Selective technique for the determination of nematophagous fungi in soils

Soil

Bid.

Eiochem.

Vol.

1, pp. 101-102.

Pergamon

Press

1969.

Printedin GreatBritain

SHORT COMMUNICATION Selective technique for the determination of nematophagous fungi in soils C.

H. E.

WYBORN,

D. PRIEST*

The Biological Laboratories,

and C. L.

DUDDINGTON

The Polytechnic,

London

ONE of the inherent disadvantages in the examination and manipulation of soil and organic debris from a biological viewpoint is the anomalous results which derive from lack of standard technique. The nematophagous fungi present in soils are often masked or inhibited when material is plated out on nutrient agar, owing to their relative slow growth rates and the apparent need for stimulation by a large free-living nematode population. Research workers have used a variety of media in their efforts to determine the presence of nematophagous fungi in soils. Their aim is clearly to determine what is present and to ascertain the relative abundance of these fungi. Further, the investigator is confronted with a personal time factor which is due to the slow emergence of these fungi under cultural conditions. Media rich in nutrients induce extensive growth of the more common moulds, encourage bacterial and myxomycete activity, and generally lead to inimical environmental conditions for the appearance of the more delicate and slow growing nematophagous fungi. In the course of examining soils for the presence of nematophagous fungi plated out on various media and stimulated with a free-living population of nematodes it was apparent that the number and frequency of positive identifications based on spore character showed considerable variation. Eren and Pramer (1965) used 2 per cent plain agar with nematode stimulation in their quantitative determination of nematophagous fungi present in soil. However, it would appear that some standardized level of nematode stimulation is desirable in order to obtain an accurate qualitative and quantitative assessment of the species present. A technique was, therefore, established to obtain a degree of uniformity and repeatability within a minimum time schedule. The various parameters taken into consideration were soil weight, medium, temperature, nematode numbers, time of addition of nematodes and the minimum time taken for positive identification of a particular species. The most convenient weight of soil (fresh weight) was found to be in the order of 0.5 g. The most suitable medium was found to be 2 per cent distilled water agar which restricted unwanted saprophytic competitors. Incubation at room temperature proved to be the most satisfactory in a temperature range 1g-30’%. The time of addition of nematodes after central inoculation of the plates with soil was found to be inconsequential over a period of O-5 days, but the greatest single factor influencing the growth and predacity of nematophagous fungi was found to be the number of nematodes added. Varying numbers of the viviparous nematode Parzagrellus redivivus (L.), raised on oatmeal porridge in Petri-dishes, were washed with sterile water and added to the inoculum after 48 hr as a suspension in 1.0 ml sterile distilled water. Five replicates of each treatment were incubated within the temperature range at a low light intensity to suppress unwanted algal * Present address: The Department

of Botany, Queensland University, Brisbane, Australia. 101

102

SHORT COMMUNICATfON

growth and examined daily over a period of six weeks. The results of this investigation are expressed in Table 1. TABLE

1.

MINI~M

TIME TAKEN LEVELS

IN DAYS FOR POSITIVE OF NEMATODE

Species 0

50

IDE~CA~ON

STlh%ULATiON

OF ~MATOPHA~US

AT ROOM

FUNGI

AT DIFFERENT

5000

10,OtM

TEMPERATURJZ

Number of nematodes added 100 500 1000

Artbrobotrys oIigospora Fres. A. conoides Drechs. Monacrosporium cionopagum

-

28 -

-

11 -

-

9 14

-

(Drechs.) Subram.

-

-

-

-

11

13

-

-

-

-

14 9

10

11 10

11 9

-

-

-

9

IO

9

9

16 -

11 15

11 -

M. psycrhophilum

(Drechs.) Cooke and Dickinson M. eliipsosporum (Grove) Cooke and Dickinson Ge~ic~iaria cystosporia

9

9

(Dudd.) Rifai and Cooke Harposporium

anguilliilae

Lohde

-

-

-

-

Meria coniospora Drechs.

-

-

-

-

The free-living nematode population

of the soil prior to addition was determined as lo/O*5 g moist soil.

The most suitable level of stimulation was in the order of 5000 nematodes per plate. This level of stimulation effected the emergence and enabled positive identification of all nematophagous fungi previously determined by the use of a wide range of nutrient media in the given sample. The 1000 level was below the threshold of stimulation for all species, and at the 10,000 level the first emergences attained epidemic proportions and swamped the nematode population. The experimental work was repeated using dead nematodes, and no replicate gave a positive identification. In order to assess the effectiveness of the technique soil was sterilized and small equivalent amounts of each of the species identified, with the exception of M. coniosportz, grown on corn meal agar in pure culture were reintroduced. The time patterns of emergence were similar and only at the 5000 level were all fungi recovered. Investigations with different soils gave comparable results. The experimental work suggests that soils investigated under laboratory conditions for the presence of nematophagous fungi require stimulation with additional nematodes to effect the emergence and a high level of predacity in the fungi present. A low nutrient status medium is desirable with nematode dosage in the order of 5000 per plate. The application of this technique would enable soil biologists to ascertain with a high level of reliability in a mi~mum time period the nematophagous fungi present in soil samples. REFERENCE EREN

J. and 99, 285.

PRAMER

D. (1965) The most probable number of nematode-trapping

fungi in soil. Soil Sci.