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Type Diseases |
Laboratory Outline |
The concept of a "Type Disease" or type of disease is useful in the diagnosis of plant diseases. There are literally thousands of diseases of the many crop plants that commonly confront the plant pathologist. Grouping diseases into categories called Type Diseases based on the plant part infected and/or obvious symptom syndrome reduces the complexities of diagnosis. Diseases within a group are often (not always!) caused by similar pathogens, have similar disease cycles, are favored by similar environmental conditions, and therefore, are often controlled with similar approaches. By categorizing diseases into these broad types, knowledge gained about one or a few diseases in the same group is often applicable to new or unfamiliar diseases. The following types of diseases will be studied in this course: Downy mildews - diseases characterized by the appearance of very fine cottony wefts or mats of fungal hyphae on infected leaves and stems that give infected plants a soft, downy appearance.
Ergot - diseases caused by a specific group of fungi that only infect the flowering parts of grasses and replace the ovary with a dark-colored, compact survival structure called a sclerotium.
Galls/Tumors - diseases characterized by localized swellings or overgrowths on any part of an infected plant depending upon the pathogen.
Leaf curl diseases - diseases are characterized by distortion of the leaves and flowers, ranging from small, barely visible blister-like areas to grossly distorted leaves.
Mosaics/Mottles - diseases characterized by a variegated pattern of different colored tissues, usually including shades of green, yellow, and white. Mosaics differ from mottles by having a distinct margin between adjacent patches of tissue, whereas the margins between colors in a mottle are diffuse and one color blends into another.
Powdery mildews - diseases characterized by the presence of a white to gray colored powdery growth on the surfaces of infected plants. Small black structures may or may not appear on the powdery material as the plant approaches maturity or as autumn nears.
Root and foot rots - diseases characterized by necrosis of the roots, crown, and lower stem region of the plant immediately above and below the soil surface (the foot region). Other symptoms may include nutrient deficiency symptoms, wilting, and stunting.
Rust diseases - diseases characterized by yellow-orange to rusty-red color of infected plants, which are often covered with small pustules containing pollen-like granules.
Seedling diseases - diseases that affect seeds during germination (seed rot), or young plants before (pre-emergence damping-off) or after germination (post-emergence damping-off). Affected plants may have a dark, watery rot of the seed, roots, or young stem.
Smut diseases - diseases characterized by the presence of black, dusty masses of spores on the host on any above-ground plant part.
Vascular diseases - diseases characterized by wilting (loss of turgidity of the host), mild to severe chlorosis, followed by necrosis and death of effected plants. Discoloration of the vascular system is the most diagnostic symptom of these diseases, although not the most common.
Yellows diseases - diseases characterized by a general chlorosis (yellowing) and stunting of the plant.
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