Plant Health Problems-Introduction and Historical Impact of Plant Health Problems

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 coffee rust. This fungal disease, caused by Hemileia vastatrix, appeared in coffee Plants in landscapes, gardens, production plantations in Ceylon (presently Sri Lanka) 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 fungus 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 leaves 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 o s 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 chlorophyll 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 spores. Many fungi produce more than one Figure 2. The Disease Triangle: One of the type of spore 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.

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