. Agricultural plant pest control : a study manual for commercial and governmental pesticide applicators . Weeds; Pests. PLANT DISEASES - PART I THE NATURE AND CAUSES OF DISEASE IN PLANTS H Introduction There are an estimated 100,000 parasitic plant diseases. Fortunately, about 80 percent occur rarely or on a limited scale and are considered relatively unimportant. The annual cost of plant diseases in the United States is five billion dollars. Plant diseases are the result of the right combination of susceptible host plants, a virulent pathogen (disease causing agent), and suitable climatic co
. Agricultural plant pest control : a study manual for commercial and governmental pesticide applicators . Weeds; Pests. PLANT DISEASES - PART I THE NATURE AND CAUSES OF DISEASE IN PLANTS H Introduction There are an estimated 100,000 parasitic plant diseases. Fortunately, about 80 percent occur rarely or on a limited scale and are considered relatively unimportant. The annual cost of plant diseases in the United States is five billion dollars. Plant diseases are the result of the right combination of susceptible host plants, a virulent pathogen (disease causing agent), and suitable climatic conditions experienced during the growing season (Fig- ure I-l). Variations fi'om characteristic or normal climatic patterns are often responsible for sudden outbreaks of certain diseases that would not normally occur. Regional climatic conditions are a major factor in determining crops that can be grown profitably and the prevalence of diseases on these crops. Plant diseases are divided into parasitic or nonparasitic: Non-parasitic or physiological diseases include: ? nutrient deficiencies or excesses, ? environmental extremes, ? air pollution and pesticide injury, ? drought, ? genetic abnormalities, and ? other physiological disorders. Environment Parasitic diseases are caused by living organisms which can multiply and spread from infected to healthy plants. Organisms commonly causing parasitic plant diseases sre fungi, bacteria, vi- ruses, mycoplasmas, and Figure I-l B Fungi Fungi are simple microorganisms that lack chlorophyll and are unable to manufacture their own food. Fungi obtain their food from living plants and animals or from decaying organic matter. There are about 8,000 parasitic fungi causing 80,000 of the known 100,000 plant diseases. Most disease-causing fiangi are inconspicuous and can be seen only with the aid of a microscope. Commonly recognized fungi include yeast, mildews, and mushrooms. Fungi may enter a plant through wounds, natural openings, o
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