aggressive mode of attack, producing pectic enzymes
to degrade the plant tissues, aided by the production
of copious amounts of oxalic acid (see later).
Rhizoctonia solani
Rhizoctonia solani(sexual stage Thanatephorus cucumeris;
Basidiomycota) is another common seedling pathogen
that attacks the basal stem tissues of plants. It is a
taxonomically complex fungus – a species-aggregate con-
sisting of strains that are differentiated by molecular
fingerprinting, by host range, and by their ability to
anastomose (fuse) with one another when colonies are
opposed on agar plates (see Fig. 9.6). Some of these
strains cause stunting diseases of cereals, such as
crater diseaseof wheat on the heavy, black clay soils
of Northern Transvaal, South Africa (Fig. 14.3). In this
disease the plants grow normally at first, but large
patches of stunted plants start to appear about 3– 4
weeks after sowing, and from this time onwards the
stunted plants make little further growth, while the rest
of the crop grows normally. The roots of the stunted
plants bear bead-like masses of hyphae at about 7–
10 cm below the soil surface, where the loose cultivated
soil meets the underlying clay soil that has not been
ploughed. From these hyphal beads the fungus pene-
trates and kills the roots, leading to the stunting syn-
drome. Similar stunting diseases of cereals are found
in the compacted sandy soils of parts of Australia,
where the roots have brown, spear-pointed ends,
caused by rotting of the root tips.
Diseases of this type have a common cause: the
compacted soil layer impedes root growth, which is
likely to increase nutrient exudation from the root
tips, promoting invasion by Rhizoctonia. In addition
to this, Rhizoctoniagrows as an extensive mycelial282 CHAPTER 14
Fig. 14.2Experimental system to study
infection of leaf petioles (living plant tissue)
from sclerotia of Athelia rolfsiiplaced at dif-
ferent distances on natural soil.Distance (cm) of sclerotia from leaf petiole0123456
Nondried sclerotia
Volatiles absent
Per cent germination 19 16 11 11 19 14 15
Per cent infection 12 0 0 0 0 0 0
Volatiles present
Per cent germination 90 73 67 51 17 14 9
Per cent infection 90 71 54 39 0 0 0
Dried sclerotia
Volatiles absent
Per cent germination 78 75 71 63 74 68 73
Per cent infection 78 75 60 56 0 0 0
Volatiles present
Per cent germination 100 100 96 95 100 98 87
Per cent infection 100 100 91 87 100 82 16
Table 14.1Germination of sclerotia
of Athelia rolfsiiand infection of leaf
petioles when sclerotia were placed at
different distances from the petioles
in the experiment shown in Fig. 14.2.
(Data from Punja & Grogan 1981.)