Disease Prevention and Control

The disease triangle

Three factors must be present for a disease to develop over time: 1) Pathogen – the agent which causes the disease; 2) Environment – the ideal growing conditions for the pathogen; and, 3) Host – the plant/soil on which the pathogen lives. 

Healthy plants are less susceptible to infection

Promoting proper plant growth and development reduces the potential for diseases to develop.  

Research prior to selecting plants aids the gardener with proper selection.  Choose plants that are naturally less susceptible or specially bred for disease-resistant qualities.  Some plants are more susceptible to infection, and others naturally exhibit very few disease problems.  Incorporating native plants into the garden reduces the chance for disease development in respective areas.

Follow recommended planting directions by placing plants in a suitable location for proper growth and development.  For example, shade-loving plants should be planted in shady areas, and sun-lovers in sunny locations.  Transplant at suitable times of the season and make sure moisture requirements are adequately met.  Some plant roots do not thrive in soggy soils while others prefer wet conditions.

The gardener should allow plenty of space between plants throughout growth and development.  Crowded stems and plants reduce airflow, allowing moisture to remain on the plant and soil too long.  Combined with a lack of sunlight, wet conditions provide the perfect environment for the development of diseases, particularly fungal diseases. 

Cultural methods interrupt the disease triangle

Thinning stems and plants allows free-air circulation and sunlight to penetrate to the lowest portion of the plant, as well as to the soil surface.  This method prevents prolonged moisture, decreasing chances for disease development. Always clean the gardening tools after use on infected plants/soils.

The routine cleaning and maintenance of garden tools is recommended.  It is also good to clean tools when switching from one garden to another.  If disease has developed unnoticed, the chances of spreading the pathogen are reduced greatly.

Diseased branches, shoots, and affected soil should be removed and discarded in the trash.  When pruning, remove infected and weakest growth areas first, clean the garden tools, and then continue with routine pruning to shape and develop trees or shrubs. 

Do not place diseased plant material and/or infected soils into the compost pile.  Pathogens may survive this environment and spread rapidly to other areas of the garden if unclean compost is utilized. 

As with compost piles, pathogens may survive on organic materials naturally found in the backyard.  Some disease organisms may survive from one season to the next.  For instance, a small, compressed pile of fallen leaves may serve as the perfect host for harboring a pathogen into the next season.  Do not allow fallen leaves and other organic yard debris to accumulate anywhere but the compost bin, unless it is diseased and requires disposal with the rest of the trash.

Fertilization and watering also influence disease development.  Proper fertilization and timely watering promote overall plant health, but improper methods increase diseases.  Some pathogens are likely to invade plants suffering from a lack of nutrition (fertilization) or from insufficient moisture, while others are prone to attacking tender plant growth.  Excessive fertilization, especially nitrogen, combined with abundant moisture may actually injure a plant and encourage infection.  Excessive watering may lead to the development of root diseases. 

Poor drainage can lead to root diseases as well.  Soil amendments and cultivation work to improve the soil, aiding in proper drainage.  Stirring slightly-moist soils adds air and distributes moisture throughout the garden area.  Cultivating the soil in this manner interrupts the pathogenic environment on the ground. 

When diseases continually attack annuals or tender perennials, plant rotation may be necessary.  The gardener needs to prepare a new garden site for planting by laying a healthy foundation for success.  By placing next year’s annuals and through transplanting this year’s healthy perennials, plants will flourish in their new disease-free locations.  Since hosts are removed from the original garden site containing diseases, soils are allowed to heal.