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94 CHAPTER 5

They are formed by relatively few fungi – mainly
Basidiomycota. At their most complex they can be up
to 1 cm diameter, with clearly defined internal zona-
tion. Sclerotia of this type are produced by omnivorous
plant pathogens such as Athelia (Sclerotium) rolfsiiand
Sclerotinia sclerotiorum, by the ergot fungus Claviceps
purpurea, and by some mycorrhizal fungi such as
Cenococcum geophilumand Paxillus involutus(although
the sclerotia of these mycorrhizal fungi are quite
small). Some other fungi produce microsclerotia less
than 100μm diameter; these are merely clusters of
melanized chlamydospore-like cells – for example, in
the plant pathogen Verticillium dahliae. Between these
extremes lie a range of types, such as the spherical or
crust-like sclerotia of Rhizoctonia solani(sexual stage:
Thanatephorus cucumeris) commonly seen as brown,
scurfy patches on the surface of potato tubers.
All sclerotia develop initially by repeated, localized
hyphal branching, followed by adhesion of the
hyphae and anastomosis of the branches. As the scle-
rotial initials develop and mature, the outer hyphae
can be crushed to form a sheath, while the interior of
the sclerotium differentiates into a tissue-like cortexof
thick-walled, melanized cells, and a central medulla
consisting of hyphae with substantial nutrient storage
reserves of glycogen, lipids, or trehalose (Chapter 7).
Sclerotia can survive for considerable periods, sometimes
years, in soil. They germinate in suitable conditions,
either by producing hyphae (the myceliogenicsclerotia
of A. rolfsii, Cenococcum, R. solani,etc.) or by produc-
ing a sexual fruiting body (the carpogenicsclerotia of
Claviceps and Sclerotinia sclerotiorum).
Nutrient depletion is one of the most important
triggers for sclerotial development. In these conditions,
sclerotia develop rapidly from pre-existing mycelia or
from sclerotial initials laid down at an earlier time,
and a large proportion of the mycelial reserves of the
fungal colony are remobilized and conserved in the
developing sclerotia. Christias & Lockwood (1973)
demonstrated this by growing four sclerotium-forming

fungi in potato-dextrose broth, collecting the mycelial

mats before they had produced sclerotial initials, then

washing the mats and placing them either on the

surface of normal, unsterile soil or on sterile glass

beads through which water was percolated continuously

to impose a nutrient-stress equivalent to that in soil

(Chapter 10). In all cases the mycelia responded by

initiating sclerotia within 24 hours, and the sclerotia

had matured by 4 days. When these sclerotia were

harvested and analyzed for nutrient content, they

contained up to 58% of the original carbohydrate in

the mycelia and up to 78% of the original nitrogen.

An example is shown in Fig. 5.10, for A. rolfsiion

leached glass beads. Such high levels of carbohydrate

conservation could only be explained if some of the

wall polymers of the fungal hyphae are broken down

and the products are remobilized into the developing

sclerotia. As we shall see later, wall polymers can be

degraded by controlled lysis and used as nutrient

reserves to support differentiation of many types of

structure, including the “fruiting bodies” of mush-

rooms and toadstools.

Nutrient-translocating organs

All fungi translocate nutrients in their hyphae, but some

fungi produce conspicuous differentiated organs for

bulk transport of nutrients across nutrient-free envir-

onments. Depending on their structure and mode of

development, these translocating organs are termed

mycelial cordsor rhizomorphs. They are quite com-

mon among wood-rotting fungi, and also among the

ectomycorrhizal fungi of tree roots, where carbohydrates

are transported from the roots to the mycelium in

soil, and mineral nutrients and water are translocated

back towards the roots. Mycelial cords are also found

at the bases of the larger mushrooms and toadstools,

serving to channel nutrients for fruitbody develop-

ment (Fig. 5.11).

Fig. 5.10Conservation of mycelial carbon

(glucose equivalents) and nitrogen (glycine

equivalents) into newly formed sclerotia

of Athelia rolfsii, 4 days after mycelial mats

were transferred to starvation conditions.

The values shown are percentages of the

original carbohydrate or nitrogen in the

mycelial mats that became incorporated

into sclerotia or that were lost by respira-

tion or by leakage into the glass beads on

which the mycelial mats were incubated.

(From Christias & Lockwood 1973.)