10 lecture 1 principles of disease managment

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10 lecture 1 principles of disease managment

  1. 1. BOT 552: PLANT DISEASE MANAGEMENT Major Principles of Control
  2. 2. Plant Pathology in Practice Goals/Objective Knowledge/Concept -diagnosis -recognition of symptoms, signs Bot 453/553 -pathogen biology -prognosis -disease cycles Bot 350/550 -principles -control -control strategies and tactics -quantitative application -cropping system management Bot 552
  3. 3. Major Principles of Plant Disease Management Strategies before pathogen is present Exclusion = attempts to prevent introduction or establishment of pathogen Strategies after pathogen is present Therapy = procedures once pathogen becomes established
  4. 4. Exclusion of the Pathogen 1. Quarantine = programs and policies to prevent introduction of pathogens. 2. Pathogen or disease-free planting material a. Certification of seed and other planting material b. Treatment of propagation material “Regulatory Plant Pathology” However…………….
  5. 5. Avoidance of the Pathogen – Avoiding disease by planting: 1. at times when inoculum is inactive or when inoculum levels are low, or 2. in geographic areas or planting sites in a local area where inoculum is absent or the environment is unfavorable. Common to grow seed in areas away for commercial production to avoid pathogens.
  6. 6. Eradication of the Pathogen – Reducing, removing, eliminating or destroying inoculum at the source, either from an area or from an individual plant in which it is already established. 1. Rotation to crops which cannot sustain pathogen 2. Treatment of source of inoculum with chemicals, heat, etc. = soil sterilization for potting mix, fumigation 3. Eradicant fungicides 4. Biological control of plant pathogen
  7. 7. Protection of the Plant – Eliminate or reduce the effectiveness of inoculum at the site of infection by imposing a barrier between the plant and the pathogen. 1. Chemical spraying or dusting of foliage 2. Seed treatment 3. Protectant fungicides
  8. 8. Resistance of Plant to Pathogen – Reduces effectiveness of inoculum, thereby reducing establishment of the pathogen Therapy applied to a diseased plant – Cure or minimize losses in diseased plants by using chemotherapeutants or by altering the severity of attack by manipulating the environment to reduce disease response.
  9. 9. Disease Triangle Host Total of all properties that affect susceptibility Pathogen Environment Total of all properties of pathogen Total of all conditions (virulence, abundance, etc.) that affect disease
  10. 10. Vanderplank’s Equivalence Theorem “Effects of host, pathogen and environment can be translated into terms of the rate parameter of an epidemic” Changes in any component has an equivalent effect on disease - More-less susceptible host All affect - More-less favorable environment amount of - More-less aggressive pathogen disease
  11. 11. Therefore, disease management principles and practices are often centered around the concept of the Disease Triangle so that management tactics often seek to manipulate one or more of the components of the disease triangle.
  12. 12. Disease Cycles Independent of host “All pathogens go through a cycle with similar events.” Knowing how particular pathogens go through their disease cycle is important in developing management strategies.
  13. 13. Understanding how disease cycles relate to disease severity is assisted by the discipline of Epidemiology Epidemiology is “the study of factors affecting the outbreak and spread of infectious diseases” Or: the study of disease in populations, how diseases increase over space, in severity, or over time.
  14. 14. A central concept to epidemiology is that different pathogen populations have different disease cycles. I. Monocyclic = single cycle (simple interest) Pathogens that complete one or even part of one disease cycle/year are called monocyclic In monocyclic pathogens the primary inoculum is the only inoculum available for the entire season. There is no secondary inoculum and no secondary infection. The amount of inoculum produced at the end of the season, however, is greater than at the start of the season so the amount of inoculum may increase steadily from year to year.
  15. 15. Disease Time Monocyclic Disease This representation of plant disease over time is referred to as a “Disease Progress Curve” Graphically, disease caused by monocyclic pathogens looks like a saturation curve.
  16. 16. Disease Time Monocyclic Disease xt = Qrt Rate of increase of disease over time can be represented by a simple interest function.
  17. 17. Examples of Monocyclic Diseases Blackleg of potato (Erwinia caratovora) Verticillium wilt Cereal Cyst Nematode
  18. 18. II. Polycyclic = multiple cycles/year (compound interest) Most pathogens go through more than one (2-30) disease cycle in a growing season and are referred to as polycyclic. Only a small number of sexual spores or other hardy structures survive as primary inoculum that cause initial infections. Once infection takes place, large numbers of asexual spores are produced as secondary inoculum at each infection site. These spores can produce new (secondary) infections that produce more asexual spores and so on. With each cycle the amount of inoculum is multiplied many fold.
  19. 19. Graphically this type of population growth is represented as a sigmoid curve Disease Time Polycyclic Disease
  20. 20. Disease Time Polycyclic Disease Rate of increase of disease over time can be represented by a compound interest function.
  21. 21. Many of these pathogens are disseminated primarily by air Or air-borne vectors and are responsible for most of the explosive epidemics in most crops Examples of Polcyclic Diseases Downy mildews Powdery mildews Late blight of potato Leaf spots Blights Grain rusts Aphid borne viruses Root-knot nematodes
  22. 22. III. Polyetic (multi-year) cycles Some pathogens take several years before inoculum they produce can be disseminated and initiate new infections. May not cause many new infections over a given area in a year, amount of inoculum does not increase greatly within a year. However, because they survive in perennial hosts they have almost as much inoculum as they had at the end of the previous year. Inoculum may increase steadily (exponentially) from year to year and can cause severe outbreaks when considered over several years.
  23. 23. Examples of Polyetic Diseases: Some diseases of trees Dutch elm disease Pear decline Citrus tristeza Fungal vascular wilts Mycoplasmal yellows Viral infections
  24. 24. Implications for Disease Management Strategies Monocyclic Diseases Reduce the amount of primary inoculum, or affect the efficiency of invasion by the primary inoculum. Polycyclic Diseases Reducing the amount of primary inoculum has less impact. Reducing the rate of increase of the pathogen more beneficial.
  25. 25. Other Concepts Related to Disease Cycles Successful Infections = symptoms Before symptoms: Incubation period = time between inoculation and penetration and appearance of the disease symptom. The length of the incubation period of different pathogens/diseases varies with: 1. the particular pathogen-host combination 2. the stage of development of the host 3. the temperature in the environment. Can make disease assessments misleading
  26. 26. Latent period = time from infection until production of new inoculum (reproduction). Duration can have a large effect on the rate of the epidemic. Affected by characteristics of the host (stage of development, age of tissue, physiological condition), the pathogen, environment (temperature, moisture).
  27. 27. PropaguleNumber Distance (m or km) Dispersal Gradient Curve Source of inoculum Gradients in pathogen densities and disease are frequently observed. Factors that affect spatial variation in the amount of incoming inoculum lead to dispersal gradients.
  28. 28. PercentageDisease Distance (m or km) Disease Gradient Curve Source of inoculum Gradients in pathogen propagule density can result in Disease gradients = change in disease severity along a straight line away from the source of inoculum.
  29. 29. The percentage of disease and the scale for distance vary with the type of pathogen or its method of dispersal, being small for soilborne pathogens or vectors and larger for airborne pathogens Disease gradients can also be caused by environmental gradients such as, variations in soil type, fertility, or gradual changes in microclimate
  30. 30. DiseaseIntensity Inoculum Dose Dose-Reponse Curve Variation in pathogen density as the result of dispersal gradients or other causes are important relative to the impact of a Dose Response on disease.
  31. 31. Disease Time Unmanaged Disease Level = Crop Loss Managed Disease Level Disease Management Monocyclic Polycyclic Effect of Management on Disease Purpose of disease management is to prevent disease from exceeding some level where profit or yield is significantly diminished.
  32. 32. Principles of epidemiology indicates that control measures can do this in only two ways. 1. They may reduce (or delay) disease at the beginning of the season (x0) or 2. They may decrease the rate of disease development (r) during the growing period.
  33. 33. Management Strategies a Sanitation b Change planting date c Partial host resistance d Eradicant fungicide e Protectant fungicide f Adult plant host resistance Effects on Disease 1. Original progress curve 2. Disease reduced by reducing x0 (a & d) or by delay of epidemic (b & e) Rate same as curve 1 3. Rate changed after disease has begun (f) 4. Rate changed from beginning (c) Zadoks and Schein, 1979
  34. 34. Ways to reduce disease (inoculum) at beginning (x0) Affects monocyclic and polycylic diseases Fumigation Certified seed Sanitation Seed treatments Quarantine Host plant resistance Ways to decrease the rate of disease development (infection rate) (r) Change the environment Fertilizer application Host plant resistance Ways to change t (see “b” on figure) Harvest early before disease becomes severe. Plant early (cereal cyst nematode)
  35. 35. Control of different diseases requires different strategies. Some pathosystems, monocyclic and polyetic diseases can be effected by use of an x0-reducing practice only. However, for most diseases more than one control procedure is used and these are often chosen to reduce x0 and r. These integrated control measures use a combination cultural methods, resistance breeding regulatory actions, chemical control measures

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