Introduction to Fungi, Third Edition

408 HYMENOASCOMYCETES: ERYSIPHALES
distinguished from Phyllactinia by its rare chasmothecia
bearing only mycelioid appendages
and by its spear-shaped conidia (Oidiopsis
anamorph).
13.8 Controlofpowderymildew
diseases
Several different strategies are employed to control
powdery mildew diseases in various crops,
chemical control and breeding for resistance
being by far the most important in commercial
terms. We describe them here in detail because
the fundamental principles apply to diseases
caused by many other groups of fungi within
the Eumycota, as mentioned elsewhere in this
book.
13.8.1 Breeding for resistance
Wheat and barley were among the first crop
plants for which resistance breeding programmes
were initiated almost a century ago,
following Biffen’s (1905) seminal work on the
genetics of resistance to wheat stripe rust,
Puccinia striiformis. Initial attempts at resistance
breeding against B. graminis as well as other
biotrophic plant pathogens used major gene
resistance based on introducing single resistance
genes into the cultivar of choice. Numerous such
major resistance genes originating from cereal
cultivars as well as wild grasses related to wheat
or barley are now available for breeding purposes
(Hsam & Zeller, 2002). Breeding for major gene
resistance has also been pursued in many other
crops susceptible to powdery mildew diseases,
such as melons and cucumbers (Podosphaera
xanthii and Golovinomyces cichoracearum; Jahn
et al., 2002), or clover (Erysiphe trifolii), hops
(Sphaerotheca macularis) and gooseberries
(Sphaerotheca mors-uvae) (see Smith et al., 1988).
Breeding for resistance in slow-growing perennial
crops such as apple or vines is obviously
rather more difficult.
Major gene resistance is usually based on
a recognition mechanism involving the hypersensitive
response (see pp. 397 398). The danger
inherent in major gene resistance breeding is
that the pathogen can overcome resistance by
mutation of its corresponding avirulence allele
to virulence. This is a frequent occurrence, e.g. in
B. graminis. On the other hand, the frequency of
virulence alleles in the field may decrease again
in pathogen populations no longer exposed to
the cultivar and its resistance gene. Further, by
co-ordinating the release of resistant cultivars,
the effect of major gene resistance in crop
protection can be maximized. For instance, the
‘green bridge’ of B. graminis (see p. 399) can be
broken if the summer and winter cereal cultivars
sown in any one year carry different resistance
genes. Further, it is possible to combine several
resistance genes in one host variety, a process
known as ‘pyramiding’. This requires the pathogen
to develop multiple virulence alleles before
it can infect such a crop variety (Hsam & Zeller,
2002). The danger, of course, lies in the creation
of multiply resistant ‘super-races’ of B. graminis
or other powdery mildews.
An interesting type of resistance in barley is
mediated by the mlo allele which, as mentioned
on p. 397, mediates the formation of very thick
papillae, restricting infection by all races of
B. graminis f. sp. hordei. This resistance is unusual
for a broad-spectrum resistance in giving
almost total control, and it is exceptional for
a single-gene resistance in that it has remained
stable in barley cultivars since it was
introduced about 20 years ago (Jørgensen, 1994;
Collins et al., 2002). In 1990, 30% of the spring
barley acreage was sown with mlo resistant
barley cultivars (Jørgensen, 1992). More
commonly, broad-spectrum resistance is based
on the combined effects of several minor genes
and it is only partial, i.e. it retards infection
and sporulation of the pathogen but does not
altogether prevent disease. It is sometimes
termed horizontal resistance because it controls
many different races of the pathogen to the
same limited level, in contrast to vertical
resistance mediated by major resistance genes
in which individual races are controlled totally,
and others not at all. Although the molecular
basis of most resistance genes is still unknown,
it is noteworthy that some genes involved in
horizontal resistance become more effective in