Agriculture Reference
In-Depth Information
Pathogens and pests can also be avoided by adapting the crop planting time or rotating
crops that harbour different suites of pests and pathogens (temporal isolation). The time
between rotations of a particular crop is usually longer in organic than conventional field crop
production (5-8 years v. 2-3 years). Successive planting of the same crop is not permitted
according to organic standards in the EU, but rotation times are minimised in organic green-
house production as only high value crops with similar cultural practices can be grown such as
tomato, sweet pepper and cucumber in rotation. Organic growers practicing short crop rota-
tions in greenhouses can face severe problems with root knot nematodes in soil-bound pro-
duction of these high-value crops.
Organic growers can avoid diseases and pests by planting susceptible crops at times of the
year when certain pests are less pervasive. For example, to avoid severe damage from late blight,
organic farmers plant early maturing potato varieties early in the growing season so that tubers
have grown to a reasonable size by the time late blight becomes pervasive (Tamm
et
al
. 1999).
Asynchrony of dispersing insect pests and susceptible crop stages is a tactic used commonly by
organic and conventional growers worldwide. For example, winter wheat production in the
northern US can be timed to avoid infestation by Hessian f fly (
Mayetiola destructor
), which lives
as an adult for only a few days. The US Department of Agriculture provides estimates of f fly-free
dates for growers sowing varieties that are otherwise susceptible to the pest. Subsistence farmers
in Malawi avoid devastation of their bean crop by sowing with the first rain. Later crops are
attacked heavily by bean f fly (e.g.
Ophiomyia spencerella
), which can kill the plant. Planting
crops before peak aphid vector f flights can reduce virus infection in US lettuce production. The
next generation of aphids is often wingless and is less likely to move between plants. Planting
dates can also be used to reduce vertebrate pest damage, such as January plantings of sunf lower
crops in India, planned to reduce the synchrony of sprouting plants and mature seeds with the
incidence of harmful birds (house crows and rose-ringed parakeets) (Mahli 2000).
Invasion biology and metapopulation theory claim that organisms can be prevented from
spreading into patchy islands when the distance between patches is large. Therefore, crop fields
can be isolated from source pools by keeping large distances between fields with the same
crop. In northern California, organic production of kiwi fruits is successful in part because of
sparse cultivation of this specialty crop. In organic agriculture with longer rotations, the patch
sizes (fields with a certain crop) are frequently smaller than in conventional agriculture.
Moreover, fields are often separated by strips of natural vegetation on organic farms. Thus,
crop plants can be pathogen-free and herbivore-free as a consequence of locating fields distant
from coloniser pools (Letourneau 1999). However, small-scale organic farmers cannot always
take advantage of this method to avoid pests and diseases, since all plots may be located very
close together. For example, fields on some small-scale organic farms are organised as 'pie-
pieces' in a circle, where all crops shift to the next small 'pie-piece' in the following season.
Although this circular arrangement is appealing to customers who buy produce at the farm,
risks of pest and disease problems are enhanced through movement of pathogens and pests
from plant residues to neighbouring seedlings of the same species. Finally, the source pool
itself can be removed by destroying weeds that act as carriers or alternate hosts of crop pests
and pathogens. Weeds and natural vegetation surrounding cropping areas may harbour
various diseases, particularly viral and bacterial diseases (Thresh 1982, Wenneker
et
al
. 1999).
Also, poplar trees may harbour lettuce root aphids, which move to adjacent lettuce crops
(Phillips
et
al
. 1999). On the other hand, non-crop vegetation can host various parasitoids and
predators that contribute to insect pest control (Barbosa 1998, Pickett and Bugg 1998), and a
judicious approach to weed control is needed. Many organic farmers remove weeds selectively
and maintain natural surroundings carefully to avoid the spread of pests and diseases into
their crops.
