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2 Even in young root zones, and persisting into the

old root zones, a fungus with hyaline (colorless)

hyphae is seen in the innermost cortical cells. This

is almost certainly an arbuscular mycorrhizal fungus

growing in the living host cells (Chapter 13).

3 Behind the root tips, Mucorand Penicilliumspecies

are found on the root surface, and they remain

associated with the surface of roots of all ages.

These fungi are assumed to grow on the soluble nutri-

ents that leak from root cells, or possibly the root

surface mucilage (a pectin-like material). However,

their hyphae cannot be seen or identified easily in

the root zone, so it is possible that they have an

initial phase of growth or periodic phases of growth

and then persist as spores on or near the root

surface.

4 A fungus with darkly pigmented hyphae is found on

the root surface in young root zones, and is seen to

occur progressively deeper in the cortex as the roots

age. This fungus is Phialophora graminicola, one of the

most abundant fungi on grass roots. It is a weak

parasite that exploits a narrow window of opportun-

ity, invading the root cortical cells as they start to

senesce, but ahead of purely saprotrophic species

(Chapter 12).

5 As Phialophoraprogresses inwards through the root

cortex, it is followed by a sequence of fungi, includ-

ing an unidentified sterile (nonsporing) fungus that

grows as hyaline (colorless) hyphae, and Fusarium

culmorumwhich is a characteristic colonizer of dead

and dying tissues but also can be pathogenic to

cereals and grasses in conditions of water-stress.

6 In the old root zones Trichoderma spp. and

Clonostachys rosea (formerly called Gliocladium

roseum) colonize the outer cortex. These are charac-

teristic soil and rhizosphere fungi. They are known

228 CHAPTER 11

Fig. 11.16Natural (nonpathogenic) cortical cell death in soil-grown wheat roots stained with acridine orange (a,c) or
neutral red (b,d). All the cortical cell layers are alive in (a) and most layers are alive in (b). The outer five (of a total six)
cell layers have died in the older root regions (c and d) but the innermost (sixth) layer always remains alive until the
root itself dies. (From Lascaris & Deacon 1991.)