HORT 1217

1. HORT 1217 Introduction to Plant Pathology

2. Plant Diseases in History Phytophthora late blight caused the potato famine in Ireland (1845 - 1846) widespread famine in northern Europe >1.5 million people immigrated to the US In 1879, downy mildew of grape was introduced into Europe from the US a new disease threatened the vineyards of Europe "Bordeaux mixture" stimulated the study of the nature and control of plant diseases plant diseases cause variable damage each year, often depending on weather patterns

3. Disease Defined Any alteration of a plant that interferes with its normal structure or function and renders it unfit for its normal use Plant disease is the rule rather than the exception. Every plant has disease problems of one sort or another. Fortunately, plants either tolerate these maladies, or they are not very serious in most years.

4. Disease Causes 1. Living factors are infectious (they spread from plant to plant). include pathogens (fungi, nematodes, bacteria, and viruses), insects, and other animals. biotic 2. Non-living factors do not spread from plant to plant. caused by chemical, physical, or mechanical factors. include nutrient deficiencies and water or temperature stress. abiotic

5. Disease Defined One must distinguish between infectious disease, caused by biotic agents, and noninfectious disease (abiotic agents). Infectious organisms can be defined as follows: Fungi Bacteria Mycoplasma Virus

6. Fungus an organism (plant) with no chlorophyll, that reproduces by means of structures called spores, and usually has filamentous growth; e.g., molds, yeasts, mushrooms.

7. Bacterium a single-celled, microscopic organism with cell walls and no chlorophyll; reproduces by fission.

8. Phytoplasma a microscopic, bacteria-like organism that lacks a cell wall, and therefore appears filamentous.

9. Virus a submicroscopic, subcellular particle that requires a host cell in which to multiply; it is not known if a virus is a living or nonliving agent.

10. Viroid a virus-like particle that lacks the outer protein coat of a virus particle.

11. Nematode a microscopic, non-segmented roundworm, usually living in soil, which feeds on plant roots and foliage.

12. Parasitic Seed Plant a higher plant with or without chlorophyll that lives parasitically on other plants, e.g., mistletoe.

14. Damage to Plant Fungi and bacteria cause such plant diseases as leaf spot and fruit, stem, or root rot. Plant viruses, viroids, and phytoplasmas often cause growth distortion, stunting, and abnormal coloration. Nematodes can cause stunting and root distortion. Parasitic seed plants cause a general weakening of the host plant.

16. CONDITIONS NECESSARY FOR DISEASE CAUSED BY PATHOGENS In order for disease to occur, three conditions must be met: Susceptible Host Virulent Pathogen Suitable Environment

17. Disease Requirement1. Host It is necessary to have a susceptible host plant. Each species of plant is capable of being infected by only certain organisms (pathogens). The plant must be in a stage of development susceptible to infection by the disease agent.

18. Disease Requirement1. Host A host or host plant is the plant infected by the pathogen. There are different levels of susceptibility, which include: Immune Susceptible Resistant

19. Host � Levels of Susceptibility Immune means that the host plant cannot be infected. Susceptible means that the host plant can be infected. Resistant means that the host may or may not be infected and that the host plant is able to prevent the pathogen from completely killing the plant i.e., the plant may be infected, but at the time of infection the host plant produces inhibitory or defense compounds that kill or prevent the spread of the pathogen.

20. Disease Requirement2. Pathogen The second requirement is the presence of an active pathogen in a stage of development conducive to infecting the host plant. If there is no pathogen present, there can be no disease.

21. Disease Requirement2. Pathogen A pathogen is a living organism, which infects a susceptible host. The severity of disease is regulated by the virulence of the pathogen The level of virulence determines the aggressiveness of the pathogen.

22. Disease Requirement3. Environment The third condition is an environment suitable for the pathogen to infect the plant. Temperature and moisture are important. Humidity is the critical factor limiting the spread of many foliage diseases The diseases of major importance are those which attack below the soil line where moisture and temperature conditions are favorable. Exception - viruses are plant pathogens that are little affected by climate.

23. Disease Requirement3. Environment Environmental conditions that favor the development of disease are those conditions that: encourage the growth and virulence of the pathogen discourage the growth and defense reactions by the host.

24. Disease Triangle Since all plants can be considered hosts and a number of pathogens survive in the landscape, it is usually the environment that regulates the severity and incidence of disease. However, the severity and incidence of disease is also determined by the susceptibility or resistance of the host and the virulence of the pathogen. The two most important environmental factorsregulating plant disease are temperature andmoisture, with moisture levels being critical. In general, cool moist conditions are going tofavor the development of disease, while hot,dry conditions will deter it.

25. Disease Triangle

26. SYMPTOMS, SIGNS,& SYNDROMES Symptom the physical expression of a change in the appearance and function of the plant.

27. Examples of Symptoms Blights sudden death of twigs, foliage, flowers. Cankers dead places on bark and cortex of stems; often discolored and raised or sunken. Galls abnormal, localized swellings on leaf, stem, or root tissue. Rots general decomposition and destruction of tissue. Necrosis death of tissue. Spots circular or irregular lesions on above-ground tissue.

29. SYMPTOMS, SIGNS,& SYNDROMES Sign the visible presence of the pathogen, such as a fruiting body or discharge associated with the disease

31. Examples of Signs Conks fungal fruiting structures formed on rotting woody plants (shelf or bracket fungi). Mycelia masses of fungal threads (hyphae) which compose the vegetative body of the fungus. Ooze (flux) viscid mass of juices composed of host and parasite substances found exuding from some diseased plants. Pycnidia minute, fungal, asexual fruiting structures, usually globose and black, formed on plant surfaces. Rhizomorphs string-like strands of fungal mycelia sometimes found under bark of trees.

34. SYMPTOMS, SIGNS,& SYNDROMES Syndrome the group of signs and symptoms which collectively characterize a disease. Familiarity with a disease's signs or symptoms is not enough to diagnose a disease; it is necessary to know the syndrome and case history. Seeing a spot on a leaf doesn't tell you much, but finding pycnidia in that spot and knowing the plant species and recent weather conditions might aid in disease diagnosis. Other times, laboratory work is necessary.

35. DISEASE DEVELOPMENT It is important to understand how plant diseases develop in order to control them. By the time it becomes obvious that a plant has a disease, it is generally too late to do anything about it in that growing season. Plants cannot be cured in the way people expect their own ills to be cured. The process by which diseases develop can be broken into five distinct phases: Inoculation Incubation Penetration

36. Disease Development1. Inoculation Introduction of the pathogen to the host plant tissue. Wind, rain, or running water can move pathogens and introduce them to a host. Also birds, insects, people, or equipment. Some pathogens move themselves short distances, but most rely on other means. Sources of inoculum include plant debris, seed, perennial plants, and soil.

37. Disease Development2. Incubation Period of development during which the pathogenundergoes changesto develop into a formwhich can penetrateor infect the new hostplant. Some fungi, for instance, grow a structure called a penetration peg that can grow through the cell walls of the plant.

38. Disease Development3. Penetration Process of getting inside the plant. It may be an active or passive process. Some pathogens produce enzymes to dissolve the cutin and cellulose layers of plant material between them and the cell contents. Some pathogens can swim through water on a plant's surface and into the plant through natural openings (such as stomata, lenticels, or hydathodes) or through wounds. Some pathogens are put inside the plant by insects, pruning tools, or driving rain.

39. Disease Development3. Penetration

40. Disease Development4. Infection Pathogen invades the plant tissue Establishes a parasitic relationship between itself and the host.

41. Disease Development5. Disease The host plant responds to the presence of the pathogen, a disease exists. The host's response results in symptoms of the disease, such as blight or necrosis. As the pathogen matures, it produces inoculum in the form of spores, virus particles, and bacterial cells that can be spread or disseminated to other adjacent, healthy plants.

42. CONTROL OF DISEASES Important to understand the disease development process when considering control options. By the time symptoms appear, the pathogen is inside the host plant and is relatively safe. Therefore, control efforts usually must occur before penetration has taken place. The overall principle in effective disease control is to keep the inoculum density of the pathogen at very low levels. Success in controlling plant disease will occur when a combination of avoidance, exclusion, eradication, protection, resistance, sanitation, and therapy.

43. Avoidance A grower can avoid certain diseases by choice of geographic area or choice of planting site. Disease can be avoided by planting at a time that does not favor disease. Using disease-free planting stock or modifying cultural practices also helps to avoid disease.

44. Exclusion A grower can inspect stock for signs of disease and reject or treat any which is suspect. Plant quarantines are designed to exclude certain pests from areas that are free of that pest. Elimination of carrier insects can exclude a disease.

45. Eradication Once a disease is established in an area, eradication is unlikely. However, significant reduction in disease inoculum can be attained by: destroying diseased plants or alternate hosts rotating crops soil solarization.

46. Protection Spraying or dusting plants with fungicides or bactericides can protect them from disease. Sometimes modifying cultural practices or the environment may protect the crop. Control of carrier insects will also protect plants.

47. Resistance Breeding and selection are used to develop resistant crops. Resistance can be enhanced through proper culture of a crop. Tolerance is another form of resistance in which the plant becomes infected but goes on to mature and yield normally.

48. Sanitation Several disinfectants can be used to surface sterilize growing surfaces. Bleach, in a 1:1 solution, is effective against viruses, bacteria, and fungal spores. It is not effective against sclerotia or other resting fungal structures. Alcohol may be used as a disinfectant of tools, but is not effective against fungal sclerotia, viruses, and some bacteria.

49. Therapy Removal of diseased parts of a plant will sometimes control the disease. Heat can be used to treat contaminated seeds and to eliminate viruses from certain types of fruit tree bud wood.

51. Disease IPM Familiarity with crops and the diseases and insects that affect them is useful in planning control programs. Some diseases occur every season; others occur sporadically. Some can be controlled easily by using proper methods; others must be tolerated. Knowing which problem falls into which category comes with experience. Knowing the proper method to use at the proper time is a part of integrated pest management.