graminis(see Fig. 9.11), which usually declines to non-
damaging levels within 1 year if a different (noncereal)
crop is sown. As we saw in Chapter 12 (see Fig. 12.2),
the alternative is to grow cereal crops continuously, year
after year, until the soil becomes naturally suppressive
to take-all.
Crop management practicescan be altered to
avoid disease. In many countries stinking smut of
wheat (caused by Tilletia caries, Chapter 14) is seed-
transmitted and must be controlled by seed-applied
fungicides. But in the Pacific northwest of the USA it
is mainly soil-borne, and if winter wheat is sown early
it reaches the most susceptible stage while the soil mois-
ture and temperature are unsuitable for germination
of the smut spores. Other simple and effective disease
avoidance practices include the liming of soil to prevent
serious clubroot disease of cruciferous crops caused
by the protist Plasmodiophora brassicae. Phosphate
deficiencycan be a major cause of yield losses. The
browning root rot disease of wheat, caused by Pythium
graminicola(Oomycota), was one of the most damag-
ing diseases in the North American prairies until the
1940s, but was virtually eliminated when phosphate
fertilizers became widely available.
Meteorological forecastinghas been used for many
years as a disease-management tool. It was developed
initially for control of potato blight(Chapter 14)
by Beaumont in 1947. By studying the relationship
between weather and blight epidemics, he established
that, after a certain date which varies from region
to region, a blight epidemic will develop within 2–3
weeks following a 2-day period in which the temper-
ature is 10°C or more and the relative humidity is
more than 75%. This 2-day period became known as
a Beaumont period, and it enabled growers to time
the application of fungicides so that major epidemics
could be avoided. This has now been refined further,
and similar forecasting methods are widely used in other
cropping systems. One example is the facial eczema
warning system for Pithomyces(see Fig. 7.20).
Sanitationcan be highly effective for controlling
or avoiding disease. Two mycoparasites, Trichoderma
harzianumand Pythium oligandrum, are best known for
their potential to control plant pathogens (Chapter 12)
but can also be a problem in commercial mushroom
production because they attack and destroy the
mycelium of Agaricus bisporus. A distinctive strain (or
strains) of T. harzianumhas become a problem in some
British mushroom sheds, when spores contaminate the
trays of compost. P. oligandrumalso has caused serious
cropping losses in some mushroom-production units.
A recent example was described from New Zealand,
where internal transcribed spacer (ITS) ribosomal DNA
(Chapter 9) was used to identify a specific strain of P.
oligandrumresponsible for a serious outbreak of mush-
room disease (Godfrey et al. 2003).
The growth of fungi such as Amorphotheca resinaeand
Paecilomyces variotiin aviation fuel storage tanks has
been particularly difficult to control. These fungi grow
on the long-chain n-alkanes of aviation kerosene
(Chapter 6) and cause problems by blocking filters
and corroding the walls of fuel-storage tanks when
they produce acids as metabolic byproducts. At least
one aircrash has been attributed to A. resinaein the
1960s, before the seriousness of this problem was
recognized. Theoretically, these fungi should not grow
in aviation fuel because they require water. But it is
almost impossible to prevent water from seeping into
fuel storage tanks or condensing during changes of
air temperature. Then the fungi grow at the fuel–water
interface. This problem is now controlled by a com-
bination of measures – lining the tank walls with
rubberized materials to prevent corrosion, regular
checking and cleaning of filters to prevent blockages
caused by fungal growth in the fuel lines, and the
addition of biocides to the fuel – often mixtures of
organoborates, or ethylene glycol monoethyl ether.
Quarantineis one of the principal methods for
preventing the spread of pathogens, but is becoming
increasingly difficult to enforce with the volume of
international trade. Most major epidemics are caused
by fungi that were introduced inadvertently from
other countries. Typical examples include potato
blight(Chapter 14) and the several major epidemics
of Dutch elm diseasein North America and Western
Europe, which have been linked to introductions of elm
timber that was not de-barked to remove the beetle
vectors (see Fig. 10.9). The devastation caused by
chestnut blight(Cryphonectria parasitica) in North
America seems to be traceable to the introduction of
an ornamental Asian chestnut tree to the New York
Zoological Garden in 1904 (see Fig. 9.13), although
recently it has been suggested that the fungus must have
been present earlier than this to have caused such exten-
sive devastation.
A unique strain of the human-pathogenic fungus
Cryptococcus neoformansis increasingly being isolated
from AIDS patients and has been traced to tropical
origins. The same strain was discovered to grow on cer-
tain types of eucalypt tree in Australia, and the regions
where the clinical strain occurred corresponded to
regions where these trees had been imported – an
association that could not have been foreseen for
quarantine purposes (Chapter 16). The spread of
Panama disease of bananas (Fusarium oxysporumforma
specialis cubense, Chapter 14) to most banana-growing
regions of the world was almost certainly caused by
transport of planting material – bananas are estab-
lished from pieces of corm at the base of the stem,
because commercial bananas are sterile triploids,
unable to establish from seeds. This fungus is thought
to have its center of origin in the southeast Asian