ficult to monitor. Without sufficient
monitoring, control measures cannot be adequately timed and growers may be left questioning the
cause of reduced yields or stock
quality. Understanding and becoming aware of the sources and sinks
of pathogens will improve the effectiveness of a holistic agricultural
system.
Most growers throughout South
Africa are affected by pathogens in
some way. The effects may be directly observed through plant mortality (e.g. Phytophthora infestans
causing late blight in potatoes) or
indirectly through costs of implementing biosecurity measures required by new regulations (e.g.
black spot caused by Phyllosticta
citricarpa in the citrus export industry). In general, the effects of
drought or flooding are obvious to
growers, whereas the effects from
pathogens may be difficult to recognize and differentiate from other
abiotic or nonpathogenic disturb-
Baiting soil for Phytophthora
ances. Improving our awareness of
pathogens within our systems can
mitigate their effects and save a lot
of time, energy, and funding.
Recognizing the effects of pathogens can be difficult because the
organisms responsible are microscopic. However, occasionally signs
of the pathogen can be observed.
These signs include physical structures of the pathogen, such as mycelia or mushrooms produced when
masses or colonies of microscopic
cells come together. For example,
Armillaria species can be implicated
as the cause of root rot in woody
plants if a mycelia fan is observed
under the bark at the base of the
stem. However, caution is necessary
because some fungi are saprophytic, meaning they live on dead material, and may have colonized the
plant after it was killed by something else. Other examples of pathogen signs include powdery mildew, white mold, or leaf and stem
rust. The presence of these signs
may suggest the cause of the disease is a fungal pathogen.
In the absence of signs, the type of
symptoms expressed by the plant
may provide insight into the cause
of disease. Symptoms are the reactions of a host to an infection from
a pathogen. Common symptoms in
plants include wilting, chlorosis,
leaf or fruit spots, or the presence
of cankers or galls. Fortunately,
there is much literature available for
growers to reference in order to
narrow down the cause of specific
symptoms on individual crop species. Although it is not absolute,
searching for resources regarding
specific symptoms on individual
crops may provide insight into the
cause. However caution is needed
because just like our own health,
self-diagnoses can be misleading.
Multiple microorganisms can cause
the same symptoms so it is best to
keep an open mind and seek diagnosis from an expert in the field.
Lesions on a bait leaf