Diagnosing plant diseases involves a systematic process of deduction to determine the cause of damage. Symptoms alone are not enough, as different diseases can cause similar symptoms. The process includes: [1] Defining the problem by identifying the affected plant and examining signs and symptoms; [2] Looking for patterns of uniform or non-uniform damage that may indicate biotic or abiotic causes; [3] Observing how damage develops over time which can indicate living or non-living causes; [4] Determining the cause by considering pathogens, insects, weather, chemicals or other factors; [5] Synthesizing information to identify the most probable cause. Forecasting plant disease epidemics involves predicting occurrences based on historical weather data

1. DIAGNOSIS OF PLANT DISEASES

2. Diagnosis of a plant disease means determining the
factors that damaged the plan.
It is determining the cause of a plant disease.
Symptoms alone are inadequate in determining the
cause of a plant disease because different diseases can
produce similar symptoms.
In diagnosing plant damage - a series of deductive
steps are followed to gather information and clues
from the big, general situation down to the specific,
individual plant or plant part.
It involves a systematic, diagnostic process of
deduction and elimination to determine the most
probable cause of the plant damage.

3. Steps in diagnosing plant damage
1. Define the problem
2. Look for patterns
3. Delineate Time-Development of damage
pattern
4. Determine causes of the plant damage
5. Synthesis of information to determine
probable causes

4. Definition of the Problem
A. Identify the Plant affected and its
characteristics
B. Examine the entire plant & its community for
signs and symptoms

5. Looking for Patterns
Involves checking the pattern of damage to
determine whether it is uniform or not.
A. Living Factors usually cause Non-Uniform
patterns of damage
B. Nonliving Factors cause Uniform patterns of
damage

6. Delineation of Time-Development of Damage
Pattern
Involves observing the change in damage
pattern over time.
A. Living factors multiply progressively. Thus
there is progressive spread of the disease with
time
B. Non-living factors do not spread with time
from one area to another. The damage is
localized in a given area.

7. Determining causes of the Plant Damage
A. Distinguish plant damage caused by living factors by looking for the
following:
i. Symptoms and signs of pathogens
ii. Symptoms and signs of insects, mites (vectors)
B. Distinguish plant damage caused by non- living factors by looking at
the following:
i. Mechanical factors - pressure exerted
ii. Physical factors – environment or weather changes.eg extreme temp
iii. Chemical factors
a. Field damage patterns
b. Injury patterns on individual plants
(i) Direct contact of toxic chemical with shoot or root
(ii) Symptoms produced after phloem or xylem translocation
of chemical
(iii) Use keys to chemical disorders in plants
C. Diagnostic clues from references, laboratory analyses

8. Synthesis of Information to Determine
Probable Causes
• Put all the information gathered together and
determine the most probable cause of the
plant disease.

9. FORECASTING EPIDEMICS
Forecasting is predicting when plant disease is
likely to occur and in what proportions.
Disease monitoring and forecasting systems
are important for management of plant
diseases.
There are 2 types of forecasting systems:
a. Empirical system
b. Fundamental system

10. Empirical system – based on studying and
comparing historical records of diseases
occurrence and the concurrent weather
(environmental) conditions in a given locality.
Based on the observations, a set of rules are
formulate that specify the specific meteorological
conditions that are used to predict diseases
occurrence
Fundamental system – disease predictive system
developed from data obtained from experiments
in the laboratory or field trial. Also called derived,
inductive or logical systems.

11. Conditions for successful forecasting
i. Use few parameters – when the number of
parameters used to predict an infection increases,
the probability of making an accurate forecast
decreases.
ii. Infection rate of pathogen should not exhibit
variability each year, such that only rate of
infection and not parameters is used to predict
epidemic.
iii. Forecasting systems should be developed only for
diseases that cannot be controlled by genetic
resistance and can completely destroy major food
crops

12. END