Parasitism and Plant Disease
Important DfiiiDefinitions: • Pathogen –a disease causing agent – Pathogenicity – Ability for an organism to interfere with one or more essential functions of another organism –causing disease – Virulence – The degree of pathogenicity of a pathogen • Parasite – Organisms that live on or in another organism and obtains its food from the later – Plant Parasite –Organism that becomes intimately associated with the a plant and multiplies/grows at the expense of the plant • Sapppyrophyte – Organisms that live on dead matter and secrete enzymes to break down material for energy Parasitism and Plant Disease Pathogen Spectrum Non‐ Obligate Parasites
Facultative Facultative Saprophytes Parasites
Obligate Parasites Obligate Saprophytes (Biotrophs) (Necrotrophs) Parasitism and Plant Disease
Parasites Saprophytes • Tend to have narrow host • Tend to have broad host range – except viruses range • Does not need to kill plant • Kills plant tissues to acquire cells to complete lifecycle nutrients for growth and • Intimate relationship with complete lifecycle plant – continuous absorption of nutrients • Secrete enzymes and toxins • Grows inter‐ and intra‐ • Grows inter‐cellularly cellu lary • “Werewolves” • Infected plants tend to be stunted, poor vigor • “Vampires” Parasitism and Plant Disease Disease Triangle
Disease occurrence is an All sides must favor disease for interactive event! the disease process to take place!
Amount of Disease
Total of conditions favoring susceptibility Host Parasitism and Plant Disease Disease Triangle Factors affecting the Pathogen: 1. Virulence 2. Population present 3. Life‐stage of propagule 4. Vectors present Parasitism and Plant Disease Disease Triangle Factors affecting the Host: 1. Level of resistance 2. Growth stage of plant 3. Genetic uniformity of neighboring plants 4. Plant Vigor 5. Plant Density 6. Structure of Plant Parasitism and Plant Disease Disease Triangle Factors affecting the Environment: 1. Conditions that favor disease 2. Conditions that promote plant growth 3. Conditions that affect dispersal
Includes levels of humidity, temperature, wind speed, and periods of rain Parasitism and Plant Disease Disease Triangle Interpretation of the triangle: ‐ Length of each side is proportional to the sum total of the chhtitiaracteristics of each component that favors disease Amount Amount ‐If one side isAmount zero, no disease ofof of ‐If one side isDisease very large, Disease disease potttilDiseaseential is large Total of conditions favoring ‐ Quantification of the triangle’s susceptibility area would represent the Host amount of disease Plant Disease Development
Diseases are a series of distinct events that occur in succession leading to the perpetuation of the disease and pathogen
This chain of events is called the disease cycle! ‐Closely related to the lifecycle of the ppgathogen Plant Disease Development
Infection Host Invasion Recognition Colonization
Pathogen Penetration The Disease Cycle Growth and/or Reproduction
Attachment Symptom Dissemination of 2° Development Incubation Inoculum Dissemination Production of 1° Inoculum Dormant Stage Dormant Period Plant Disease Development
PiPrimary Events: 3 1. Inoculation 2. Penetration 4 3. Establishment of Infection 2 The 5 4. Invasion Disease 5. Growth and reproduction of the pathogen Cycle 6 (Colonization) 6. Dissemination of the pathogen 1 7. Survival in the absence of 7 the host Plant Disease Development
1. IliInoculation ‐ Initial contact of Inoculum Sources pathogen with a ‐ Survival in perennial susceptible plant site plants, weeds, ‐ Innoculum: pathogen contaminated soil, soil brought into infection debris, seeds, court transplants, and vectors ‐ May be spores, sclerotia, mycelial fragments, Arrival of inoculum bacteria, or viruses 1. Passively (i.e. wind) ‐ Primary inoculum – 2. Chemotaxis – organisms primary infection within the soil are ‐ Secondary inoculum – attracted to plant roots secondary infection 3. Vector‐transmitted (i.e. insects) Plant Disease Development
2. Penetration 2.4 Recognition between host and pathogen 2.1 Attachment to host ‐ Triggers plant resistance genes –if ‐ Fungi and bacteria produce present –promotes /prevents gelatinous substances to help infection them stick to leaf surface 2.5 Penetration 222.2 Spore germination ‐ Direc t – FtiFormation of appressorium, ‐ Mainly Fungi –spore germinates and penetration peg forming germ tube and moves ‐ Indirect along host to find opening/weak point ‐wounds ‐ wind/growth cracks, wind blown sand, lesions 232.3 Appressorium Formation caused by other pathogens, ‐ Fungi only – Formation of vectors appressorium and “softening enzymes” and ppprepares for ‐natural oppgenings ‐ Stomata, penetration into plant hydrathodes, lenticels Plant Disease Development 2. Penetration (cont.)
Agrios, 1997 Plant Disease Development 2. Penetration (cont.)
Photos from: Read, N.D. , Kellock, L.J., Knight, H., Trewavas, A.J. (()1992b). Contact sensing during infection by fungal pathogens. Plant Disease Development 3. Establishment of Infection ‐ Infection: Process by which pathogens establish contact with susceptible cells and procure nutitrien ts
‐ Successful infections result in symptoms –viiblisibly ddtetec tbltable changes in the plant
‐ Obliga te fflungal parasites form haustoria ‐ intracellular “vampires”
‐ PthPathogen releases enzymes, ttioxins, and growth regulators
Photos from: Read, N.D. , Kellock, L.J., ‐ Plan t reacts with ddfefense Knight, H., Trewavas, A.J. (1992b). Contact mechanisms sensing during infection by fungal pathogens. Plant Disease Development
4. IiInvasion Pathogens spread through plant
Fungi and bacteria: Spread by intracellular and intercellular growth by using enzymes and hormones, lllidocalized
Nematodes: Move intercellularly, localized
Viruses, Viroids, and xylem/phloem limited bacteria: Move cell to cell intracellularly, systemic Plant Disease Development
5. GthGrowth and reprodtiduction of the pathogen (()Colonization) ‐ Pathogens continue to spread until the infection is stopped or the plant is dead
‐ Only fungi and nematodes can actively move
‐ All other pathogens rely rapid reproduction and being passively moved Plant Disease Development 5. Growth and reproduction of the pathogen (Co lon izati on) ‐ Reproduction: ‐ Fungi –spp,ores, inter‐ and intra‐, surface and interior ‐ Bacteria –cell division, inter‐ and intra‐, surface and interior ‐ Viruses –intra‐, inside cells only ‐ Nematodes –inter‐ and intra‐, surface and interior ‐ Parasitic Plants – seeds, exterior of plant only Rate varies on pathogen present, environment, and host Plant Disease Development
6. Dissemination of pathogen
Agrios, 1997 Plant Disease Development 7. Survival of pathogen without a host Bacteria: same way as fungi; infected plants, seeds, tubers, (Overwintering stage) and plant debris; in the bodies of Fungi: mycelium in cankers, bud insect vectors. Survive better in scales, seeds, tubers, and plant debris; spores; and sclerotia large slimy colonies than as small groups. ‐ Soil inhabitants –survive in soil indefinitely ((ppy)saprophytes) Viruses: survive only in living ‐ Soil transients –survive in plant tissues; roots of perennial soil for short period of time plants, seeds of some hosts, and (parasites) insect vectors.
Parasitic Plants: Seeds and Nematodes: Survive as eggs in vegetative from on host the soil; or lifestages that are dormant in seeds and bulbs Plant Disease Development
7. Survival of pathogen without a host
Agrios, 1997 Plant Disease Development
Disease Epidemics: 1° inoculum Monocyclic: completes 1 disease cycle in a year 1° infection ‐ 1° inoculum is only inoculum for entire year ‐ Disease increases year to year as inoculum builds Over‐seasoning Stage
Polycyclic: 2or more disease cycles in a year 1° infection ‐ most pathogens 1° inoculum ‐ disseminated by air, and airborne vectors ‐ create explosive epidemics –Late 2° inoculum Bligg,ht, Powdery Mildew, and Rusts 2° infection
Polyetic – requiring two or more years to complete lifecycle (considered monocyclic) ‐ typical of many vascular wilt Over‐seasoning Stage pathogens –Dutch Elm Disease