(Slides shown.)
Diseases Affecting the Success of Tree Crop Plantings
G. F. GRAVATT and DONALD C. STOUT
Division of Forest Pathology, Plant Industry Station, Beltsville,
Maryland.
Mass plantings of many trees of the same kind frequently result in an increase in the severity of insect pests and diseases. Leaf diseases, for instance, spread quickly through such plantings when weather conditions favor growth of the causal organisms. Plants on sites unfavorable to a specific tree species also are responsible for disease increases. Chinese chestnuts grown on a site where they are subject to early-fall and late-spring frosts will fail. Not only will crops be reduced by the killing of buds or blooms, but the twigs, or even whole trees, may be killed by freezing. The blight fungus develops rapidly on such injured trees and may mislead people into thinking that the blight fungus is the primary cause of the killing.
Still another factor that determines the damage by diseases, and thus the success or failure of nut tree plantings, is the ignoring of soil and fertilizer requirements. Trees weakened by drought, because they are on a site having a soil too shallow for good root growth, are much more subject to attack even by weakly parasitic fungi than those growing on a site with deeper soil. Innumerable dying twigs and branches with fungi growing on them are sent to the U. S. Department of Agriculture or State experiment stations with requests that the disease be identified, when the real trouble is lack of water for the roots. Weak trees are much more subject to winter injury than vigorous ones.
Trees require a good supply of plant food materials and water to produce profitable crops. Tho heaviest bearing chestnut trees we have observed were grown in an irrigated orchard in California and in a poultry yard in the East where chicken droppings actually formed a mulch under the trees. However, if you wish to kill a young chestnut tree quickly, just apply a very heavy application of chicken manure; the point is that trees must become adjusted to chicken manure by gradual applications.
Another way to damage a tree is to keep it growing late in the fall by cultivation and fertilizers so that it does not harden off properly. Many plantings, representing heavy investments, fail because of lack of organic matter in the soil. This is related to water-holding and water-supplying capacity of the soil, and lack of proper fertilizer. Dr. Harley L. Crane and his assistants, in their work with tung and pecan trees, have shown the vital need for certain elements on some soils. Trees weakened by the lack of these elements are early prey for some diseases. The element most frequently deficient is nitrogen, but sometimes boron, copper, or iron is lacking; or the elements are not in balance, because of the excess of some, or the lack of others.
By adjusting the various soil, water, and site factors necessary for a continuous, vigorous growth of trees, many so-called disease conditions are eliminated. Many fungi and viruses, however, will attack trees in the pink of condition; a few of the more important of these are treated in the following sections.
+Chestnut Blight+