Diseases of both shade and forest trees have the same pathogens, but the trees differ in value, esthetics, and utility. In forests, disease is significant only when large numbers of trees are seriously affected. Diseases with such visible symptoms as leaf spots may be alarming on shade trees but hardly noticed on forest trees. Shade trees with substantial rot may be ornamentals with high value, whereas these trees would be worthless in the forest. Emphasis on disease control for the same tree species thus requires a different approach, depending on location of the tree.
Forest trees
From seed to maturity, forest trees are subject to many diseases. Annual losses of net sawtimber growth from disease (45%) are greater than from insects and fire combined. Young, succulent seedlings, especially conifers, are killed by certain soil-inhabiting fungi (damping-off). Root systems of older seedlings may be destroyed by combinations of nematodes and such fungi as Cylindrocladium, Sclerotium, and Fusarium. Chemical treatment of seed or soil with formulations containing nematicides and fungicides, and cultural practices unfavorable to root pathogens help to avoid these diseases.
Roots rots are caused by such fungi as Heterobasidion (= Fomes) annosus (mostly in conifers) and Armillariella (= Armillaria) mellea (mostly in hardwoods). These fungi cause heart rot in the roots and stems of large trees and also invade and kill young, vigorous ones.
In natural forests, leaf diseases are negligible, but in nurseries and plantations, fungal infections cause severe defoliation, retardation of height growth, or death. Scirrhia acicola causes brown spot needle blight and prevents early height growth of longleaf pine in the South; it defoliates Christmas tree plantations of Scotch pine (Pinus sylvestris) in northern states. Fungicides and prescribed burning are used successfully for control.
Oak wilt is a systemic disease, with the entire tree affected through its water-conducting system. The causal fungus, Ceratocystis fagacearum, spreads to nearby healthy trees by root grafts and to trees at longer distances by unrelated insects. The sporulating mats of the fungus develop between bark and wood, producing asexual and, sometimes, sexual spores, which are disseminated by insects. Control is possible by eradicating infected trees and by disruption of root grafts by trenching or by chemicals.
Stem rust diseases occur as cankers or galls on coniferous hosts and as minor lesions on other ones. A few, such as white pine blister rust and southern fusiform rust, are epidemic, lethal, and economically important. Others of less immediate importance (such as western gall rust) are capable of serious, widespread infection. Resistant varieties are favored for control. Other control measures include pruning out early infections and spraying nursery trees with chemicals during periods favoring needle infection.
Stem infections by numerous fungi, resulting in localized death of cambium and inner bark, range from lesions killing small stems in a year (annual) to gross stem deformities (perennial), where cankers enlarge with stem growth. Chestnut blight, first known in the United States in 1904, destroyed the American chestnut as a commercial species; and is an example of the annual lesion type. The less dramatic or devastating Nectria canker destroys stems of timber value, and is an example of the perennial lesion type.
All tree species, including decay-resistant ones such as redwood, are subject to ultimate disintegration by fungi. Decay fungi (Hymenomycetes) are associated with nondecay fungi (Deuteromycetes) and bacteria. These microflora enter the tree through wounds, branch stubs, and roots, and are confined to limited zones of wood by anatomical and wound-stimulated tissue barriers. The extent of decay is limited by compartmentalization of decay in trees. Trees aged beyond maturity are most often invaded by wood-rotting fungi; losses can be minimized by avoiding wounds and by shortening cutting rotations. Losses from rot are especially serious in overmature coniferous stands in the western United States, Canada, and Alaska. See also Forest pest control; Wood degradation.
Shade trees
Many shade trees are grown under conditions for which they are poorly adapted, and are subject to environmental stresses not common to forest trees. Both native and exotic trees planted out of natural habitats are predisposed to secondary pathogens following environmental stress of noninfectious origins. They are also susceptible to the same infectious diseases as forest trees. Appearance is more important than the wood produced, and individual value is higher per tree than for forest trees. Thus, disease control methods differ from those recommended for forest trees.
The most important and destructive shade tree disease known is Dutch elm disease, introduced from Europe to North America before 1930. The causal fungus, Ophiostoma ulmi (Ceratocytis ulmi), is introduced to the water-conducting system of healthy elms by the smaller European elm bark beetle (Scolytus multistriatus) or the American elm bark beetle (Hylurgopinus rufipes). One or more new and more aggressive strains of the fungus have arisen since 1970. More devastating than the original ones, they are destroying the elms in North America and Europe that survived earlier epidemics. Effective means of prevention are sanitation (destroying diseased and dying and dead elm wood); insecticidal sprays; disruption of root systems; and early pruning of new branch infections. Of much promise are resistant varieties of elm, systemic fungicides, and insect pheromones.
The most common bacterial disease of shade trees is wetwood of elm and certain other species. It is reported to be caused by a single bacterial species (Erwinia nimipressuralis), although causal associations of other bacteria are now suspected. The bacteria are normally present in the heartwood of mature elms and cause no disease unless they colonize sapwood by exterior wounds. Fermented sap under pressure bleeds from wounds and flows down the side of the tree. Sustained bleeding kills underlying cambial tissue. Internal gas pressure and forced spread of bacterial toxins inside living tissues of the tree can be reduced by strategic bleeding to avoid seepage into bark and cambium.
A second bacterial disease of elm, elm yellows, is caused by a mycoplasmalike organism, considered to be a unique kind of bacterium. Elm yellows is as lethal as Dutch elm disease but is more limited in distribution. The pathogen is carried by the elm leafhopper (Scaphoideus luteolus), which sucks phloem juice from leaf veins. Spread of disease also occurs through grafted root systems. Control measures include early destruction of infected trees, disruption of root systems, and insecticidal sprays. Injection with tetracycline and other antibiotics helps to slow the progress of the bacterium.
The most common and complex diseases of shade trees are diebacks and declines (such as maple decline). Many species show similar patterns of symptoms caused by multiple factors, but no single causal factor is known to cause any one of these diseases. Noninfectious agents of shade tree diseases are drought, soil compaction, mineral deficiency, soil pollution from waste or salt, air pollution, and so on. Trees affected experience chlorosis, premature fall coloration and abscission, tufting of new growth, dwarfing and sparseness of foliage, progressive death of terminal twigs and branches, and decline in growth. Such trees are often infested with borers and bark beetles, and infected by branch canker and root rot fungi. Noninfectious stress predisposes trees to infectious disease that is caused by different kinds of weakly parasitic fungi as secondary pathogens. See also Plant pathology.
