Diseasecausing Pathogens

Biotic diseases are the result of infection of a host plant by a pathogenic or disease-causing agent under environmental conditions favorable to the pathogen. The most common disease-causing pathogens important in orchard ecosystems include bacteria, fungi, and viruses. Disease severity is dependent on a number of factors, including tree cultivar, vigor and maturity, environment, and soil texture and quality. Generally, symptoms of pathogenic infection first appear in the region where the tree has been infected, such as bacterial and fungal infections associated with foliage, blossoms, fruit, and twigs. Such infections result in localized lesions on foliage and fruit. Trees are especially vulnerable to infection during bloom because blossoms provide an excellent entry way for infection, leading to wilt of blossoms and blossom stems. Foliar infections are important because photosynthesis declines due to destruction of leaf tissue; degeneration of chloroplasts due to infection also can lead to lowered levels of photosynthesis. Toxins associated with pathogenic infection can interfere with enzymes involved in photosynthesis. Further more, pathogenic infection of leaves by bacteria and fungi destroy foliar cuticle and epidermis, leading to increased transpiration rate and uncontrolled water loss, resulting in wilt, unless compensated by water absorption and translocation. The trunk and branches are a third area where pathogenic infections caused by fungi or bacteria commonly occur. Infection leads to canker formation that causes decline in food and water transport as cambial cells are destroyed, resulting in reduced growth, wilting foliage, and loss of crop and/or yield if fruit wood is destroyed. Infections in a fourth area, the root system and/or crown, by soilborne fungi or bacteria disrupt translocation of water and nutrients from soil. Here, the first obvious symptoms are not necessarily at the site of infection but instead are above ground. Some common symptoms include poor shoot growth, yellow foliage, dieback of terminals, and decreased productivity. Often these infections are confused with water-related stress. Bacterial or fungal infections of fruit itself can lead to economic losses, as fruit can be destroyed due to rot. Viral infections are generally insect vectored or spread from grafting and budding with infected rootstock or scion wood. The infection moves rapidly through the phloem, often leading to phloem degeneration and problems with transport of organic molecules produced in foliage following photosynthesis. Symptoms of viral infection include yellowing of foliage along leaf veins, premature leaf drop, and reductions in growth, yield, and fruit quality. Ultimately, physiological responses induced by pathogenic organisms infecting deciduous tree fruit can affect size, shape, appearance, and overall quality of fruit; induce distortions of leaves and premature leaf drop; cause dieback; reduce tree vigor; and lead to death of trees.

0 0

Post a comment