91-67 Plant Diseases Adapted by Edmond L. Marrotte Consumer Honiculturist Depattment of Plant Science @;,~r,.'i' ~ -====:::..... -.' :.;;.ncJ •• ~.::,"f:::=== UNfVERSITY OF CONNECTICUT COOPERATIVE EXTENS ION SYSTEM • C-/br-..r AK""'U~ff .nJ NJ,,, ..III,,.,,ra, · Table of Contents What is a Plant Disease? . How are Diseases Classified? 1 How Do Plants Work? 2 What Causes Plant Diseases? 3 Major Groups of Fungi .. 6 Bacteria 7 Viruses and Virus-like Organisms 7 Nematodes .. .. .. 8 Parasitic Plants 8 How are Diseases Recognized? 8 How to Diagnose Plant Diseases 9 Steps to Follow ....... 10 How Do Diseases Develop? . 12 How are Plant Diseases Controlled? 16 Cultural Control of Plant Diseases 16 What to Plant . 16 How to Plant 16 Routine Care of Plants 17 Chemical Control of Plant Diseases 17 When is the Best Time to Apply Chemicals? 17 How Should rhe Chemical be Applied? . 18 Fungicides Used in Plant Disease Control 18 Adapted from Plant Di"am by Otis C. Maloy. Extension Plant Parhologist. Washington State University Isn«ti in forthtrona ofCoopt:rarive Ex~nsjon work. Acts ofMay 8 and Junr 30, 1914. in coop­ eration with the U.S. D~artmenl ofAgriculture . Kirltlyn Kur, Dirtftor, Cooptrative Extmsion System. The UnilJmity ofConn«ticut . Slam. The Conn«ticul Cooptrotive Exunsum sys~m offm itJ programr to jJn'$ons r~ardksJ of race. cowr, n4tWnA/ origin. so: or disability and ;s an eqU4i opportunity employ". ( Plant Diseases What is a Plant Disease? taneollS effect of an injury. It is necessary to dis­ tinguish between disease and injury and not con­ Many attempts have been made (0 define what a fuse normal life processes, such as maturation plant disease is. but all definitions have to be and falling of leaves, with diseases. One common qualified CO some extent. A plant disease IS not a confusion is that causal agent and disease are the tangible object as is an insect or, a weed. Ie ~~ same. Some believe that a fungus, bacterium or not have absolute, fixed dimensIOns but vanes In nematode is a disease, but it is not. It is a causal its appearance in a great many ways. agent of a disease. Broadly defined. a plant disease is any condi­ The agent causing a plant disease is called a rion in which a plant is different in some way plant pathogen whether the agent is living (para­ from a normal (i.e., healthy) plant in either struc­ sitic) or nonliving (nonparasitic), but mually the ture or funccion. The diseased plant may be term pathogen is resuicted to the former. shorter, have more branches or fewer leaves man normal- it differs in structure. It may wilt and die prematurely, or not produce flowers or ffuic­ it differs in function. Plant diseases affect all plants to some extent. They generally cause minor damage, bur when all of the required interacting and contributing faclOcs are present. they can be cawlrophic. Famines caused by blights and mildews are recorded from biblical times. Late blight of porawes led [0 the great famine in Ireland in the mid-lSDDs. and chestnut blight eliminated the American chestnut as a commercial tree in the United States in a few decades. In commercial crops, even minor losses to disease can be serious. The science thar deals with the n3Wfe and con­ trol ~ f plant diseases is called plant pathology. Plant pathology also involves other sciences, such as mycology. bacteriology. plant physiology. nematology, virology. etc. Although disease is sometimes referred to as a Figure 1 condition, it is not a static, constant condition Disease Symptoms but an ever-changing one. Disease is a series of changes in the plant caused by some agenc. It is How are Diseases Classified~ physiological. affecting all or part of the func­ Plant diseases can be classified or categorized in a tions of the plant. It is abnormal to the plant and number of ways. One of the earliest and most not the normal termination of some particular popular ways is to separate diseases on the basis function (for example. normal dropping of of the crop affected. Thus. there are diseases of leaves). Diseases are always harmful to the plant tree fruits, small fruits, vegetables, forest trees, in the long run even though there have been cereals and grasses, ornamentals, etc. Another sys­ attempts to benefit from certain diseases. Exam­ tem is to categorize diseases on the basis of the ples are the virus-induced color break in tulip causal agent. In this system we have fungus dis­ and [he virus-caused shortening ofinternocles of eases, bacterial diseases, virus diseases, nematode cherries. diseases, nutrient deficiencies, etc. Other ways The causal agent of a disease has a prolonged diseases can be classified are on the plant part influence as compared to me sudden, instan- affected, e.g. , root diseases, stem diseases or leaf Plant Diseases diseases; or on the symptom or symptoms produced. such as blights. mildews. decays or ( mosaics. The name applied to a given plant disease generally includes three significant pieces of infor­ mation: (I) the plant attacked. (2) the plant part affected and (3) how it is affected. For example. peach leaf curl. alfalfa leaf spot. potato leaf roll and black stem rust of wheat. are names that help to develop a mental image of how a disease affeces a plane. Figure 1 illustrates some of the ways that me name of a disease is often derived from the way the plant is affected. How Do Plants Work? Figure 2 Since plant diseases modify or interfere wim Pans of the Plant the normal functioning of a plant, we must have some idea of how a normal plant works. Figure 2 role co elevate leaves from the ground where they is a simplified diagram of the major parts of a arc effectively exposed to sunlight. In woody plant and their components. Cenain types of plants. there is a layer of meristematic cells called cells and tissues do certain kinds of jobs in a the cambium which gives rise to xylem, or water­ plant. Knowing how a plant functions will often conducting tissue, on the inside and to phloem, aid in the diagnosis of plant problems. Briefly. or food-conducting tissue, on the outside. Bark the main parts of the plant are: develops from the outer cells of the phloem. Roots--Anchor plant: absorb water and The leaf is the food factor of the plant. The nucrienes; score food. nutrients from the soil move upward in the Stern-Supports foliage and conducts xylem and into the leaf veins. These veins ter­ nucrienes from roocs co leaves; water and minate at the leaf margin as special structures nuuienes move upward through the xylem; called hydathodes. The nutrients diffuse out of manufactured food moves downward through the xylem in the veins inco the palisade and the phloem. mesophyll cells. These cells contain the green pig­ Leaves--Absorb C02 and light; synthesize ment called chlorophyll. The plant surface is sugar. covered with a one-cell-thick layer called the epidermis. On the leaves and some stems, the Flower-Sexual organs of the plant. epidermis is covered by a waxy cuticle. Also, on Fruit- Reproduce the plant. the leaf surface. especially on the underside. are For a plant to grow and develop normally. natural openings called stomata. through which mineral nutrienes must be obtained from the soil. gases are exchanged with the atmosphere. Carbon These nutrients are dissolved in the soil water and dioxide in th e air moves through the stomata inco absorbed by rOot hairs which are simply exten­ the openings in the spongy mesophyll and dif­ sions of the epidermal cells. The dissolved fuses inco these cells. Here, carbon dioxide and nucriencs then diffuse inward to the xylem cells water, in the presence of light, are converted into where they can move upward in the plant. (he simple sugar, glucose, with oxygen as a by­ Secondary roots originate from the peri cycle product. This process is called photosynthesis. which is meristematic tissue Ilke the cambium of The glucose diffuses through the cells into the the stem. These secondary roots force their way phloem of the leaf vein. From here it is trans­ through the cortex and cause small natural ported to various parts of the plant to be used for wounds through which some disease agents can growth and energy, or it is stored as glucose or enter. one of many other compounds. The oxygen dif­ Nutrients move up the plant in the xylem of fuses back into the chambers of the mesophyll the stem. The stem serves mainly a supporting and our inm the atmosphere. 2 Plant Diseases Seven major plant funclions can be identified plant communities are [feated in which there are ( that are affected by disease. Table 1 lists these combinations of different plants in a natural asso­ functions and some of the cypes of diseases affect­ ciation even though the larger of the control ingthem. measure is a single plant species. The highest level In considering how diseases affect plants and of plant organization is the biome. This includes how control measures are brought into play, it is all of the plant life (and many include the ani­ necessary to have some idea of how plants and mals) in a large geographic area. For example, the their parts are organized. Figure 3 illusuates the western coniferow forests comprise a biome. It is various levels of organization of plants. Diseases rare when the biome is considered in the concrol are usuaUy described in terms of how they affect of a plant disease. cells, tissues or organs. Cells are killed, tissues are over- or underdeveloped.