L.) 
Oak spp. (Quercus spp. ) 
Turkey (Q. cerris L.) 
Osage orange (Maclura pomifera (Raf.) Schneid.) 
Peach (Prunus persica (L.) Batsch.) 
Peony (Paeonia spp.) 
Plum (Prunus domestica L.) 
Canada (P. nigra Ait.) 
Rose (Rosa spp.) 
Tree of heaven (Ailanthus altissima (Mill.) Swingle) 
® Verticillium sp. isolated. 
Widespread (Parker 1959) 
Mass. (Edson & Wood 1936) 
Eastern U.S. (Hutchinson 
1931 )a 
Va. (Harrar 1937) 
Widespread (Parker 1959) 
Widespread (Parker 1959) 
Calif. (McCain 1963) 
Mass. (Holmes 1957) 
Canada (Quebec 1961) 
Cards (Quebec 1961) 
Canada (Quebec 1961) 
Italy (Goidanich 1935), Hol- 
land (van der Meer 1926) 
Va. (Gruenhagen & Fordyce 
1963 ) 
Pa. (Armstrong 1941) 
Italy (Goidanich 1935) 
Calif. (McCain 1963) 
N.Y. (Martin 1926) 
NW states (Bedwell & Childs 
1938), Calif. (Mielke 1935) 
Tenn. (Felix 1955) 
N.Y. (Hibben 1959) 
Canada (Quebec 1961), Italy 
(Goidanich 1934) 
Russia (Krangauz 1958) 
Hungary (Georgescu et al. 
1959) 
Italy (Goidanich 1934) 
Canada (Quebec 1961) 
Conn. (Waterman 1941) 
Widespread (Parker 1959) 
Kans. (Martin 1926; Rogerson 
1957 ) 
Widespread (Parker 1959) Canada (Quebec 1961) 
Canada (Quebec 1961) 
Canada (Quebec 1961) 
Italy (Goidanich 1935) 
