Tree and Shrub Diseases

for the most part caused by fungi, bacteria, and viruses, as well as factors of a nonliving nature. They cause large losses to agriculture and forestry, sometimes leading to the destruction of trees over large areas.

Woody plants have many defensive adaptations; for example, they are covered with bark, are capable of healing wounds, and can secrete various substances into the air and soil, such as phytoncides, which are dangerous to harmful microorganisms. Pathogens of trees and shrubs usually enter the plant through damage-caused and natural openings-such as stomata and lenticels—and cause pathological changes; this also occurs in herbaceous plants. However, certain changes are specific to woody plants because of the predominance of woody tissue in them and because the course of certain diseases may last many years.

Most dangerous for seedlings are fusarium wilt and ver-ticilliosis, accompanied by wilting, and gray mold. Somewhat older plants (15–20 years) and full-grown trees are often infected with rot, particularly root rot, sapwood rot, and mixed rot, as well as cankers. Root rot causes rapid death of trees or promotes windfall. The pathogens of root rot are most often honey fungus (Armillaria mellea), which causes rotting of the root sapwood (more rarely, of the trunk), and root fungus, which causes central rot of the roots. Mixed rots, most dangerous of whose pathogens is Fomitopsis pinicola, envelop the sapwood and central part of the trunk, causing the rapid death of the trees and promoting windfall. Pathogens which cause cankers can be rust fungi, ascomycetes, and bacteria. The most dangerous disease for coniferous seedlings and young trees is pine twisting rust, which infects needles and twigs. (The pathogen is the rust fungus Melampsora pinitorqua.) In this disease the shoots of the young plant become curved. Pine-twig blight is a very widely distributed coniferous disease.

The shoots of seedlings and young deciduous plants are often infected with rot of the above-ground parts (the pathogen is the fungus Phytophthora omnivora), root rot (the pathogen is the fungus Rosellinia quercina), leaf spots, powdery mildew, and necroses (often caused by ascomycetes—for instance, by Clithris quercina in oaks, by Endoxylina astroidea in ashes, and many other incomplete fungi). Among grown deciduous trees, mycosis of the vessels is often found (the pathogens are the fungi Grophium ulmi, Verticillium alboatrum, and others), as are rots caused by various pore fungi and virus diseases.

General methods of fighting tree and shrub diseases consist in the observance of sanitary and agrotechnical rules in nurseries, cultures, and tree stands. Special methods include chemical defenses (disinfecting the soil and seeds, spraying the planting area, and others), sanitary pruning, treating the tree (by surgery, the use of antibiotics, and so on), and the selection of resistant breeds.