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