Plant Pest and Disease Management: The Basics
The most important component of any home gardener’s insect and disease management regimen is taking the time to directly observe plant problems as frequently as possible, a practice known as scouting. Following along with the UMass Extension Landscape Message offers clues as to what specific insect and disease problems may be active on plants at a particular period during the growing season, but outbreaks that go unnoticed and unmanaged can become an irreparable problem.
First steps: What plants, which problems?
While taking an inventory of plants in the garden, it is important to be familiar with how these plants should appear when healthy. If there is a history of pest or pathogen problems, it is helpful to understand the life cycles of these organisms, so as not to encourage re-infestations. Some fungal pathogens may overwinter in soil or debris and planting of susceptible plants year after year in the same place would only lead to re-emergence of the pathogen (impatiens downy mildew, for example). Other biotic stressors may not survive after a year or two without a host plant to infect and so a simple rotation planting will suffice in controlling future outbreaks. In some cases insect feeding may be a survivable condition, while at other times it may lead to irreversible decline. Only an accurate disease or pest diagnosis and careful planning can ensure the prolonged health of plants grown in areas with a history of stress, or in response to a new introduction.
How do pests and pathogens come in to the garden?
Many insects can fly to new host plants within certain distances, and most fungi can disperse airborne spores to susceptible plants. But how are new problems introduced into garden settings? Nursery plants may harbor unwanted organisms, within infested soils, planting containers and seed. It is crucial to give your plants the best start in life. Starting seeds and transplants in good quality soil with a plenty of water and the proper amount of mulch can make a significant difference as plants establish at a new site. Strong, healthy plants are better able to resist infection and infestation by harmful pathogens and pests, and have a better chance at surviving stress caused by environmental factors such as drought.
Familiarize yourself with symptoms and signs
Symptoms are external and internal reactions of a plant as a result of disease or insect infestation (e.g. wilting, yellowing/browning foliage, resin/sap production, general dieback, among others). Signs are pathogen or its parts on or within the plan (e.g. spore-bearing structures, bacterial ooze, among others). Symptoms of pest and pathogen damage can be very general and highly conspicuous, or they can be fairly specific but cryptic without careful scouting. Water stress from drought conditions may have the same appearance as a vascular wilt disease, because internally the plant is experiencing the same physiological effect: a lack of water. This is a situation in which only the homeowner knows how thoroughly the plant has been irrigated, if similar symptoms have appeared in previous years, and if any potentially diseased plant material or organic matter was introduced recently. Timing is critical. Fungal leaf pathogens, for instance, often need extended periods of leaf wetness and are unlikely to be active in dry and hot weather (like this past May). Leaf spots caused by fungi often appear as circular, necrotic lesions and may even produce visible spore-bearing structures that can be identified. Bacterial leaf spots, on the other hand, are inhibited by the leaf veins, and may appear angular and water-soaked (e.g. bacterial leaf spot on oakleaf hydrangea).
How to use growing degree days and phenology
Growing degree days (GDD) are used to track insect development based on daily temperatures. This is the most accurate way to assess the timing that insect pests will be active, and when treatment (if any) should occur. Growing degree day minimums and maximums can be found for important plant pests. The minimum GDD indicates the time when the insect will begin to be active, and the maximum GDD is the end of the window of time in which any control should be implemented. Some insects may have multiple periods during which time control can be implemented. Observing the stages of plant development can be an even more accurate way to track insect development, as plants are exposed to the same environmental conditions. Recording insect and disease development timing in relation to plant phenology in order to tailor an integrated pest management program for the home garden can be a rewarding experience and may sharpen observational skills.
