Agriculture Reference
In-Depth Information
tional diversity and synthetic chemical inputs. Consequently, plant pathogens are frequently
suppressed in organic farming systems by enhanced microbial complexity and activity, brought
about by regular soil amendment with recalcitrant organic materials like mature composts
and manure (Mäder
et
al
. 2002, van Bruggen and Termorshuizen 2003, Litterick
et
al
. 2004).
Curative control
There are limited options for curative control allowed under organic agriculture guidelines,
which vary from country to country. Curatives are inputs to the crop production system that
are applied after a pest or pathogen has established in the crop, and threatens to reduce yields
if action is not taken. Table 4.1 provides a representative list of botanically derived pesticides,
microbial agents and other naturally available materials typically approved under organic
standards. These materials vary in their toxicity levels and non-target effects.
In many countries, copper fungicides are allowed for persistent problems such as the
control of late blight on potatoes and downy mildew on grapes. Similarly, sulfur fungicides are
used to control powdery mildew on various crops and scab (
Venturia
inaequalis
) on apples and
pears. The number of sulfur sprays may even exceed that of synthetic fungicides in conven-
tional apple production, but the environmental impact may still be lower (Spruijt-Verkerke
et
al
. 2004). The environmental impact of copper can be significant, considering the broad
impact spectrum and the tendency to accumulate in soil. Finally, some synthetically produced
curatives, such as pyrethroids, are allowed for certain uses as an exception to the rule. However,
the organic regulations are adjusted constantly, and curative applications are becoming more
restricted. For example, copper fungicides are already banned in many countries.
Various plant extracts are allowed under most organic guidelines, provided that they are
not formulated in petroleum-based synergists or carriers. However, they are only rarely used,
primarily as insecticides (Table 4.1). Compost extracts are used more frequently, and are com-
mercially formulated these days (Litterick
et
al
. 2004). They can be very effective in disease
control, depending on the starting material, the composting and fermentation procedures,
and the final microbial activity.
Curative biological control can be accomplished by inundative release of selected biocon-
trol agents. Although many specific biological control agents against plant pathogens, insect
and nematode pests have been identified, relatively few species have been registered for field
use, primarily parasitoids and predators for insect and mite control, and some fungi and
bacteria for insect and pathogen control. Biocontrol of soilborne pathogens has been success-
ful under controlled environmental conditions using simplified potting mixes, but has often
failed when selected microorganisms were added to field soil (Fravel 1999). This is also the
case, even more so, for foliar microbial biocontrol agents due to the increased exposure to the
elements. We recently noticed that the bacterial biocontrol agent
Pseudomonas
f luorescens
did
not survive as well in organically as in conventionally managed soil (Hiddink
et
al
. 2005). It
may be more difficult to get a biocontrol agent established in a microbially diverse organic soil
than in a microbially impoverished conventional soil, as could be expected from invasion
biology theory.
One might expect that organic growers would use biological control proportionally more
than conventional growers, but surveys indicate that biological control agents are rarely applied
on organic farms (Langer 1995), with the exception of
Bacillus
thuringiensis
(Bt) for caterpillar
control and various parasitoids and predators in greenhouse production. In most countries,
organic regulations allow the application of biological control agents, provided that no petro-
leum-based synergists or carriers are used in the formulation. It is possible that the greater bio-
diversity in organic agroecosystems (in the open air) reduces the effectiveness of inundative
biological control agents through intraguild predation or competition.
