Introduction to-plant_pathology

Introduction to Plant Pathology AND environmental impact

Disease = disturbance from plant pathogen or environmental factor that interferes with plant physiology Causes changes in plant appearance or yield loss Disease results from : Direct damage to cells Toxins, growth regulators, or other byproducts that affect metabolism Use of nutrients and water or interference with their uptake

Disease Pyramid The interaction of components of plant disease can be expanded to include time and humans. Time is often considered as the fourth component of plant disease development. The four components together can quantify the amount of disease. The human equation can affect the three components of the disease triangle and should be considered as a fifth component in disease development.

Host Factors All plants can be considered hosts Degree of genetic uniformity – crop plants – inbred lines Age – affects disease development depending on plant-pathogen interaction There are different levels of susceptibility, which include: Immune - cannot be infected. Susceptible - can be infected. Resistant - may or may not be infected, and is the plant able to prevent the pathogen from killing it. ie. defense compounds

Pathogen Factors Amount of inoculum Pathogen genetics Virulence of the pathogen Type of reproduction: Monocyclic polycyclic Ecology and mode of spread Air Soil Seed Vector dependency

Environmental Factors Moisture Temperature Effect of human culture practice Monoculture Amount of inoculum: seed quality, disease residues, rotation, alternate host Introduction of new pathogens

Disease Development Every infectious disease requires a series of sequential events in order for disease to develop. Specific characteristics are unique for each disease . General events are: dispersal of the pathogen to the host penetration and infection of the host invasion and colonization of the host reproduction of the pathogen pathogen dispersal pathogen survival between growing seasons and/or in the absence of a host

Fungi Diverse and widespread Filamentous (hyphae) form a network of mycelium (lots of hyphae) Recognized by reproductive structures (mushrooms, rusts, conks, etc.) Most of the 100,000 spp. are saprophytes Live on dead organic matter Approximately 8,000 species attack plants Plant pathogens

Fungal Diseases Reproduction by sexual and asexual means Spread through a variety of methods wind/water blown spores rhizomorphs Sclerotia (overwintering) Include organisms from Kingdom Protista, that are now classified outside the Kingdom Fungi: Downy mildews Pythium Phytophthora Clubroots

Symptoms Initially, similar to drought & starvation: Plants appear off-colour Weakened & susceptible to attack Wilting and dieback occur later Younger plants usually killed rapidly Older plants decline over time (years) Roots have brownish streaks

Bacteria Prokaryotic microscopic organisms Free living single cells, or Filamentous colonies Reproduce via binary fission 2 daughter cells are identical to mother cell Don’t usually produce resistant resting spores Need host or growth medium to survive For rapid spread, plant infecting bacteria usually require: Warmth Moist conditions

Bacterial Diseases Less common than fungal or viral diseases They can be either: parasites, saprophytes, or autotrophs Symptoms include: Cankers, Wilts, Shoot Blights, Leaf Spots, Scabs, Soft Rots, & Galls Generally, cannot invade healthy tissue; need wound or opening to infect. Control methods usually cultural in nature (don’t use antibiotics on large scale)

Bacterial Diseases Bacterial galls: In some cases, toxic materials are produced that cause plant tissues of roots, stems or leaves to grow abnormally as in crown gall. Bacterial leaf spot disease: The bacteria usually enter through leaf stomata. Symptoms include water-soaking, slimy texture, fishy or rotten odor, confined initially between leaf veins resulting in discrete spots that have straight sides and appear angular .

Gene on gene action! Evolution of the plant–bacterial pathogen interaction. (a) Plants have evolved receptors that could recognize PAMPs and triggers basal defence. (b) Bacterium injects effector protein through type III secretion system (T3SS) to interfere with defence signalling or response. (c) Plant responds to infection by generation of immune receptors encoding for nucleotide-binding (NB), MAP kinase, leucine-rich-repeat (LRR) R-proteins that recognizes effector protein and triggers an acute defence response usually involving hypersensitive response (HR) and programmed cell death

Disease Development Infections occur through leaf scars and wounds. These give rise to small cankers in which the bacteria survive the winter. Rain or water splash, and pruning tools spread the bacterium. Bacteria overwinter in active cankers, in infected buds, and on the surface of infected and healthy trees and weeds. The bacterium reproduces best between 21 ºC and 25 º C. The disease seems to be more severe after cold winters and prolonged spring rains.

Viruses Viruses are "submicroscopic" entities that infect individual host plant cells. Viruses are obligate parasites: They can only replicate themselves within a host's cell. In the virus infected plant, production of chlorophyll may cease (chlorosis, necrosis) Cells may either grow and divide rapidly or may grow very slowly and be unable to divide

Viral Diseases > 400 viruses infect plants; few are economically important pathogens The infection remains forever Viruses are transmitted from plant to plant by living factors: insects, mites, fungi and nematodes Or non-living factors: rubbing, abrasion or other mechanical means (including grafting or other forms of vegetative propagation) Occasionally transmitted in seed.

Virus Disease Symptoms The symptoms of most virus diseases can be put into four categories: Lack of chlorophyll formation in normally green organs Stunting or other growth inhibition Distortions Necrotic areas or lesions

Nematodes Microscopic roundworms Barely visible with naked eye No segments Up to 4mm long Clear or transparent Feed with stylet Pierce plants (pests) Kill arthropods (beneficials)

Nematode Diseases Plant pathogenic nematodes = pests Infect roots & bulbs (below-ground) Foliar nematodes (above-ground) Also vectors of plant viruses As they feed, they weaken & stress plants – also predispose to other problems Causes bulb & root decline, and root knots Spread by splashing water, and infested soil & plant parts

Shoot Nematodes ( Aphelenchoides spp.) Foliar nematodes feed inside leaves between major veins causing chlorosis and necrosis. Injury is most often seen at the base of older foliage. When plants with a net-like pattern of veins become infested with foliar nematodes, the tissues collapse in wedge-shaped areas and then change color.

Root Nematodes Moisture and nutrient stress symptoms and general stunting are common (by killing meristem tissue) Root lesion nematodes ( Pratylenchus spp.) Burrowing nematodes ( Radopholus similis ) destroy root cortex tissues as they feed Root-knot nematodes ( Meloidogyne spp.) inject growth-regulating substances into root tissues as they feed, stimulating growths called galls or knots

Environmental and cultural factors affecting buildup of fungal and bacterial plant pathogens Moisture Temperature Dispersal agents Soil pH Other

Moisture Activates resting stages Affects germination of spores and penetration into host Water on leaves Humidity Splashing water distributes inoculum Leaf wetness = best indicator but difficult to measure

Moisture Activates resting stages Affects germination of spores and penetration into host Water on leaves Humidity Splashing water distributes inoculum Leaf wetness = best indicator but difficult to measure Rainy, cloudy conditions = important for spread and growth of many diseases

Temperature Affects growth rates Some pathogens adapted to certain temp. ranges Refrigeration = important for management

Dispersal Agents Bacteria, fungi are limited in mobility, need to be moved by: Water Wind People, machinery Insects, other animals

Soil pH specific requirements for many soil-borne pathogens Other Widespread planting of genetically homogeneous crops can favor epidemic

Management of Plant Disease – Strategies Eliminate or reduce initial inoculum, or delay its introduction (preventive) Slow the rate of increase, shorten exposure to favorable conditions

Management of Plant Disease Sanitation Fungicides Host plant resistance Crop rotation Cultural practices Temperature Biological control Organic amendments Improved plant health and nutrition

Sanitation (aimed at excluding pest) Avoid infested sites Clean soil, planting material, tools, etc. Inspection and quarantine Remove infected debris Tissue culture can provide disease-free planting material

Fungicides Bactericides, if target is bacteria Dusts, sprays, fumigants, etc. Foliar, soil, seed, wound, or post-plant application Preventative – slows rate of increase Insecticides may also be useful for managing insect vectors

Host Plant Resistance Caution: pathogens can have multiple isolates Vertical resistance – against some genotypes of a pathogen Horizontal resistance – not limited to certain genotypes, across all isolates Host genetic diversity is important to slow epidemics

Crop Rotation Useful vs soil-borne diseases Residues of some plants (e.g., cabbage family) may be toxic to some pathogens

Cultural Practices to Minimize Spread of Disease Favorable irrigation practices (drip vs overhead) Timing of Planting Wider row spacings Eradicate alternate hosts for viruses Important to minimize water and humidity to limit disease spread Moisture management

Temperature Heat for soil sterilization Hot water treatment of planting material Solarization Refrigeration to slow disease progress in harvested material

Management of Plant Disease Sanitation Fungicides Host plant resistance Crop rotation Cultural practices Temperature Biological control – Rhizobacteria may interfere with colonization of plant roots by fungi and bacteria Organic amendments (avoid diseased plants in mulch, etc.) Improved plant health and nutrition

Introduction to-plant_pathology

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