Field Guide of Discovery-based Exercises for - Aseanipm ...

Exercise No. 5.01 174
DISEASE TRIANGLE RELATIONSHIP: UNDERSTANDING
SPREAD OF DISEASES IN ORGANICALLY-GROWN
VEGETABLES
BaCKGroUND aND raTIoNalE
Disease is a function of host, pathogen, and environment;
all are components of a disease triangle. Disease triangle is
based on an equivalence theorem, which states that effect of
environment, pathogen, and host can each be translated into
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Field Guide of Discovery-based Exercises for Organic Vegetable Production
terms of epidemic rate parameter. A result is that changes in anyone of disease triangle components
(e.g., from a more to less susceptible host, from a favorable to an unfavorable environment, or
from a more aggressive to a less aggressive pathogen) all have an equivalent effect on an epidemic.
Therefore, it is not surprising that disease management is centered on this equivalence theorem and
that much disease predictive systems are based on one or more components of a disease triangle 175 .
For participants to effectively manage common diseases of organically-grown vegetables, they must
develop a conceptual definition of a disease. The concept of a disease and factors associated with
their occurrence are important tools in developing management strategies for diseases. As a way
of synthesizing results of a disease triangle exercise, role-playing may be conducted. This method
will allow facilitators to gauge how deep participants’ understanding is of initial participatory
discussions on plant diseases topics. Similarly, this activity will provide participants an opportunity
to clear some gray areas or misconceptions about aforesaid equivalence theorem.
In farmer field schools (FFSs), this activity is aimed at developing farmers’ basic knowledge on
diseases and their occurrences as affected by components of a disease triangle. This exercise was
developed to serve this purpose.
How long will the exercise take?
when is this exercise most
appropriate?
ɶ During FFS, TOT, and
VST sessions as starting
activity of an ‘Integrated
Disease Management’
topic; and
ɶ When farmers want
to learn how organic
vegetable diseases
develop.
• Thirty minutes to one hour field walk observation and simulation exercise; and
• Thirty minutes to one hour brainstorming session.
174 Adapted from Callo, Jr. D.P., A.G. Castillo, and C.A. Baniqued (eds). 2001. Field Guide of Discovery-based Exercises for Corn Production. SEAMEO
Regional Center for Graduate Study and Research in Agriculture (SEARCA), College, Laguna, Philippines. pp360-362. pp380-383.
175 Johnson, K.B. 1987. The role of predictive system in disease management. In: Teng, P.S. (ed). 1987. Crop Loss Assessment and Pest Management. The
American Phytopathological Society, Minnesota, U.S.A. pp176-190.