14 BULLETIN 24, U. S. DEPARTMENT OF AGRICULTURE. 
As with all species, the demand of cottonwood upon light varies 
with the amount of moisture in the soil, and occasionally trees sur- 
vive under shade, provided moisture* conditions are exceptionally 
favorable. 
SUSCEPTIBILITY TO INJURY. 
WIND. 
Cottonwood is fairly windfirm, but in exposed situations it is 
likely to suffer from breakage on account of the brittleness of its 
branches. Windfall is common only on rather wet, poorly drained 
sites. where the roots lie near the surface. In well-drained soils cot- 
tonwood sends a rather stocky taproot, well reenforced with spread- 
ing laterals, down to a depth of 4 to 6 feet. 
FUNGI. 
Damage from fungi is not serious in the Mississippi River Valley. 
On unfavorable sites, however, especially in plantations outside its 
natural habitat, injuries from this source are often more or less pro- 
nounced. Of the several diseases, the most common and injurious is 
the “ rust.” caused by a fungus (Uredo melampson meduse Thiim.), 
which is said materially te check the growth of the tree. This and 
many other leaf fungi are likely to do considerable injury to young 
trees in the nursery. In such instances it is advisable to burn the 
diseased leaves at the end of the growing season. Spraying with 
Bordeaux mixture is often effective. 
Of the fungi attacking the wood of cottonwood, Fomes applanatus 
(Pers.) Gill? (Elvingia megaloma (Lév.) Murrill),is one of the most 
noticeable. but seldom gains entrance to perfectly sound trees. It 
attacks both heartwood and sapwood, causing a white rot that weakens 
and sometimes kills the tree. Wounded and fire-scarred trees are 
most liable to such injury. Other fungi attack the twigs and 
branches of cottonwood. These include species of Cytospora and 
Nectria, which cause dead spots or cankers in the bark, resulting in 
the death of branches beyond the point affected. Very little is 
known of some of these fungi, but as a rule they seldom do serious 
damage in cottonwood stands, and expensive measures for combat- 
ing them are rarely justified. The ordinary procedure in case of 
widespread fungous attacks is to cut out all diseased trees and burn 
or remove them. 
The cottonwood in the southern United States is subject to damage 
by the mistletoe, Phoradendron flavescens (Pursh.) Nutt.2 Injuries 
i Heald. F. D. A disease of cottonwood due to Elringia megaloma, Nebr. Agr. Expt. 
Sta. Rept... vol. 19, pp. 92—100, 1906. é 
2Bray, W. L. The mistletce pest in the Southwest. Bureau of Plant Industry Bul- 
letin 166, February, 1910. 
