untitled

blasticidin-S, and reduced binding of kasugamycin to

the ribosome.

Validamycin Ais used to control several diseases

caused byRhizoctonia, including seedling diseases,

black scurf of potato, and sheath blight of rice, but

it is inactive against most fungi and bacteria. It has

no effect on Rhizoctoniain rich culture media, but it

causes abnormal branching and cessation of growth in

weak media. Validamycin has been found to be con-

verted to validoxylamine in fungi, and this compound

is a strong inhibitor of trehalase, the enzyme that

cleaves the fungal disaccharide trehalose to glucose (see

Fig. 7.6).

Antibiotics implicated in biocontrol

The roles of antibiotics in interfungal interactions

were discussed in Chapter 12, so here we consider

only the practical applications of these compounds in

disease control. Several new biocontrol products are

undergoing field trials or have been marketed in the

last few years. One of these, marketed as GlioGard™

but now as SoilGard, consists of small, air-dried

alginate beads containing fermenter-grown biomass

of Trichoderma virensand wheat bran as a food base.

This product is thought to act mainly by producing

the antibiotic gliotoxin. The dried granules are mixed

with soil-less rooting media in glasshouses and left for

several days after the medium is moistened, before

seedlings or rooted cuttings are transplanted into the

beds. The timing of these operations is crucial, because

the fungus must grow on the wheat bran substrate

and produce gliotoxin which diffuses into the rooting

medium, protecting the plants from pathogenic attack.

If there is a delay in planting then the antibiotic levels

start to decline and the degree of protection is reduced.

Rhizosphere bacteria also produce many antifungal

compounds. Table 17.4 shows the results of “non-

selective” screening (on Trypticase Soy agar) of the dom-

inant bacteria from a range of plants by a commercial

company (Leyns et al. 1990). The most common bac-

teria with antifungal properties included Pseudomonas

spp. (some of which produce the antibiotics pyrrolni-

trin, pyoluteorin, phenazines, and diacetylphloroglu-

cinol), Xanthomonas maltophilia(unknown antibiotics),

Bacillus subtilisand other Bacillusspp. (some of which

produce iturins, mycosubtilins, bacillomycin, fengy-

mycin, mycobacillin, and mycocerein), and Erwinia

herbicola(which produces herbicolins). Some of these

bacteria are used in commercial biocontrol formulations.

For example, Bacillus subtilisis marketed for seed

treatment of cotton and soybeans, to protect against

damping-off fungi. Others have been implicated in

PRINCIPLES AND PRACTICE OF CONTROLLING FUNGAL GROWTH 349

H 2 N N

O

O

NH 2

NH 2

Blasticidin-S

N COOH

H

N

C

N

CH 3

NH

O

HOH 3 C

OH

HO

HO

HOH 2 C

HO

OH

CH 2 OH

OH

OH

OH

O

H

N

C

Validamycin A

NH 2

OH

OH OH

OH OH

CH 3

O

O

N

H

HOOC C

Kasugamycin

NH

N

H

O

NH 3 +

H 2 N

H

H H

Polyoxin D

N

O

OHOH

H

NH

COOH

OCC

O

C

H

O

C

H

O H

O C

H

H

C

H

H

C

O COOH

Fig. 17.10Japanese agricultural antibiotics.