untitled

plate (Table 12.1). For example, in the top data line

of the table, Pythium oligandrumwas detected in 18 (of

a total 28) different soil samples that were sprinkled

onto agar precolonized by Fusarium culmorum, but

P. oligandrumwas detected on only three plates pre-

colonized by Trichoderma, and on none of the agar

plates precolonized by Rhizoctonia. These data indicate

that different mycoparasites are selectively favored

by different “host” fungi – at least in laboratory

conditions.

A mycoparasite unique to Britain!

Well, that’s not exactly true. There is a mycoparasite

that has only ever been recorded in Britain, and it is

very common. Pythium mycoparasiticum(see Fig. 10.2)

is a relatively slow-growing parasite that has been

recorded frequently on precolonized agar plates, but

only if the soil is serially diluted with sand to dilute

out the spores of more aggressive mycoparasites,

which outcompete P. mycoparasiticumin “raw” soil

(Deacon et al. 1991). It almost certainly occurs in

many countries, and this example serves to illustrate

that other fungi await discovery by using novel isola-

tion techniques.

Pythium oligandrumas a biological control agent

Pythium oligandrum and the other mycoparasitic

Pythiumspp. have the potential to be used for biocontrol

of plant pathogens, especially as seed treatments to con-

trol seedling pathogens. In this respect, P. oligandrum

grows rapidly on cheap commercial substrates such as

molasses, and it produces large numbers of oospores

in shaken liquid culture. These oospores have been

applied to seeds of cress, sugar beet, and other crops,

using a commercial seed-coating process, and were

reported to give good protection against seedling dis-

ease caused byPythium ultimum in experimental con-

ditions (McQuilken et al. 1990). A commercial powder

formulation of P. oligandrumhas been developed in

the former Czech Republic. However, it is not used

widely, if at all, because of technical problems. The

major problem is that the oospores of P. oligandrum

germinate slowly and poorly because they exhibit con-

stitutive dormancy (Chapter 10). Only a maximum

250 CHAPTER 12

Table 12.1The ability of different mycoparasites to grow across colonies of other fungi on agar plates. (Data from
Mulligan & Deacon 1992.)

Number of soils yielding the

Mycoparasite Precolonizing fungus mycoparasite (max. 28)

Pythium oligandrum Fusarium culmorum 18

Trichoderma aureoviride 3

Rhizoctonia solani 0

Botrytis cinerea 12

Clonostachys rosea F. culmorum 20

T. aureoviride 27

R. solani 26

B.cinerea 25

Trichoderma harzianum F. culmorum 2

T. aureoviride 0

R. solani 22

B. cinerea 13

Papulasporasp. F. culmorum 0

T. aureoviride 11

R. solani 3

B. cinerea 19

Note: Soils were collected from 28 different locations in Britain and were stored air-dried. Twenty-eight plates of potato-dextrose agar

were inoculated with Fusarium culmorum, and similarly 28 plates were inoculated with Trichoderma aureoviride, Rhizoctonia solani, or Botrytis

cinerea. When these fungi had covered the agar plates a small amount of air-dried soil was added, and the plates were incubated

to detect any mycoparasites that grew across the fungal colonies. All the mycoparasites either grew completely across the established

fungal colonies or not at all. The results show that four mycoparasites are common in soil (detected in at least 18 of the 28 soils tested).

But, with the exception of Clonostachys roseawhich grew across any fungus, the different mycoparasites are selectively favored by dif-

ferent “host” fungi.