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Focus of this course • First half - diseases • Second half - weeds • Many important plant diseases are caused by pathogens and this will be the main emphasis of our studies Plant Disease defined: • It is a disruption of normal plant function in a host plant due to a causal agent • The disruption results in a morphological change in the host plant, this is what we term symptoms. • The causal agent may be biological or physiological (ie. Chemical or physical) Disease vs. injury • Disease is a continous (chronic) or persistent condition. • Injury is acute and/or short lived Physiological agents • These are non-living and include things such as cold, chemicals, elements, pollution, and etc. • they can’t reproduce • are not infectious • and are called physiopaths Biological agents • These are living organisms such as fungi, bacteria, viruses, etc. • they can reproduce • they are infectious and can spread through a host population • they are called pathogens Physiological agents • Symptoms usually uniformly distributed over entire plant • typically occur as tip or marginal leaf scorch • more likely to occur on older leaves Biological agents • • • • Leaf lesions scattered or sporatic lesions usually have halo but not always spread readily to surrounding plants margins of leaf spots are typically smooth Host plant disease relationship • All plants are susceptible to one or more diseases • plant diseases have always occurred and continue to occur in nature but are not a problem due to diversity • man made ecosystems tend to be monocultures Epiphytotics • Plant epidemics • monocultures pave the way for this to occur • pythium in turfgrass is an example History of plant disease • 1500 BC - first description of cultural controls, especially planting dates • 950 BC - first use of burning as a cultural control • 470 BC - Pliney the Elder uses olive oil for control of blight in grapes • 1660 AD - connection between barberry and wheat rust discovered History of diseases continued: • 1824 - sulfur recommended for powdery mildew on grapes • 1845 - Irish potato famine • 1858 - “Diseases of Cultivated Plants” published • 1868 - deBary - proved association of fungus with specific plant disease Still more to come! • • • • • 1880 - first commercial sprayer 1882 - Bordeaux mix & Paris green 1883 - Koch’s postulate published 1885 - USDA creates mycology section 1940 - synthetic organic fungicides for protecting military clothing • 1946 - tomato blight Koch’s Postulate • Acceptable proof of causation – organism must be associated with all cases of the disease – organism must be isolated from diseased plant – organism must then be able to produce disease in a healthy innoculated plant – organism must be re-isolated from the “artificially” infected plant Symptoms and Signs • Symptom is defined as any visible evidence of disease expressed by the plant. • Sign is any visible evidence manifested by the pathogen itself. • Symptoms are usually specific for a particular plant. Classifying symptoms • Local vs. systemic • Local occurs in only in one plant organ or tissue • Systemic occurs in two or more plant organs or tissues Classifying symptoms • Lesional vs. habital • Lesional - a specific structural change, obvious damage or wounds • Habital - an abnormal growth habit Classifying symptoms • Primary vs. secondary • Primary symptoms occur at the point of infection • Secondary symptoms occur away from the point of infection Classifying symptoms • Necrotic vs. chlorotic • Necrotic symptoms cause eventual death of the tissue • Chlorotic symptoms involve a loss of or failure to produce color & may or may not result in death of the tissue Classifying symptoms • Hypoplastic vs. hyperplastic • Hypoplastic symptoms are the results of underdeveloped growth • Hyperplastic symptoms are the result of over developed growth Types of necrotic symptoms • Hydrosis - water soaked appearance due to rupturing of cell membranes and resultant “leaking” of the cytoplasm (cell sap) • wilt - loss of turgor pressure, usually a secondary symptom due to plugging of zylem tissue by an organism • dieback - death of the branch system of trees or shrubs Leaf showing hydrosis Types of necrotic symptoms • Scorch - sudden death of the tissue due to physiological agents • blight - sudden death of the tissue due to pathogen(s) • spot - lesions which vary in size, shape, and color • rot - slow decay/mushiness ending in mummification blight scorch Leaf spot More leaf spot Seeing spots yet? Fungal leaf spot and perithecium Healthy cherry on right, rot on left Mummification following rot Types of necrotic symptoms • Canker - oblong lesion with a sunken center usually occurring on stems • damping-off - seeding rot often occurring on the stem near the soil line • scald - blanching of foliage due to high light intensities and standing water stem canker Types of chlorotic symptoms • Chlorosis - failure or partial failure of green color. • Yellowing - loss of green color after it has developed • (difference is academic to some extent) Chlorosis/mottling • Camellia mosaic virus Types of hypoplastic symptoms • Albication - albinism • suppression - failure of organs to develop when and where they usually develop • etiolation - spindly growth with poor color • dwarfing - stunting of the whole plant or specific organ(s) • rosetting - failure of internodes to elongate Rosetting caused by bermudagrass mites Types of hyperplasitic symptoms • Prolepsis - premature organ development • abscission - premature drop of organ(s) • russeting - roughening of the epidermis of organ(s) due to excess suberin deposits • discoloration - expression of pigments other than chlorophyll (bronzing, purpling) • tumefactions - gall formation • fasciculation - clustering of organs, witches broom Discoloration/purpling gall • Cedar-apple rust • Gall on left • Telial horns (i.e. fruiting body) protruding from gall on right Crown gall Types of hyperplastic symptoms • Callus - bulging growth of stem cambium due to lesions • curling\cupping - overgrowth of tissue on one side of a leaf or petal • scab - raised, rough, cracked lesion • fasciation - flat, fan-shaped, over development of stems, leaves, flowers Apple scab on pyracantha Exudates • Ooze - liquid around lesions • slimeflux - fermenting ooze • gummosis\resinosis - thich, viscous secretions around lesions • odors - many diseases have a distinctive odor associated with their exudates