This
is the time of year, around mid summer, when we can expect an increase of the
moisture content in our weather. Arizona has it worse. They, and New Mexico, frequently
call it their “summer monsoon”. It accounts for 35 to 45% of the state’s annual
precipitation. In Mexico, it accounts for 60% of the annual rainfall in Sonora.
is the time of year, around mid summer, when we can expect an increase of the
moisture content in our weather. Arizona has it worse. They, and New Mexico, frequently
call it their “summer monsoon”. It accounts for 35 to 45% of the state’s annual
precipitation. In Mexico, it accounts for 60% of the annual rainfall in Sonora.
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| Cloud development typical of summer monsoons. |
The
summer monsoon is not well understood but we know that it moves inward from the
Pacific Ocean because the land heats up faster than the Pacific Ocean. The
monsoons mostly stop around the Phoenix area but we do see an increase in clouds
building from the South and, more importantly, an increase in our relative
humidity.
summer monsoon is not well understood but we know that it moves inward from the
Pacific Ocean because the land heats up faster than the Pacific Ocean. The
monsoons mostly stop around the Phoenix area but we do see an increase in clouds
building from the South and, more importantly, an increase in our relative
humidity.
So
why is this important? Because it causes an alteration in some plants
susceptibility to diseases.
why is this important? Because it causes an alteration in some plants
susceptibility to diseases.
This
all goes back to the plant disease triangle. The plant disease triangle was
probably one of the most important concepts I was ever taught in plant
pathology while in school. The disease triangle relates to a three-legged stool;
each leg coinciding with an increased potential for disease development.
all goes back to the plant disease triangle. The plant disease triangle was
probably one of the most important concepts I was ever taught in plant
pathology while in school. The disease triangle relates to a three-legged stool;
each leg coinciding with an increased potential for disease development.
These
three factors are: the predisposition of the plant to disease, the presence of
the disease organism, and an environment that is conducive to the development
of disease. When the summer monsoons come into play, all three of these legs of
the stool become activated. Let me explain.
three factors are: the predisposition of the plant to disease, the presence of
the disease organism, and an environment that is conducive to the development
of disease. When the summer monsoons come into play, all three of these legs of
the stool become activated. Let me explain.
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| Plant disease triangle |
Predisposition
of the plant.
Healthy plants normally ward, or can fend off, diseases. Just like us, the
healthier we are, the better able we are to withstand diseases. The plants more
susceptible to diseases are those that are weakened, in poor health, old,
malnourished or in some way compromised. This is why I encourage you to feed
your plants on a regular basis. This helps to maintain their health. As summer
temperatures increase, plants have a reduced capacity to withstand diseases.
The summer monsoon comes at a time when plants have been hit hard with high
temperatures and are under a lot of stress.
of the plant.
Healthy plants normally ward, or can fend off, diseases. Just like us, the
healthier we are, the better able we are to withstand diseases. The plants more
susceptible to diseases are those that are weakened, in poor health, old,
malnourished or in some way compromised. This is why I encourage you to feed
your plants on a regular basis. This helps to maintain their health. As summer
temperatures increase, plants have a reduced capacity to withstand diseases.
The summer monsoon comes at a time when plants have been hit hard with high
temperatures and are under a lot of stress.
Presence of
the disease organism.
We can make some assumptions. The assumption I like to make is that most common
disease organisms are always present in our garden or landscape. Two factors
that contribute to the possibility that a disease may become a problem are the
amount of inoculum (that is the total amount of bad disease organism) present and
the virulence (the strength) of that organism. If the disease is highly
virulent, it will not require a lot of that organism to be present for the
disease to occur. If there is a lot of inoculum present, the potential for
disease is high.
the disease organism.
We can make some assumptions. The assumption I like to make is that most common
disease organisms are always present in our garden or landscape. Two factors
that contribute to the possibility that a disease may become a problem are the
amount of inoculum (that is the total amount of bad disease organism) present and
the virulence (the strength) of that organism. If the disease is highly
virulent, it will not require a lot of that organism to be present for the
disease to occur. If there is a lot of inoculum present, the potential for
disease is high.
Environment. The third
factor is the physical environment surrounding the plant and the disease
organism. Fungal diseases, our most common plant diseases, require a wet
environment to prosper. Bacterial and viral diseases, less so. When the
humidity increases, the moisture in the environment increases, and the
propensity for diseases to occur increase. If the other two factors in this
three-legged stool are leaning towards a disease problem, higher humidity may
send it over the top.
factor is the physical environment surrounding the plant and the disease
organism. Fungal diseases, our most common plant diseases, require a wet
environment to prosper. Bacterial and viral diseases, less so. When the
humidity increases, the moisture in the environment increases, and the
propensity for diseases to occur increase. If the other two factors in this
three-legged stool are leaning towards a disease problem, higher humidity may
send it over the top.
The
adage in plant disease control is to manage plants and their environment to
minimize disease problems by considering all three legs of the stool. Eliminate one of the legs and the disease can be managed or controlled.
adage in plant disease control is to manage plants and their environment to
minimize disease problems by considering all three legs of the stool. Eliminate one of the legs and the disease can be managed or controlled.
These
summer monsoons come at a time when plants are under a lot of stress due to
high temperatures, high light intensities, poor soils, poor irrigation
practices and a host of other things. The disease organism is present. Count on
it. They are there and ready for any opportunity that presents itself. Now, the
summer monsoons come, increasing humidity. Our biggest disease problems will
come from fungal diseases that prefer higher humidity, higher temperatures and
plants that are susceptible.
summer monsoons come at a time when plants are under a lot of stress due to
high temperatures, high light intensities, poor soils, poor irrigation
practices and a host of other things. The disease organism is present. Count on
it. They are there and ready for any opportunity that presents itself. Now, the
summer monsoons come, increasing humidity. Our biggest disease problems will
come from fungal diseases that prefer higher humidity, higher temperatures and
plants that are susceptible.