Agriculture Reference
In-Depth Information
virulence (Ridout
et al
., 2006). The authors suggested that multiple copies of related
but distinct AVR effector paralogues might enable populations of
B. graminis
f. sp.
hordei
to rapidly overcome host resistance genes while maintaining virulence (Ridout
et al.,
2006).
Fungicides are an integral part of crop production in most parts of the world. Without
fungicides, crop losses in the region of 10% for wheat and rice, for example, would be
considerably higher (Oerke, 2006). In soybean, yield losses due to the rust
Phakopsora
pachyrhizi
are reported to be in the region of 80%, with projected losses in the Mato
Grosso and Bahia areas of Brazil alone amounting to 2.2 million tonnes ($487 m). These
losses were incurred despite the use of fungicides at a cost of $544 m (Yorinori
et al.,
2005). One of the problems with repeated, high frequency use of fungicides is that fungi
can become insensitive to them. The development of fungicide resistance in pathogen
populations has been a major problem since the use of single-site mode of action fun-
gicides became widespread. For example, the benzimidazole group of fungicides was
introduced in the 1970s and disruptive selection for resistance occurred quickly in a
number of pathogens leading to a loss of fungicide effi cacy in the fi eld. The mid-1990s
saw the introduction of the Quinone outside Inhibitor (QoI) group of fungicides, which
includes the strobilurins. These fungicides quickly took a large share of the cereal
fungicide market because of their effectiveness against important cereal pathogens.
Although the risk of resistance developing was predicted to be moderate, the fi rst signs
of resistance were reported within two years in the wheat powdery mildew pathogen,
Blumeria graminis
f. sp.
tritici
, in northern Germany. By 2000, resistance to this group
of fungicides in populations of wheat powdery mildew was widespread throughout north
west Europe and in 2002, isolates of
Mycosphaerella graminicola
(anamorph
Septoria
tritici
, the cause of septoria leaf spot) resistant to strobilurin fungicides were detected
in fi ve European countries (Leadbeater, 2005). Resistance to QoI fungicides has also
been detected in other cereal pathogens, including
Pyrenophora tritici-repentis
on
wheat and
Drechslera teres
on barley (Leadbeater, 2005). Moreover, in a recent study,
4200 isolates of
Alternaria solani
were collected in the 5-year period from 2002 to 2006
from 11 potato-producing states in the United States. Of these isolates, 96% exhibited
reduced sensitivity to QoI fungicides and/or had the F129L mutation in the cytochrome
b
gene (Pasche & Gudmestad, 2008).
In the past 50 years, pesticide use has greatly increased the quantity and improved
the quality of food for the increasing world population. However, as pesticide use
increased, so did concern about their adverse effects on non-target organisms, including
humans. Pesticide poisoning of non-target organisms has been identifi ed as a cause of
mortality in fi sh, reproductive failure in birds and illness in humans (Rao
et al.,
1993).
Increasing public concern about the accumulation of pesticides in the environment and
the impact on non-target organisms has led to the introduction of rigorous regulatory
processes (Holm
et al.,
2005; Stark, 2008). Nevertheless, there is continued concern
over the impacts of pesticides on wildlife, including invertebrate populations, wild plants
and farmland birds (Best & Burn, 2005). These concerns have led to reviews of active
substances used in plant protection, with the resulting withdrawal of an increasing num-
ber of crop protection products from the market (Richardson, 2005). This has created
problems and in some situations, effective control measures are no longer available to
meet all the challenges posed by pathogens, pests and weeds (Richardson, 2005).
