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FUNGI AS PLANT PATHOGENS 283

Fig. 14.3Crater disease of wheat caused by Rhizoctonia solaniin Northern Transvaal, South Africa (Deacon & Scott
1985). (a) Aerial view showing extensive areas of stunted plants. (b) A disease crater, showing the sharp boundary
between the healthy crop and a stunted patch. (c) The boundary between the loose surface soil and the heavily com-
pacted clay soil occurs at about 5– 8 cm depth. (d) Beads of Rhizoctonia on the first-formed seminal roots; these roots
are important because they would normally penetrate deep into a soil profile, so that water can be tapped from a
depth when the surface soil layers have dried. (e) Comparison of normal healthy plants and plants within a crater.

(a) (b)

(c) (d) (e)

network in the lower, undisturbed soil layer. The
disease patches represent the areas where this fungal
network is present beneath the soil surface, and these
patches are spread progressively along the plough
lines (Fig. 14.3), increasing in extent from year to
year. The most effective way of controlling these
diseases is by deep-ploughing, although this is seldom
economic on heavy clay soils. However, on the sandy
soils of Australia the use of a tine that rips the soil about
10 –12 cm below the level at which the seed is sown
can allow the roots to penetrate deeply and greatly
improve crop yields.

Decline and replant diseases

Several perennial fruit crops, including strawberry,

apple, cherry, and avocado, show a progressive decline

in yield as the plants age, until production is no

longer economic. If the declining plants are removed

and replaced by others the new plants often grow

poorly or die. These decline andreplant diseasesare

closely related, indicating a common cause. As we saw

in Chapter 10, the young “feeder roots” of plants

often have a short life span before they senesce and