Introduction to Fungi, Third Edition

HYPHAL AGGREGATES
19
Fig1.16 Development of sclerotia. (a) The loose type, as seen in Rhizoctonia (Moniliopsis) solani.(b)HyphaofBotrytiscinerea showing
dichotomous branching on a glass coverslip to initiate the terminal type of sclerotium. (c) Later stage of sclerotium formation
in B. cinerea.The hyphae have become melanized and are growing away from the glass surface.They are embedded in a glucan
matrix (arrows). (d) Mature sclerotia of B. cinerea on a stem of Conium. Some sclerotia are germinating to produce tufts of
conidiophores. (e) Sclerotia of Claviceps purpurea from an ear of rye (Secale cereale). Rye grains are shown for size comparison.
(a) and (b) to same scale.
potato tubers. In pure culture, sclerotial initials
arise by branching and septation of hyphae
(Fig. 1.16a). These cells become filled with dense
contents and numerous vacuoles, and darken to
reddish-brown. The mature sclerotium does not
show well-defined zones or ‘tissues’. It is made
up of a central part which is pseudoparenchymatous,
although its hyphal nature can be seen.
Towards the outside, the hyphae are more
loosely arranged; a rind of thick-walled hyphae
is absent (Willetts, 1969).
The terminal type
This form is characterized by a well-defined
pattern of branching. It is produced, for example,
by Botrytis cinerea, the cause of grey mould
diseases on a wide range of plants, and by the
saprotrophic Pyronema domesticum (see p. 415).
Sclerotia of B. cinerea are found on overwintering
stems of herbaceous plants, especially
umbellifers such as Angelica, Anthriscus, Conium
and Heracleum. They can also be induced to form
in culture, especially on agar media with a high