Agriculture Reference
In-Depth Information
Diseases, pests, and resistance
to these
Introduction
Apples and pears are subject to a large number of diseases and pests. Some are
very obvious and may cause distinctive damage. Others are virtually symptom-
less other than leading to reduced growth and cropping.
The impact of the different diseases and pests may be reduced by confining
susceptible cultivars to regions of climate unsuitable for the pathogen. It may
alsobereducedbytheuseofquarantineand'planthealth'proceduresdesigned
to ensure healthy planting material. Control of damage where conditions are
suchastomakethisaseriousthreatmaybeachievedbychemicalandbiological
control agents and by the deliberate breeding of resistant cultivars.
With the world-wide expansion of apple and pear growing, new pest and
disease problems have arisen. Consumer fears of pesticide residues and con-
cerns about ecological impacts have placed significant constraints on chemical
control. Biological and 'integrated' control methods to reduce chemical inputs
may require sophisticated localized monitoring of both pathogens and envi-
ronmental factors. Genetic resistance may break down as new strains of disease
organisms and pests evolve. Control of disease and pest incidence is thus very
complex. Emphasis in this chapter is given to the pests and diseases of greatest
importance in Europe, North America and Australasia and genetic resistance
to these. Discussion of chemical and biological control and of specific pests
and diseases prevalent in Asia, Africa and South America is more limited.
Virus and MLO diseases
Most commercial cultivars of
Malus
and
Pyrus
are highly tolerant of viruses
which cause severe effects on non-commercial forms of these genera (Posnette,
). This presumably reflects a historic process of selection for tolerance; for
example, a number of quince clones discarded by Hatton for incompatibility
