Background On Tree Fruit Diseases And Their Control

Diseases develop when a pathogenic organism encounters a susceptible host in an environment that allows infection to occur. Diseases can be controlled by eliminating any one of the three factors required to complete a disease cycle: pathogen, susceptible host, or conducive environment. Pathogen exclusion is the most common method used to control virus diseases in tree fruit and is often accomplished by selecting and planting only virus-free trees. For other diseases, host suscepti bility is eliminated or minimized by using disease-resistant germplasm or by protecting otherwise susceptible tissue with fungicides or bactericides. Most fungal and bacterial diseases are dependent on rainfall for dissemination and infection, so environmental conditions required for disease development sometimes can be avoided by careful selection of planting sites and cultural management practices that hasten drying after dews and rains.

Integrated pest management (IPM) systems use multiple approaches to minimize pest damage. IPM systems seek to affect the pathogen, the host, and the environment in ways that minimize opportunities for diseases to become established. For example, an IPM approach for controlling fire blight in apples should include alternating small blocks of highly susceptible cultivars with blocks of more resistant apple cultivars so as to minimize potential spread of inoculum within orchards. If infections occur, inoculum for the next year can be reduced by pruning out infections as they appear and by removing cankers during winter. Host susceptibility is reduced by avoiding excessive nitrogen fertilization and by spraying trees during bloom to prevent primary infections of blossoms. Using integrated approaches for managing difficult diseases is usually more effective than depending on a single practice. However, where effective fungicides are available, fungicides alone may provide the most cost-effective way of controlling fungal diseases.

Diseases are easiest to manage if control measures are applied before the pathogen becomes well established in the orchard. Thus, most control measures are aimed at controlling the initial infections in spring. With many fungi and bacteria, pathogen numbers can escalate exponentially with each reproductive cycle. Sanitation measures or pesticide applications that are effective when applied early in the season may be only partially effective when applied to a population in the log phase of epidemic development.

Careful timing of pesticide applications is essential for cost-effective control, especially early in the growing season. Pathogen levels and availability of inoculum cannot be easily assessed with any visual methods, so timing of controls must be based on tree phenology, weather conditions, or models that predict optimum control timing. Later in the growing season, timing of fungicide cover sprays is often adjusted to coincide with insecticide spray timing so as to minimize application costs by controlling both insects and diseases with a single trip through the orchard.

Fungicides are available for controlling most fungal diseases of apples and stone fruit. Although fungicides have been used for many years, fungicide strategies are constantly evolving and changing as new fungicides are introduced and older products are discontinued. New cultivars often have different disease susceptibility patterns than the cultivars that they replace. And new discoveries in plant and pathogen biology continuously contribute to improvements in disease control strategies.

Bacterial diseases occur more sporadically than the major fungal diseases. Bacterial diseases are notoriously difficult to control because bacteria reproduce rapidly when conditions favor their development, and few bactericides are registered for treating fruit trees. Controlling bacterial diseases usually requires a combination of pathogen exclusion, sanitation measures to reduce inoculum levels where the pathogen is already present, and carefully timed antibiotic sprays to protect plants during periods of peak susceptibility.

Viral diseases can be prevented only by exclusion. No pesticides are available for controlling virus diseases in plants.

Many diseases cannot be identified with certainty based on field symptoms alone because various pathogens produce similar symptoms. The pattern of disease occurrence within a tree or an orchard sometimes provides clues that can help with diagnosis. However, accurate disease diagnosis is often possible only by isolating the pathogen into pure culture and identifying it via microscopic examination in the laboratory. This is especially true for pathogens causing summer fruit rots on apples, postharvest decays, cankers, and nondescript leaf spots.

0 0

Post a comment