Dr. Sharon M. Douglas Department of Plant Pathology and Ecology The Connecticut Agricultural Experiment Station 123 Huntington Street, P. O. Box 1106 New Haven, CT 06504

Phone: (203) 974-8601 Fax: (203) 974-8502 Founded in 1875 Email: [email protected] Putting science to work for society Website: www.ct.gov/caes

PLANT HEALTH PROBLEMS

Introduction and Historical Impact of plant disease on human culture is the disease Plant Health Problems . This fungal disease, caused by Hemileia vastatrix, appeared in coffee Plants in landscapes, gardens, production plantations in Ceylon (presently ) fields, forests, and interiorscapes are subject in the 1860’s and subsequently destroyed to a wide variety of problems that threaten the coffee industry in that nation. As a part their health. These problems can affect the of the British Empire, Ceylon was one of the aesthetics of the plant or can pose more primary sources of coffee for England, a serious consequences which result in plant nation whose citizenry drank coffee as their disfigurement, crop loss due to reductions in favorite boiled beverage. However, as a yield and quality, and plant death. result of the coffee rust epidemic, tea was Throughout time, plant diseases have had planted in its place and coffee became very profound effects on the history of human difficult to obtain. Thus, the English had to civilization and culture and plant health modify their drinking habits and as a result problems continue to impact our daily lives. of a plant disease, England became known Plant diseases affect food, fiber, and as a nation of tea drinkers. ornamental plants as well as those in natural areas. Notable among these diseases is Plant diseases have also changed the potato late blight. This disease, caused by composition of the forest and landscape. Phytophthora infestans, was responsible for Until the early 1900’s, the American widespread epidemics throughout Ireland chestnut was one of the most dominant and and Europe in the 1840’s. This devastating important hardwood tree species in the disease not only resulted in famine, but was forests of the eastern United States. It was also responsible for the emigration of 1.5 prized for its commercial value as a source million people from Ireland to the United of lumber, pulpwood, poles, tannins, States and Canada. Late blight continues to railroad ties, and edible nuts. With the threaten potato production in many regions introduction of the Cryphonectria of the U.S. as new strains of the fungus parasitica, a species which was not native to develop. the U.S., chestnut trees became infected with the chestnut blight fungus and the tree Another interesting but less dramatic was almost completely eliminated from the example which illustrates the impact of a forest. Today, sprouts continue to grow

from old stumps although they usually or cultural care. Living disease agents, succumb to disease. called biotic agents or plant pathogens, include microorganisms such as fungi and Dutch elm disease is another example of a bacteria. Both abiotic and biotic agents will disease that changed our city and town be described in greater detail in the section streets and greens. The fungus Ophiostoma “Types of Disease Agents.” ulmi, along with one of the insects that transmits it, were introduced to the U.S. on How Does Disease Occur? logs imported from Europe. The American elm, Ulmus americana, was highly In order for disease to occur, three factors susceptible to these exotic pests and quickly must be present. Because of this, disease is succumbed to infection. Since many of the often pictured as a triangle having three elm trees were planted in rows along city equal sides. Each side of the triangle is streets and parks, the fungus easily spread necessary in order for disease to occur. One from tree to tree through root grafts and side of the triangle represents the host plant, feeding activities of the beetle. the second side represents the causal agent or factor, and the third side represents the Because of the diversity of plant health environmental conditions that are necessary problems and causal factors, it is important in order for the other two sides to interact to learn to recognize them, understand what (Figure 1). When one or more sides of the causes them, and why and when they occur. triangle are missing, the triangle collapses It is also helpful to understand their and disease will not occur (Figure 2). For importance or relative impact. This example, scab of crabapple is a very information is helpful in order to prevent the common disease in the Connecticut problems from occurring or when they do landscape. The “disease triangle” for this occur, to properly manage them. disease consists of the host plant, a susceptible variety of crabapple, the causal What is Disease? factor, the fungus Venturia inaequalis, and the proper environment, typically a cool, Plant disease can be defined in many ways wet spring during which the young, but one of the simplest definitions describes emerging stay wet for extended disease as any condition in a plant caused by periods of time. When all three of these living and non-living agents that interferes factors are present, scab will develop. If one with its normal growth and development. component is missing, perhaps the spring Diseases or plant health problems can weather is hot and dry and not favorable for impact plants in many ways since all parts disease development, disease will not occur of a plant can be affected including flowers, since one side of the triangle is not present leaves, fruits, seeds, stems, branches, and the triangle collapses. growing tips, and roots.

Many different factors can cause plant health problems. These factors can be divided into two groups based on whether they are living or non-living. Non-living disease agents, often called abiotic agents, include factors such as environmental stress Plant Health Problems S. M. Douglas 2 The Connecticut Agricultural Experiment Station (www.ct.gov/caes)

Unlike abiotic agents, biotic agents are able to spread from plant to plant. This is an important attribute since the number of C t a diseased plants can increase over time as a s u s o a direct result of the growth, multiplication, H l t A n g and movement of the causal agent. la e P Disease n t Fungi The majority of plant diseases are caused by fungi. There are well over 100,000 different Environment species of fungi and only a surprisingly small portion of them are capable of causing plant diseases. Fungi are similar to plants Figure 1. The Disease Triangle: All but lack and the conductive or components are present and disease occurs. vascular tissues that are found in ferns and seed plants. Fungi are small, usually microscopic, organisms that consist of a mass of filaments or threadlike strands Plant Host called hyphae. The primary means of Environment reproduction and spread of fungi are by . Many fungi produce more than one Figure 2. The Disease Triangle: One of the type of during their life span and this components is missing (Causal Agent) so often influences how diseases are spread. disease does not occur. The primary ways that fungi infect plants is Types of Disease Agents through direct penetration of tissues although they also infect through natural As previously mentioned, the two main openings such as stomates, hydathodes, and categories of agents capable of causing plant lenticels or through wounds. In most cases, disease are abiotic and biotic, non-living and fungi require free water on plant surfaces in living factors, respectively. In natural order to infect. Because of this settings, it is not uncommon for plants to be environmental criterion, fungal diseases are affected by both abiotic and biotic problems frequently more common after periods of and it is often difficult to determine which wet weather or when overhead irrigation is came first. However, in many cases, plants used. Fungi are primarily spread by wind, which are initially stressed by abiotic factors splashing water (from rain or irrigation), will be weakened and therefore predisposed insects, and through cultural practices (e.g., to biotic problems. For example, on pruning shears, on pots, or in rhododendrons whose roots have been contaminated soil). weakened by drought stress are more susceptible to the fungal root rot caused by Bacteria Armillaria than their healthy counterparts. Bacteria are very different than fungi and are single-celled microorganisms that do not Biotic Agents have an organized nucleus. As with fungi, only a small percentage of bacteria found in nature are capable of invading living plants Plant Health Problems S. M. Douglas 3 The Connecticut Agricultural Experiment Station (www.ct.gov/caes)

and causing plant disease. Bacteria have activities of phloem-feeding insects such as cell walls and most plant pathogenic leafhoppers. Phytoplasmas can also be bacteria are rod-shaped. Bacteria reproduce spread mechanically by grafting infected primarily by cell division. This can occur in plant parts onto healthy plants. a short period of time and their initial presence or growth within a plant is usually Viruses and Viroids not visible. Viruses are unique plant pathogens since they consist of nucleic acid and a protein Unlike fungi, bacteria cannot invade plant coat and have no cellular structures. tissues that are intact and healthy. As a Additionally, viruses are unable to replicate consequence, most infections occur through or reproduce without the aid of the wounds. Bacteria also infect through components of the plant host cell. Viroids wounds made by insects during their feeding are even more simplistic than viruses since activities and through natural openings in a they lack a protein coat and only consist of plant such as nectaries or stomates. Bacteria nucleic acid. are spread from plant to plant by splashing water (from rain or irrigation), by insects, Because of the nature of these disease and through a variety of cultural practices agents, wounds are necessary in order for (e.g., as contaminants on pruning shears, in viruses and viroids to infect. Therefore, the plant or soil debris in pots). Bacteria can primary means of spread is through the also be transmitted by seeds from infected feeding activities of a number of insects, plants. predominantly aphids, whiteflies, and hoppers. Viruses can also be spread by Phytoplasmas nematodes and in infected pollen. Human Phytoplasmas are a relatively new type of activities are also very important for spread disease agent which are closely related to of these disease agents. Included among bacteria but they lack a rigid cell wall. these are grafting and mechanical These organisms used to be called transmission associated with the handling of mycoplasma-like organisms or MLO’s. As infected plant material. with bacteria, phytoplasmas have no organized nucleus and are microscopic and Nematodes unicellular. They can be irregular and Nematodes are tiny, translucent amoeba-like or spiral in shape. roundworms, oftentimes just barely visible Phytoplasmas are tissue-specific and only to the naked eye. As with fungi and live in the phloem or the nutrient-transport bacteria, only a small portion of nematodes system of their plant hosts. Most found in nature are parasitic to plants. Most phytoplasmas are incapable of living outside nematodes have three life stages: egg, larva, of their plant host or insect vectors. and adult. The latter two stages are most damaging to plants. Plant parasitic Since phytoplasmas can’t survive as free- nematodes are obligate pathogens and have living microorganisms, they are incapable of developed specialized structures called infecting plants without “outside” stylets that allows them to pierce plant cells assistance, through insect vectors or and extract cell contents. mechanical means of transmission. As a consequence, the primary way that Nematodes can infect plants through direct phytoplasmas are spread is through the penetration. This usually occurs at the tip of Plant Health Problems S. M. Douglas 4 The Connecticut Agricultural Experiment Station (www.ct.gov/caes)

a root. They also infect through wounds, Environmental disease agents result in a through natural openings, and through the wide variety of symptoms. For example, activities of vectors such as some insects. drought or dry soil conditions result in root Nematodes are also spread by infected plant damage and death. Non-woody feeder roots, material and by contaminated soil and plant usually located in the top 15 inches of soil, debris. are particularly sensitive and are the first ones affected. Without moisture, these roots Abiotic Agents shrivel and die. When these roots become Plant health problems attributed to abiotic nonfunctional, a water deficit develops since agents can also be referred to as disorders the roots cannot provide water to the top of rather than diseases. Both terms are used to the plant. Symptoms of drought vary with describe the same types of abnormalities in a the plant species and the severity of the plant although disorder usually implies the water deficit but are often not evident until causal factor is non-living whereas disease some time after the event has occurred- even usually implies the causal factor is a living as much as one or two years later! agent. Abiotic disease agents can be Symptoms include loss of turgor in needles categorized as being cultural or and leaves, drooping, wilting, yellowing, environmental. These types of agents are premature leaf or needle drop, bark cracks, often overlooked as probable causes of plant and twig and branch dieback. Leaves on health problems because they are very deciduous trees often develop marginal difficult to identify since they cannot be scorch and interveinal necrosis whereas cultured or viewed microscopically. As a needles on evergreens turn brown. Drought- consequence, the ability to pinpoint the stressed trees and shrubs can also exhibit causal factor requires close review and general thinning of the canopy, poor growth, examination of the cultural and and stunting. In extreme cases, drought can environmental history of the plant in result in plant death. question. Cultural Factors Environmental Factors Cultural factors associated with plant health Many types of environmental factors cause problems are quite diverse. Among the plant diseases. Among these are unusual common factors are site and soil attributes precipitation patterns resulting in drought or (e.g., pH, organic matter, drainage, soil water-logged soils, limited snow cover, type), planting practices (e.g., preparation of excessive winds, lightning, hail, late spring the rootball and planting hole, planting too or early autumn frosts, and extreme deep or to shallow), plant hardiness, temperature fluctuations, especially during construction activities resulting in soil the winter. Air pollution is another compaction or severing of roots, and important factor. Among the pollutants mechanical injuries from lawn mowers and encountered in Connecticut are ozone, string trimmers. Other types of problems hydrogen fluoride, sulfur dioxide, ethylene, result from incorrect or improperly timed and peroxyacyl nitrate. Some of these pruning, incorrect mulching, fertilizing compounds are more problematic in practices (e.g., incorrect timing, glasshouses and others primarily occur inappropriate applications resulting in outdoors. toxicities or deficiencies), and watering practices (e.g., time of day, too much or too little, frequent, shallow watering). Plant Health Problems S. M. Douglas 5 The Connecticut Agricultural Experiment Station (www.ct.gov/caes)

diseased plant from its healthy counterparts. Another type of cultural agent that can result Plants can exhibit a variety of symptoms, in plant damage and death involves some of which are associated with a specific chemicals. Included in this category are causal factor, but more commonly, can be deicing salts and misapplied pesticides, associated with many different factors. particularly herbicides. For example, when Symptoms can also occur on many different misapplied, herbicides can result in damage parts of a plant. Additionally, it is not which varies with the particular compound uncommon for a diseased plant to exhibit and plant species. Symptoms can develop more than one type of symptom. For several days to weeks after exposure or in example, the initial symptoms of Septoria some cases, not until the following spring. leaf spot of tomato appear as distinct spots Symptoms include chlorosis, necrotic approximately ¼ inch in diameter with dark spotting, marginal scorch, twisting, growth margins and tan centers. These spots are abnormalities, leaf/needle drop, dieback, usually scattered over the surface of the leaf. general decline, and plant death. However, as the disease spreads, the leaves quickly develop a blighted appearance as the As with environmental factors, cultural leaves turn completely brown and shrivel. factors can affect plant health in many ways and result in a wide range of symptoms. For Some of the common symptoms that we example, when mulches are applied too encounter on diseased plants are listed and close to the base of a plant and too thick, defined in this section. These terms provide they can result in root and crown rots and the vocabulary or terminology to describe asphyxiation of roots, respectively. This can what we see when a plant does not appear cause plant decline and death. healthy or normal.

For more detailed information on many of Common Symptoms of Plant Disease: these abiotic disease agents, please refer to Leaf spot- dead, discolored or injured areas Fact Sheets available upon request or on the of tissue which usually have distinct CAES website (www.ct.gov/caes). margins; spots often appear on leaves or fruit Blight- rapid yellowing, browning, collapse, Types of Plant Health Problems and death of leaves, shoots, stems, flowers, or the entire plant A significant factor which influences the Chlorosis- yellowing of leaves and stems ability to manage plant health problems lies which are normally green with the ability to recognize a problem when Necrosis- browning or blackening of areas it occurs. One of the most important ways on a plant indicative of the death of plant to identify a problem is by the symptoms cells that are produced by the affected plant. A Wilt- loss of turgor or drooping of leaves, symptom is defined as the response of the shoots, or the entire plant due to lack of plant to the presence of a disease agent, water regardless of whether it is non-living or Distortion- twisting or other abnormal traits living, abiotic or biotic. Symptoms are the of leaves, stems, and shoots external and internal reactions of a plant as a Mosaic- uneven pattern of yellow, light result of disease. The presence of a green or dark green, usually on leaves symptom on a plant distinguishes the Plant Health Problems S. M. Douglas 6 The Connecticut Agricultural Experiment Station (www.ct.gov/caes)

Canker- dead area on a stem or branch; can hand, there are diseases which can be fatal if be sunken, swollen, or discolored and are left uncontrolled. For example, tar spot of usually distinguished from adjacent healthy maple is usually not serious enough to tissues by color require control measures even though it can Rot- breakdown and decay of plant tissue, result in premature defoliation. In contrast, often used to describe conditions in roots Phytophthora root rot of zucchini is a and fruit disease which interferes with water-uptake Dieback- death of the tips of leaves, shoots, and can seriously debilitate and eventually and stems; failure of branches to develop, kill the plant if left unchecked. especially in the spring Witches’ broom- abnormal proliferation of Another part of diagnosis involves assessing shoots from the same point on a plant the severity of the problem. This resulting in a bushy, broom-like appearance assessment is made by gathering Gall- a swelling or abnormal growth of plant information about the nature of the problem: tissues; can develop on leaves, stems, and is it a foliar or a root problem, is it localized roots to one part of the plant or is it systemic? It Stunt- abnormally small sized plant parts is also helpful to determine the level of the due to the failure of those plant parts to disease: how many plants are involved or grow to full size; often used to describe an how long have they been symptomatic? entire plant Disease diagnosis based solely on symptoms Strategies for Managing Plant Health can sometimes be misleading and can lead Problems to improper, ineffective controls. In circumstances where different causal agents Regardless of the plant host or particular incite the same or similar symptoms on a type of disease that you encounter, the host plant, accurate diagnosis requires concepts of disease prevention and identification of the causal agent. Since management are fundamentally the same. most biotic agents are microscopic, accurate Management of plant diseases involves a identification is not possible without the two step process that first requires accurate necessary equipment. In these cases, diagnosis and assessment of the severity of samples may be submitted to the Plant the problem. This is followed by Disease Information Office of the implementing strategies to minimize the Experiment Station for diagnosis. Diagnosis impact of the disease. can involve light microscopy and histochemical staining, isolation on artificial 1. Disease Diagnosis: media, soil extraction, electron microscopy, The first step in disease management is studies of host range, and indicator plants. knowing what you’re trying to control- is it The office also utilizes serological tests and a disease caused by a fungus or is it a variety of other procedures as necessary. associated with the weather, the site, or your Information about the Plant Disease cultural care? Accurate diagnosis is very Information Office and about how to prepare important since it determines two things: the and submit samples can be obtained by need for control and the type of control. calling the Office or can be found on the Some plant diseases are merely aesthetic CAES website. and under normal circumstances don’t require control measures. On the other 2. Management Options: Plant Health Problems S. M. Douglas 7 The Connecticut Agricultural Experiment Station (www.ct.gov/caes)

A common misconception to disease control Planting- dig and prepare the planting hole is that chemical sprays, dusts, and soil correctly, making sure the plant is not drenches are the only effective means of planted too deep or too shallow reducing the effects of plant disease. Rootball preparation- for balled and However, chemical control is only one burlapped stock, the burlap and wire basket component of a multifaceted approach should be removed, if possible; for which includes: culture, sanitation, container-grown stock, the rootball should resistance, biological, and chemical be scored and teased apart before planting— components. this applies to both woody and herbaceous material It is important to realize that the goal of Crop rotation- it is helpful to purposefully disease management is not necessarily to alternate the crop species or closely related completely eliminate diseases but to manage plants grown in a specific area, especially them such that they remain at acceptable when the disease agent is soilborne levels. Additionally, in most cases, prevention is the best strategy for disease c. Plant care control. Fertilizing- appropriately timed applications to maximize plant growth and vigor and to Culture: A key opportunity for disease avoid plant stress due to deficiencies or management focuses attention to cultural toxicities manipulations which help to minimize Watering- maintain adequate soil moisture conditions favorable for disease for the plant species; this usually translates development. These include numerous to approximately one inch of water per methods that modify the plant’s growing week; in the absence of natural rainfall, conditions in order to optimize growth and irrigation should be used and depending vigor. upon soil type, this is best delivered as a deep soaking; avoid overhead irrigation or a. Plant and Site Selection water plants early in the day to allow foliage Hardiness- often an overlooked aspect of to dry before nighttime disease prevention; most of Connecticut is Mulching (summer mulch)- properly applied in USDA Zone 6 (some Zone 5); this is an mulch helps with weed control, soil important factor for consideration when temperature moderation, soil moisture trying new species retention, and spread of disease; summer Plant Requirements vs. Site Characteristics- mulches should not be applied too thick or - it is important to match the conditions too close to the stem or crowns of plants required by a particular plant with the Winter Protection- winter mulches, physical attributes of the intended site as closely as barriers, and applications of anti-transpirants possible; special attention should be given to or anti-desiccants can be effective in soil type and pH, drainage, and light levels protecting plants from heaving during freeze-thaw cycles and from drying winds. b. Planting Practices Spacing- use the correct spacing for the An example of cultural manipulations that particular plant species; too-close spacing help to reduce disease can be illustrated for can promote disease by compromising plant winter injury and desiccation of vigor and by inhibiting drying and air rhododendron, a common problem in circulation Connecticut. Rhododendrons are more Plant Health Problems S. M. Douglas 8 The Connecticut Agricultural Experiment Station (www.ct.gov/caes)

prone to this type of injury as well as to These practices significantly help to reduce fungal leaf spots when they are not properly the amount of overwintering inoculum maintained or when stressed by root injury which will be available to infect the newly from drought. These problems can be emerging tissues in the spring. minimized by maintaining an acidic soil pH, fertilizing in early spring, and watering Resistance: This management option during periods of drought and just before the utilizes resistant or tolerant cultivars or ground freezes in the fall. species of plants to minimize or avoid disease. When available, genetic resistance Sanitation: This option for disease is probably the most desirable and effective management focuses on minimizing the management tool since it circumvents the introduction of disease agents through plant need for additional controls. It is especially selection and by eradication of diseased important for diseases caused by viruses, plants or plant parts as a means to reduce the nematodes, and by soilborne and wilt potential for spread of biotic agents. pathogens since these are all extremely Plant Selection- use of healthy, disease-free difficult to control with other means. seeds, seedlings, cuttings, and transplants; Although genetically resistant plants are not Prune and Remove Infected Plants and available for all plant and all diseases, Debris- symptomatic plants or infected plant breeding programs are underway and the parts should be promptly removed to availability of these types of plants is minimize disease spread; it is also helpful to expected to increase in the near future. remove the debris of annual plants and to cut back the tops of perennials after they Examples of effective use of genetic have been killed by frost at the end of each resistance include cultivars of crabapple growing season; this practice helps to reduce with resistance to scab and rust, cultivars of the amount of overwintering inoculum; rose with resistance to powdery mildew and Groom Plants- remove spent flowers and black spot, and cultivars of tomato with leaf debris during the growing season to resistance to Verticillium and Fusarium minimize inoculum buildup and spread; wilts, tobacco mosaic virus, and nematodes. Use Clean Equipment- all pruning tools, pots, flats, and equipment should be Biological: This management tool employs thoroughly cleaned and disinfested with living agents (usually antagonists or 10% household bleach (1 part bleach: 9 competitors of the causal agent) to control parts water), 70% alcohol, or a commercial plant diseases. Effective biological controls compound such as Greenshield ®; take advantage of the natural competition of Scout- check plants on a regular schedule in living organisms for limited resources or order to monitor for buildup of diseases and ecological niches. Thus, two organisms plant abnormalities. cannot occupy the same space at the same An example of sanitation as an essential time, they cannot consume the same component for disease control can be resource (e.g., food source) at the same time, illustrated for Brown Rot, a common and and in some cases, one organism produces destructive fungal disease of stone fruits in compounds that are inhibitory to the growth Connecticut. This involves removing and and development of the other organism. The destroying mummied fruit on the ground or availability of biological controls is remaining on a tree and pruning and somewhat limited at present although this removing dead and/or cankered twigs. Plant Health Problems S. M. Douglas 9 The Connecticut Agricultural Experiment Station (www.ct.gov/caes)

option for disease management shows “Biorational” pesticides: these pesticides considerable promise for the future. are defined as products that are considered to be environmentally friendly because they An example of biological control is the have minimal harmful effects on non-target introduction of hypovirulent (“less” organisms and the environment; they are virulent) strains of the chestnut blight frequently more “user friendly” than fungus. These strains compete with virulent traditional pesticides; examples include strains and keep them from causing killing neem oil, insecticidal soap, horticultural oil, cankers on infected trees. More detailed and potassium bicarbonate. information on this topic can be found in numerous Fact Sheets about chestnut blight Biological pesticides: these pesticides are on the CAES website. Several commercial living agents which are used to control biological control agents have recently specific pathogens which are also living become available and are registered for organisms; the control agents can be control of some root rot and foliar diseases. antagonists (e.g., they secrete compounds or Since these products contain living their by-products alter the environment and organisms, the directions for storage and use make it unfavorable for the growth of the of these products are different than those for pathogen) or competitors (e.g., they occupy conventional pesticides. Thus, careful the same niche or site or compete for the attention to the label particularly important. same food source) of the causal agent; examples include Ampelomyces quisqualis Chemical (Pesticides): Although it is and Trichoderma harzianum. possible to successfully manage many disease problems without the use of “Chemical” pesticides: these are pesticides, there are situations where considered “traditional” pesticides with pesticide usage is important and highly traditional modes of action; examples successful. Chemical disease control uses include strobilurins, sterol inhibitors, pesticides (fungicides, bacteriocides, and benzimidazoles, coppers, and sulfurs. nematicides) to limit the effects of biotic agents. Fungicides are the most common Protectant vs. Systemic Fungicides: Most chemicals used for disease control. In most fungicides are protectants and must be cases, however, the degree of control present on the surface of the plant in depends upon the proper selection, timing, advance of the causal agent in order to and method of application of the compound. prevent infection. Their primary mode of In this regard, selection of the appropriate action is to inhibit fungus spores from fungicide is contingent upon accurate germinating or to kill spores after they diagnosis of the problem since fungicides germinate. These compounds do not stop or vary in their efficacy; some fungicides are cure a disease after it has started since they toxic to all or most kinds of fungi whereas are not absorbed or translocated within the others affect only specific types of fungi. plant. On the other hand, systemic Another way of looking at pesticides is as fungicides are absorbed through the foliage “plant medicines”, these are compounds or roots and are translocated within the used to protect or cure plants from infectious plant. These compounds have a therapeutic agents. (curative) or “kickback” mode of action since they can kill or inhibit growth of Categories of Pesticides: Plant Health Problems S. M. Douglas 10 The Connecticut Agricultural Experiment Station (www.ct.gov/caes)

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®, Bravo®, and Ortho Multipurpose Garden 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.

June 2008 (revision)

Plant Health Problems S. M. Douglas 11 The Connecticut Agricultural Experiment Station (www.ct.gov/caes)