untitled

of this, the normal hosts are found to be resistant

to parasitism if they are grown in liquid culture

media, but susceptible when grown on agar plates. The

hyphae from liquid culture were found to have the three

sugar residues on their surface, presumably because the

glycoproteins that cover them were not produced in

liquid culture conditions.

Potential control of potato black scurf by Verticillium
biguttatum
The mitosporic fungus Verticillium biguttatum is a
biotrophic mycoparasite with potential for use as a
biocontrol agent of the specific strains of Rhizoctonia
solani(anastomosis group AG4) that cause black scurf
disease of potatoes. This disease is characterized by
the familiar black or brown crusts that develop on
the surface of potato tubers. The crusts are sclerotial
masses of Rhizoctoniaand they significantly reduce the
marketability of the crop even though the damage is
only superficial. Like all biotrophs, V. biguttatumhas
relatively little effect on the growth of its fungal host.
It penetrates from germinating spores, produces haus-
toria within the host hyphae (Fig. 12.11), and then
forms a limited mycelium outside the host, where it
sporulates. The infected host hyphae grow more or less
unimpeded, but even a localized infection of a host
colony can markedly suppress the production of

sclerotia over the colony. Apparently V. biguttatum

creates a nutrient sink towards itself within the host

mycelial network, counteracting the normal nutrient

sink towards developing sclerotia.

V. biguttatumcan be grown easily on normal labor-

atory media, and it produces abundant spores for

use in biocontrol. These spores have been shown to

reduce black scurf in field experiments, but there is one

significant limitation: V. biguttatumneeds a relatively

high temperature (at least 12°C) for growth, whereas

Rhizoctoniacan start to grow at about 4°C so it colo-

nizes potato plants earlier in the growing season,

before the mycoparasite can grow. This problem

would not be important if potato tubers could be

inoculated later in the growing season, because sclerotia

are produced only when the potato skins begin to

mature. Agronomists in the Netherlands are exploring

potential ways of doing this by lifting the potato

tubers before the skins mature, inoculating the tubers

with V. biguttatum, and then returning the potatoes to

the soil until the skins mature – a technique termed

“green crop harvesting” (van den Boogert et al. 1994).

Ampelomyces quisqualis, a biotrophic mycoparasite

of powdery mildew fungi

Ampelomyces quisqualisis one of several naturally

occurring biotrophic mycoparasites that are being

246 CHAPTER 12

Fig. 12.11Mycoparasitism of Rhizoctonia solani(anastomosis group 4) by Verticillium biguttatum. (a) Spores of V. bigut-

tatumgerminate near the host hyphae and grow towards the host in a spiral manner, then penetrate the living hyphae

to produce club-shaped haustoria (arrowheads). (b) A similar image, showing that the parasitized host hyphae are still

alive; they have intact vacuoles, and videotaped sequences show active protoplasmic streaming. (From van den Boogert

& Deacon 1994.)

(a) (b)