peninsula and to have been disseminated by traders
in the nineteenth century. The development of tech-
nologies for raising sterile “tissue-culture” plantlets
(actually derived by meristem culture) should overcome
such problems in the future.
More recent examples can be cited. A serious root-
rot disease of raspberry canes caused by Phytophthora
fragariaevar. rubiis now found in most raspberry
production areas of the world. It seems to have been
spread in vegetative propagating material (canes) of new,
improved cultivars. Consistent with the spread of this
pathogen from a single source in recent times is the
finding that restriction fragment length polymor-
phisms (RFLPs, Chapter 9) are virtually identical in
pathogenic strains across the world (Stammler et al.
1993). The same was true in the past for the spread of
P. fragariaevar. fragariae, the cause of red core disease of
strawberry plants; and for a particular form (biovar. 3)
of the crown gall bacterium, Agrobacterium tumefaciens,
on grape vines. This pathogen has been discovered to
grow as a systemic, symptomless endophyte, so in the
past it would have escaped detection in “visibly clean”
vegetative propagation material.
This catalogue of examples illustrates an important
point: many diseases can be controlled easily and
cheaply, without the need for toxic chemicals, but
simply by “good housekeeping.”Biological and integrated controlSeveral examples of biological control have been dis-
cussed in this book, so here we review the subject
in broader terms. Biological control (biocontrol)
can be defined as the practice in which, or process
whereby, an undesirable organism is controlled
by the activities of other organisms. This definition
covers both naturally occurring biocontrol and the
practical methods used to achieve control, such as:- the purposeful introduction of one organism to con-
trol another;- the purposeful exploitation and management of
naturally occurring biocontrol systems; - the purposeful change of an environmental factor to
promote the activities of natural biocontrol agents; - any combination of these, because often it is neces-
sary to change some environmental factor in order
to favor an introduced biocontrol agent.
- the purposeful exploitation and management of
Examples of the purposeful introduction of a control
agent include the use of Phlebiopsis giganteato control
root rot of pine (see Fig. 12.9); the use of hypoviru-
lence to control chestnut blight (see Fig. 9.15); the use
of mycoparasites such as Trichodermaspp. as seedling
protectants (see Fig. 12.5), and the use of various fungi
to control insect pests (Chapter 15).
The purposeful exploitation of naturally occurring
biocontrol was seen in the decline of cereal cyst nem-
atode (Chapter 15), the exploitation of soils naturally
suppressive to Pythiumdiseases of cotton (Chapter 12),
and the manipulation of turf pH to control the take-
all patch disease of turf grasses (see Figs 12.16–12.18).
The third approach to biocontrol involves chang-
ing an environmental factor to promote the activities
of naturally occurring biocontrol agents. A classic
example of this is shown in Table 17.1, from the work
of Olsen & Baker (1968) on pasteurization of soilin
glasshouse cropping systems. Plant-pathogenic fungi
tend to build up during the course of a cropping
season in glasshouses, so the soil needs to be treated
to destroy pathogens before a new crop is sown.
Traditionally this has been done by raising the soil
temperature to 65–70°C for 30 minutes, using steam–air
mixtures (aerated steam) rather than by treatment
with steam alone (100°C). The use of steam–air mix-
tures is not only cheaper but also more effective,
as shown by the experimental results in Table 17.1
and Fig. 17.1. In this experiment, trays of soil naturally
contaminated with the seedling pathogen Pythium
ultimum were treated at different temperatures then
sown with a thick crop of bell pepper seedlings. A small
inoculum of another pathogen, Rhizoctonia solani,
was placed in one corner of each tray, to simulate a
surviving pocket of inoculum that might be found just340 CHAPTER 17
Disease caused by
Rhizoctonia solani Area of disease (cm^2 )
caused by resident
Temperature Area (cm^2 ) Linear spread (cm) Pythium speciesNo treatment None <1 103
100°C 253 18 0
71°C 65 9 0
60°C 3 2 0Table 17.1Disease caused by Rhizo-
ctonia solaniwhen introduced into one
corner of seedling trays containing soil
treated at different temperatures to
eradicate a natural resident soil popu-
lation of Pythium. (Data from Olsen &
Baker 1968.)