Regardless of the plant host or particular type of disease that you encounter, the concepts of disease prevention and management are fundamentally the same. Management of plant diseases involves a two-step process that first requires accurate diagnosis and assessment of the severity of the problem. This is followed by implementing strategies to minimize the impact of the disease.
Plant Disease Diagnosis
The first step in disease management is knowing what you’re trying to control—is it a disease caused by a fungus or is it associated with the weather, the site, or your cultural care? Accurate diagnosis is very important since it determines two things: the need for control and the type of control. Some plant diseases are merely aesthetic and under normal circumstances don’t require control measures.
On the other hand, there are diseases which can be fatal if left uncontrolled. For example, tar spot of maple is usually not serious enough to require control measures even though it can result in premature defoliation. In contrast, Phytophthora root rot of zucchini is a disease which interferes with water uptake and can seriously debilitate and eventually kill the plant if left unchecked.
Another part of diagnosis involves assessing the severity of the problem. This assessment is made by gathering information about the nature of the problem: is it a foliar or a root problem, is it localized to one part of the plant or is it systemic? It is also helpful to determine the level of the disease: how many plants are involved or how long have they been symptomatic?
Disease diagnosis based solely on symptoms can sometimes be misleading and can lead to improper, ineffective controls. In circumstances where different causal agents incite the same or similar symptoms on a host plant, accurate diagnosis requires identification of the causal agent. Since most biotic agents are microscopic, accurate identification is not possible without the necessary equipment. In these cases, samples may be submitted to the Plant Disease Information Office of the Experiment Station for diagnosis. Diagnosis can involve light microscopy and histochemical staining, isolation on artificial media, soil extraction, electron microscopy, studies of host range, and indicator plants. The office also utilizes serological tests and a variety of other procedures as necessary
A common misconception to disease control is that chemical sprays, dusts, and soil drenches are the only effective means of reducing the effects of plant disease. However, chemical control is only one component of a multifaceted approach which includes: culture, sanitation, resistance, and biological and chemical components.
It is important to realize that the goal of disease management is not necessarily to completely eliminate diseases but to manage them such that they remain at acceptable levels. Additionally, in most cases, prevention is the best strategy for disease control.
Culture: a key opportunity for disease management focuses attention to cultural manipulations which help to minimize conditions favorable for disease development. These include numerous methods that modify the plant’s growing conditions in order to optimize growth and vigor.
Plant and Site Selection
Hardiness: often an overlooked aspect of disease prevention; most of Connecticut is in USDA Zone 6 (some Zone 5); this is an important factor for consideration when trying new species.
Plant Requirements vs. Site Characteristics: it is important to match the conditions required by a particular plant with the attributes of the intended site as closely as possible; special attention should be given to soil type, pH, drainage, and light levels.
Spacing: use the correct spacing for the particular plant species; too-close spacing can promote disease by compromising plant vigor and by inhibiting drying and air circulation.
Planting: dig and prepare the planting hole correctly, making sure the plant is not planted too deep or too shallow.
Rootball preparation: for balled and burlapped stock, the burlap and wire basket should be removed, if possible; for container-grown stock, the rootball should be scored and teased apart before planting—this applies to both woody and herbaceous material.
Crop rotation: it is helpful to purposefully alternate the crop species or closely related plants grown in a specific area, especially when the disease agent is soil borne.
Fertilizing: appropriately timed applications to maximize plant growth and vigor and to avoid plant stress due to deficiencies or toxicities.
Watering: maintain adequate soil moisture for the plant species; this usually translates to approximately one inch of water per week; in the absence of natural rainfall, irrigation should be used and, depending upon soil type, this is best delivered as a deep soaking; avoid overhead irrigation or water plants early in the day to allow foliage to dry before nighttime.
Mulching (summer mulch): properly applied mulch helps with weed control, soil temperature moderation, soil moisture retention, and spread of disease; summer mulches should not be applied too thick or too close to the stem or crowns of plants.
Winter Protection: winter mulches, physical barriers, and applications of anti-transpirants or anti-desiccants can be effective in protecting plants from heaving during freeze-thaw cycles and from drying winds.
An example of cultural manipulations that help to reduce disease can be illustrated for winter injury and desiccation of rhododendron, a common problem. Rhododendrons are more prone to this type of injury as well as to fungal leaf spots when they are not properly maintained or when stressed by root injury from drought. These problems can be minimized by maintaining an acidic soil pH, fertilizing in early spring, and watering during periods of drought and just before the ground freezes in the fall.
Sanitation: this option for disease management focuses on minimizing the introduction of disease agents through plant selection and by eradication of diseased plants or plant parts as a means to reduce the potential for spread of biotic agents.
Plant Selection: use of healthy, disease-free seeds, seedlings, cuttings, and transplants.
Prune and Remove Infected Plants and Debris: symptomatic plants or infected plant parts should be promptly removed to minimize disease spread; it is also helpful to remove the debris of annual plants and to cut back the tops of perennials after they have been killed by frost at the end of each growing season; this practice helps to reduce the amount of overwintering inoculum.
Groom Plants: remove spent flowers and leaf debris during the growing season to minimize inoculum buildup and spread.
Use Clean Equipment: all pruning tools, pots, flats, and equipment should be thoroughly cleaned and disinfested with 10% household bleach (1 part bleach: 9 parts water), 70% alcohol, or a commercial compound such as GreenshieldÒ .
Scout: check plants on a regular schedule in order to monitor for buildup of diseases and plant abnormalities.
An example of sanitation as an essential component for disease control can be illustrated for Brown Rot, a common and destructive fungal disease of stone fruits in Connecticut. This involves removing and destroying mummied fruit on the ground or remaining on a tree and pruning and removing dead and/or cankered twigs. These practices significantly help to reduce the amount of overwintering inoculum which will be available to infect the newly emerging tissues in the spring.
Resistance: this management option utilizes resistant or tolerant cultivars or species of plants to minimize or avoid disease. When available, genetic resistance is probably the most desirable and effective management tool since it circumvents the need for additional controls. It is especially important for diseases caused by viruses, nematodes, and by soilborne and wilt pathogens since these are all extremely difficult to control with other means. Although genetically resistant plants are not available for all plant and all diseases, breeding programs are underway and the availability of these types of plants is expected to increase in the near future.
Examples of effective use of genetic resistance include cultivars of crabapple with resistance to scab and rust, cultivars of rose with resistance to powdery mildew and black spot, and cultivars of tomato with resistance to Verticillium and Fusarium wilts, tobacco mosaic virus, and nematodes.
Biological: this management tool employs living agents (usually antagonists or competitors of the causal agent) to control plant diseases. Effective biological controls take advantage of the natural competition of living organisms for limited resources or ecological niches. Thus, two organisms cannot occupy the same space at the same time, they cannot consume the same resource (e.g., food source) at the same time, and in some cases, one organism produces compounds that are inhibitory to the growth and development of the other organism. The availability of biological controls is somewhat limited at present although this option for disease management shows considerable promise for the future.
An example of biological control is the introduction of hypovirulent ("less" virulent) strains of the chestnut blight fungus. These strains compete with virulent strains and keep them from causing killing cankers on infected trees. More detailed information on this topic can be found in numerous Fact Sheets about chestnut blight on the CAES website. Several commercial biological control agents have recently become available and are registered for control of some root rot and foliar diseases. Since these products contain living organisms, the directions for storage and use of these products are different than those for conventional pesticides. Thus, careful attention to the label particularly important.
Chemical: although it is possible to successfully manage many disease problems without the use of pesticides, there are situations where pesticide usage is important and highly successful. Chemical disease control uses pesticides (fungicides, bacteriocides, and nematicides) to limit the effects of biotic agents. Fungicides are the most common chemicals used for disease control. In most cases, however, the degree of control depends upon the proper selection, timing, and method of application of the compound. In this regard, selection of the appropriate fungicide is contingent upon accurate diagnosis of the problem since fungicides vary in their efficacy; some fungicides are toxic to all or most kinds of fungi whereas others affect only specific types of fungi. Another way of looking at pesticides is as "plant medicines," these are compounds used to protect or cure plants from infectious agents.
Categories of Pesticides
"Biorational" pesticides: these pesticides are defined as products that are considered to be environmentally friendly because they have minimal harmful effects on nontarget organisms and the environment; they are frequently more "user friendly" than traditional pesticides; examples include neem oil, insecticidal soap, horticultural oil, and potassium bicarbonate.
Biological pesticides: these pesticides are living agents which are used to control specific pathogens which are also living organisms; the control agents can be antagonists (e.g., they secrete compounds or their by-products alter the environment and make it unfavorable for the growth of the pathogen) or competitors (e.g., they occupy the same niche or site or compete for the same food source) of the causal agent; examples include Ampelomyces quisqualis and Trichoderma harzianum.
"Chemical" pesticides: these are considered "traditional" pesticides with traditional modes of action; examples include strobilurins, sterol inhibitors, benzimidazoles, coppers, and sulfurs.
Protectant vs. Systemic Fungicides: most fungicides are protectants and must be present on the surface of the plant in advance of the causal agent in order to prevent infection. Their primary mode of action is to inhibit fungus spores from germinating or to kill spores after they germinate. These compounds do not stop or cure a disease after it has started since they are not absorbed or translocated within the plant. On the other hand, systemic fungicides are absorbed through the foliage or roots and are translocated within the plant. These compounds have a therapeutic (curative) or "kickback" mode of action since they can kill or inhibit growth of pathogens after they have invaded the plant host.
Trade Name vs. Common Name: the common name of a pesticide is the name assigned to the active ingredient of the pesticide. In contrast, the trade name of a pesticide is the name assigned by the manufacturer or distributor of a particular product. For example, chlorothalonil is the common name of a fungicide which is sold by the trade names Daconil ,Thalonil, and Ortho Multipurpose Fungicide. Therefore, a single common name or active ingredient may be available under many different trade names.
When using pesticides for disease control, it is very important to thoroughly read and comply with the label. This applies to information on host plant, dosage rates, safety precautions, and days to harvest intervals, when applicable.
An example of effective fungicide applications can be illustrated for control of apple scab, the most troublesome disease of apples in Connecticut every year. For this disease, the fungus has two distinct cycles of infection—if the pathogen is essentially controlled with properly selected and timed fungicides during the first cycle of infection in the spring, the second cycle does not occur and fungicide sprays are unnecessary for the remainder of the season.