Introduction to Fungi, Third Edition

386 HYMENOASCOMYCETES: PYRENOMYCETES
Fig12.50 Diagrammatic representation of the
relationship between wheat yield (solid line), the
severity of take all infection (dashed line) and the
occurrence of 2,4-diacetylphloroglucinol-producing
Pseudomonas spp. (dotted line) on a field with successive
wheat cultivation.The insert shows the molecular
structure of 2,4-diacetylphloroglucinol. Based partly
on Parry (1990) and Weller et al.(2002).
infects the roots, causing blackening and decay.
As a result, the cereal plants appear stunted and
produce small, non-fertile heads which appear
bleached and are therefore called ‘white heads’.
The fungus overwinters on cereal stubble and
infects the new crop mainly in spring. The infection
mechanism is not entirely clear, and the
fungus may enter host roots directly from dark
melanized hyphae (runner hyphae) or melanized
hyphopodia which may be aggregated to form
infection cushions or mycelial mats (Butler &
Jones, 1949). A hyphopodium is defined as an
appressorium produced from a vegetative hypha
rather than a germinating spore (Fig. 12.49). The
turgor pressure in hyphopodia is around 1.5 MPa
(¼ 15 bar), i.e. considerably less than in appressoria
of M. grisea (Money et al., 1998).
Like M. grisea, G. graminis is a complex of
strains with different but overlapping host
ranges, and varieties tritici, avenae, graminis and
maydis are distinguished. The conidial state of
Gaeumannomyces is a Phialophora, but since the
assignments of anamorphs is difficult, it may be
best to speak of the Gaeumannomyces Phialophora
complex (Bryan et al., 1995). Gaeumannomyces
is closely related to Magnaporthe (Bryan et al.,
1995).
Take-all is considered to be the most important
cereal disease in temperate climates. If
wheat is grown on a field for about four successive
years, the disease will build up to very
high levels, causing crop losses in excess of 50%
(Polley & Clarkson, 1980), but if cultivation is
continued the disease will decline to an acceptable
base level (Fig. 12.50). If a crop rotation
is carried out, the antagonistic properties of
the soil are lost. The reasons for this remarkable
phenomenon are now beginning to be understood,
and it is clear that the establishment of an
antagonistic soil microflora plays an important
role. In particular, fluorescent Pseudomonas
spp., i.e. pseudomonads which produce watersoluble
substances which fluoresce green
or yellow, have been implicated as agents of
take-all decline (Weller et al., 2002). The metabolite
2,4-diacetylphloroglucinol (Fig. 12.50)
produced by them seems to be responsible for
suppression of G. graminis in the rhizosphere
(Raajmakers & Weller, 1998). Wheat roots injured
by Gaeumannomyces are colonized extensively
by Pseudomonas spp., thereby explaining why a
severe take-all attack must occur in order to
render the soil suppressive.
12.10 Glomerellaceae
Like the Magnaporthaceae, the family
Glomerellaceae is a group of plant-pathogenic
fungi which cannot be assigned to any order at
present, although it is certain that it belongs to
the core group of Pyrenomycetes like all other
groups described in this chapter (Wanderlei-Silva
et al., 2003). The family comprises only one