Introduction - Disease Control in Crops - page 3

Table 1.2 Estimates of actual losses
due to diseases in worldwide production
of wheat, maize and cotton for the years
1964/65, 1988-90 and 2001-03 (adapted
from Oerke, 2006, © Cambridge University
Press, reproduced with permission).

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the result, not of increased area of cropped land, but of increased inputs, including
pesticides (Hazell & Wood, 2008). In the period from 1963 to 2002, cereal yields
increased by 114% globally, although the annual rate of growth fell from 3.14% in the
period 1963-1976, to 0.84% in the period 1989-2002 (Hazell & Wood, 2008). In the
period from 1960 to 2004, pesticide sales worldwide increased more than 10-fold to some
$30 billion (Oerke, 2006). However, despite this increased pesticide use, crop losses as a
result of pests, diseases and weeds have not fallen signifi cantly in the past 40 years.

1.2
Plant disease can be controlled using a variety of approaches. The fi rst line of defence
is the exclusion of the pathogen through plant quarantine and, for example, the use of
pathogen-free propagating material. The next line of defence is to exclude, eliminate
or reduce pathogen inoculum. This can be achieved in various ways, including cultural
control (see Chapter 2), use of host plant resistance (see Chapter 6) and chemical control.
Cultural methods provide the foundation for disease control in crops. However, in many
parts of the world, diverse ecosystems have been replaced with simple agro-ecosystems,
which are more vulnerable to pathogen attack. The planting of large areas of genetically
uniform crops facilitates pathogen spread and in conjunction with widespread use of
race-specifi c resistance, leads to the appearance of new strains of the pathogen able to
infect and colonise previously resistant crop varieties. Cereal powdery mildews are a case
in point. These fungi can evolve rapidly to overcome host resistance without apparent
loss of fi tness (Bronson & Ellingboe, 1986; Brown, 2003). In some interesting work, the
barley powdery mildew fungus Blumeria graminis f. sp. hordei was found to possess
a repertoire of avirulence ( AV R ) genes, which function as effectors and contribute to

Problems associated with controlling plant disease