Agriculture Reference
In-Depth Information
and frequent fungicide and insecticide applications). On organic farms, cereal crops are still
profitable as they are grown less intensively. Moreover, they are essential crops in the longer
rotation schemes. Some of these crops (oats, rye, triticale) are also used as winter cover crops
to reduce nitrate leaching. Because of the large differences in production practices for cereal
crops at organic versus conventional farms in Europe, many comparative studies of farming
systems have focused on these crops. Most comparisons have involved single (experimental)
farms side by side (Rabbinge and Zadoks 1989). In a few cases, replicated experimental treat-
ments were compared (Hannukala and Tapio 1990), and even fewer publications covered large
surveys on many farms (Tamis and van den Brink 1998 1999).
From all these studies it is apparent that fungal root and foot rots caused by
Fusarium
species are similar or reduced in organic cereal crops. The same holds for take-all disease
caused by
Gaeumannomyces
graminis
and for eyespot and sharp eyespot caused by
Pseudocer-
cosporella
herpotrichoides
and
Rhizoctonia
cerealis
, respectively (van Bruggen 1995). In some
studies, the lower disease levels at organic farms were associated with lower N application rates
(van Bruggen and Termorshuizen 2003), higher microbial activity and diversity (Hiddink
et
al
. 2005), or greater populations and/or diversity of soil microfauna (Mäder
et
al
. 2002, van
Bruggen 1995). Another reason for suppression of root diseases in organic cereal crops could
be enhanced competition by arbuscular mycorrhizae, which are generally more abundant in
organic than in conventional crops, due to the lower available N and P in organically managed
soils (Oehl
et
al
. 2004).
The lower N application levels in organic cropping systems also have a tremendous inf lu-
ence on above-ground diseases and pests. Powdery mildew, snow mould and stripe rust of
wheat were less severe in a long-term organic than in a neighbouring conventional experimen-
tal farm in the Netherlands, despite regular fungicide applications in the conventional system
(Daamen
et
al
. 1989). This was attributed to lower N levels in the organic wheat tissue, but
could also be due to a more open canopy structure and less conducive microclimate. In an
extensive survey of 150 Dutch wheat fields, most above-ground diseases (snow mould, powdery
mildew,
Septoria
leaf and glume blotch,
Fusarium
scab) were less severe in the organic than in
the conventional and integrated farming systems (Tamis and van den Brink 1998). The differ-
ences were significant for snow mould, glume blotch, and
Fusarium
scab. Contrary to the two-
farm study mentioned above (Daamen
et
al
. 1989), there was no significant difference for
stripe rust, while leaf rust and powdery mildew on the ear were significantly more severe in
organic than in conventional farms (Tamis and van den Brink 1998). Incidences of diseases
that were higher in conventional farms than in organic farms were again positively correlated
with N application rates (Tamis and van den Brink 1998). The higher severity of leaf rust and
powdery mildew on the ears of organic than on those of conventional wheat plants may also
have been related to N concentrations, since N is released from soil organic matter in the
summer time, and can be higher in organic than in conventional farms at that time.
Plants high in N can also support large aphid populations (van Emden 1966, Thresh 1982)
and are often more susceptible to virus infection. Thus, when high soil nitrate concentrations
coincide with aphid f flights, the population may grow explosively. Unfortunately, organic
farmers have little control over the time when N is released in soil, and in some seasons aphid
populations may be as high in organic as in conventional farming systems (Daamen
et
al
. 1989,
Piorr and Hindorf 1986). Leaf miners and cereal leaf beetles were generally more numerous in
the conventional farming system and were associated with high N application rates (Daamen
et
al
. 1989). In an extensive field survey, no significant differences were found in populations
of aphids, leaf miners and cereal leaf beetles between organic and conventional farms, but
there were enormous variations among years (Tamis and van den Brink 1998). Nevertheless, it
is important to try to keep mineral N concentrations minimal, also at organic farms. This can
