Plant Disease Defined
Plant disease is the abnormal growth and development of a plant:
Growth and development of the plant does not live up to the normal expectations. Some standard of "normal" must be assumed.
A diseased plant is incapable of carrying out its normal physiological functions to the best of its genetic potential.
Basic Concepts In Plant Pathology
Many different living and nonliving entities can have a negative affect on plants.
A. Infectious Diseases (caused by biotic organisms):
a. Fungi
b. Prokaryotes:
i. Bacteria
ii. Mycoplasmas
c. Viruses and viroids
d. Nematodes
e. Parasitic Higher Plants
B. Non-Infectious (caused by abiotic factors)
a. Temperature extremes
b. Moisture extremes
c. Light extremes
d. Nutrient extremes
e. Soil acidity or alkalinity (salt problems)
f. Pesticide toxicity
g. Air conditions: pollution, strong winds, etc.
h. Improper cultural practices
2. A disease episode requires the interaction of three components: The Disease Triangle
A. The host must be susceptible to disease, that is at the proper age and physiological state for infection and development of disease to occur. Healthy, strong growing non-stressed plants are less susceptible to disease than plants under stress.
B. The pathogen must be virulent (able to cause disease) not in a state of dormancy and present in a certain minimum population level.
C. The environment must be conducive (favorable) for the development of disease:
temperature, moisture, nutrients, wind, etc. must all favor the pathogen.
D. The degree to which these three components interact, relate to the severity of the disease episode (e.g., if the host is highly susceptible, the pathogen highly virulent and the environment highly conducive, then the disease will be very severe).
E. Successful disease control depends on the integrated use of available control methods.
Symptoms and Signs
1. Symptoms are the response of the plant to attack by a disease causing agent.
A. Examples: leaf spots, wilting, stunting, chlorosis, necrosis, etc.
2. Signs are the visual presence of some structure formed by the pathogen on the host.
A. Examples: mycelium, spores, fruiting bodies, bacterial ooze, etc.
Organisms Associated with Diseased Tissue
1. Primary organism - the organism is directly responsible for the disease.
2. Secondary organism - the organism(s) is taking advantage of weakened tissue.
3. Disease complexes and organism succession.
A. Disease complex refers to the situation where the disease is caused by more than one organism. Disease complexes are especially common in turf.
B. Organism succession refers to the fact that plants are colonized over time-by many different organisms. For example, when plants are healthy, they are colonized by nonpathogenic symbionts When the plants become diseased, they are first colonized by primary pathogens, then by secondary organisms, and eventually by other saprophytes. Saprophytic organisms are in association with healthy plants. The primary disease causing agent is only operating by itself for a short period of time. Secondary organisms may be weak-pathogens or pathogens. Weak pathogens are organisms that are not aggressive and (typically) do not cause disease by themselves.
C. Disease complexes add to difficulty in disease management as identification and control of one organism may accelerate activity of another organism in the complex.
D. Organism succession makes primary pathogen identification difficult. The diseased specimen must be examined relatively quickly after disease symptoms begin; otherwise, secondary pathogens or saprophytes are all that can be found. Accurate diagnosis of the causal agent is required for effective use of chemical control measures. Use of an inappropriate chemical will not only be ineffective against the disease agent, but can also lead to additional disease problems by killing beneficial microorganisms in the environment.
Steps In Diagnosis of Plant Disease
Questions to ask grower or manager.
1. Identify the plant species affected: genus, species, and cultivar (whenever possible).
A. In a mixed turf stand, identify the species and which are diseased or not diseased.
B. In the landscape, note if (and what) other types of plants are affected.
2. Observe the symptoms:
What plant parts are affected?
Is the disease spreading or localized? On the plant and to other plants.
Try to determine when the symptoms first appeared.
3. Determine the environmental conditions prior to and during symptom development.
Temperature - day and night.
Moisture - air (humidity) and soil.
Wind, hail, dust, blowing sand, etc.
4. Determine the growing conditions:
A. Determine soil type - sand, silt, clay, drainage, etc.
B. Where is plant growing? Lawn, home garden, organic garden, landscape, courtyard, greenhouse, golf course, commercial nursery, indoors, etc.
C. What is the exposure - sun, shade, etc.
D. What is the proximity of the plant to structures such as, other plants, buildings, sidewalks, roads, walls, etc..
E. What is the irrigation history? How, how much, how often, time applied, etc.
F. What is the fertilization history? What, how, how much, how often, etc.
G. What is the history of chemical use? what, how, how much, how often, etc.
5. Additional Questions for;
A. Home gardens:
a. Size of garden.
b. Crop rotation.
c. Proximity to structures.
B. For turfgrass:
a. Presence of other plants in or nearby the turf area.
b. Terrain (slope).
c. Age of turf and how it was established (seed, sod, plug).
d. Maintenance practices: mowing, aeration, de-thatching, raking, etc.
C. For commercial fields:
a. Acreage.
b. Crop Rotation.
c. Past problems in the field.
d. Percent of plants affected and distribution of diseased plants in the field (scattered, one side, in one area, etc.)
e. Proximity to structures.
f. Crops growing nearby.
6. Microscopic examination of the tissue - dissecting, light and electron microscopy.
7. Isolation and identification of associated microorganisms.
A. Plate sample on culture media.
a. General ; water agar, PDA, nutrient media, etc.
b. Organism specific - bacteria, actinomycetes, fungi, etc.
c. Selective - type (e.g., group of fungi, such as oomycetes, etc.), genus, etc.
B. Moist chambers.
C. Diagnostic kits:
a. Detection of fungi.
b. Identification of bacteria.
c. ELISA for virus detection and identification.
8. Diagnostic resources:
A. APS compendium
B. UC IPM guides
C. Extension publications
D. Host indexes
E. Reference books: identification of fungi, bacteria, nematodes; plant disease handbooks; textbooks; etc.
F. Past experience
9. After diagnosis - what next?
A. Determine the major contributing factors for disease development.
B. Identify the means by which the organism -operates and survives (over winters).
C. Ask yourself
a. Can anything be done now?
b. Is damage significant to warrant action?
c. What action should be taken now? Later?
d. What management practices can the grower/home owner reasonably do? Cost? Equipment? Labor?
e. Can the problem be avoided in the future? How?
D. Make recommendations:
a. Cultural practices
b. Variety selection
c. Chemicals
Principles of Plant Disease Control
1. Avoidance - Avoiding disease by planting at a time when, or in areas where the pathogen is
ineffective, rare or absent.
2. Exclusion - Reducing, inactivating, eliminating or destroying the pathogen at the source.
3. Protection - Preventing infection by use of a toxicant or other barrier between the host and the pathogen.
4. Disease Resistance - Use of plant genetic resistance or tolerance.
5. Therapy - Reducing the effect of the pathogen in an already infected plant.
6. Trap Crop - Establish plants attractive -to insect vectors on the borders or the main crop, then destroy the trap crop and the vector.
Major Plant Disease Control Methods
1. Cultural Control - examples:
A. Sanitation - pruning, removal of debris, removal of diseased plants, sterilizing tools, washing hands, etc.
B. Use of disease-free planting material.
C. Choice of planting location.
D. Time of planting.
E. Choice of irrigation method and schedule.
F. Choice of fertilizer: type, timing, application method, schedule.
G. Crop rotation.
H. Use of green manures or cover crops.
2. Biological Control - the use of living organisms that are antagonistic to pathogens.
A. Stimulate beneficial organisms in the environment with soil amendments or other cultural practices.
B. Add beneficial organisms to the soil or plant environment.
3. Resistance or Tolerance - host plant genetic control.
A. Generally the most effective means of control when available.
B. Must be continually monitored as pathogens will develop virulence to tolerant plant material.
4. Chemical Control - Act to eliminate, reduce or remove the pathogen at the source (eradication); to prevent disease (protection), or to cure disease (therapy).
A. Examples: fungicides, bactericides, nematicides, soil fumigants
B. Chemical used must be less toxic to the plant than to the organism(s) they are designed to control.
C. Most fungicides are actually fungistats, which means that the chemical limits the activity of the fungus, but doesn’t kill it. The disease will return when the chemical is no longer active and the conducive environment for fungal activity reoccurs, Thus, management of diseases with fungicides often requires repeat applications.
D. Effective chemical use depends on:
a. Choosing the right chemical.
b. Applying the chemical in the right way, at the right time, and in the right concentration.
c. Reading and following the label directions very carefully.