Agriculture Reference
In-Depth Information
CHAPTER 18
APPLE SCAB: ROLE OF ENVIRONMENT IN PATHOGEN AND
EPIDEMIC DEVELOPMENT
A.L. JONES AND G.W. SUNDIN
18.1 INTRODUCTION
Apple scab (apple black spot) continues to be a serious disease in virtually all apple-
producing regions worldwide despite the steady flow of research reports on its
biology, epidemiology and management for more than a century; for reviews see
MacHardy (1996) and Aylor (1998). Largely due to these efforts, the disease has
been satisfactorily managed but lasting solutions have been elusive. Periodic
outbreaks of problems with fungicide resistance (Köller, 1994) and the breakdown
of resistance in scab-resistant cultivars (Parisi and Lespinasse, 1996) are constant
reminders that scientists are only ahead of the pathogen temporarily.
The early research on the epidemiology of apple scab provided a strong
foundation for future studies. The use of controlled environmental chambers for
defining the environmental factors that favour infection was pioneered by Keitt and
Jones (1926). Later, Mills (1944) used data from these studies to develop one of the
first practical disease predictive systems. The Mills system established a rational
basis for developing management strategies for scab. Much of today's scientific
literature on apple scab represents the efforts of many scientists to re-evaluate,
embellish and update the research contributions of these early scientists.
The objective of this chapter is to provide a modern assessment of the
epidemiology of apple scab and is organized as a running narrative of research on
the Mills system, the foundation for many of the epidemiological studies on apple
scab in modern times.
18.2 AETIOLOGY OF APPLE SCAB
In most apple-producing regions
Venturia inaequalis
(Cooke) G. Wint., the cause of
apple scab, has one sexual infection cycle per year and a series of asexual infection
cycles. In these regions
V. inaequalis
may also produce conidial primary inoculum
in lesions on wood, shoots, and outer and inner surfaces of bud scales (Becker
et al.
,
1992; Holb
et al.
, 2004; Moosherr and Kennel, 1995; Stensvand
et al.
, 1996). The
sexual cycle is highly significant in causing scab epidemics because the number of
ascospores potentially available from apple leaf litter is considerably greater than the
number of conidia potentially available from diseased wood, shoots and buds, and
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