fungus (B). They are asked to study the plate and to recordtheir own observations independently of anyone else. Aftera set period their records of observations are collected upand everyone is invited to compare observations. Twothings emerge from the ensuing discussion. Firstly, itreveals something about the nature of observations: e.g.some people utilize their microscope to aid observation,but in the absence of specific instructions many do not;different people select different features to emphasise;some people offer interpretations rather than obser­vations; etc. Secondly, it provides the opportunity to reachsome kind of consensus (guided by the lecturer) fromwhich to tackle the next stage in the proceedings - for­mulating hypotheses to explain the salient facts. Again, thestudents are asked to work on their own and write downand hand in their suggestions before the question is open­ed up for class discussion. Among the points which usuallyemerge here are: that many people are satisfied with onehypothesis, while others offer two or three; that somehypotheses are simple and others complex; thathypotheses often come in pairs - one the mirror imageofthe other; that many of the hypotheses offered are looselystated and some are implausible; etc. There is an oppor­tunity here for the lecturer to show his erudition by mis­quoting William of Occam as he focuses attention on thetwo firs t order hypotheses (i.e. that B releases a chemicaldiffusate into the agar which inhibits A - OR - that Bremoves a nutrient factor from the agar which A requiresfor growth) in preparation for the next step: designing ex­periments to test these hypotheses. At this stage people'sdifferent programmings start to emerge with a vengeance:chemists desire a complete analysis of the agar medium;biochemists want to mash up and fractionate the fungi;ecologists prefer to subject the combatants to a variety ofenvironmental regimes; etc. But, with luck, a few studentswill have sufficient perception to come up with suggestionsfor the simple experiments which the lecturer had in mindall along and for which, bless my soul , the laboratorytechnician has just the very things needed! This saga con­tinues into the next practical, with another round of obser­vations, drawlnq of conclusions, and the liberal use of"What if?" and "Where do we go from here?" typequestions. Most of our students seem to approve of the ex­ercise . If you would like to try it on your students, write tothe senior author for cultures, preparation notes and stu­dent hand outs.

REFERENCE

(1) Merrill, W. (1978). Innovative teaching in plant pathology. An­nual Review of Phytopathology. 16: 239-61.

How long have there been plant viruses inAustralia?

Adrian Gibbs and Paul GuyResearch School of Biological Sciences,Australian National University, Canberra.

Until recently very few viruses had been isolated fromAustralian native plants, and, with few exceptions, theviruses isolated from crops in Australia have been foundpreviously in other parts of the world. These facts led White(1) to attempt to "stimulate further research" by suggesting

41

that "all plant viruses now in Australia were introducedsince European settlement of the Australian continenttowards the end of the eighteenth century". However asWhite (1) indicated, evidence to refute or support this ideais difficult to obtain.

One line of evidence could come from a study of strainvariation in viruses, because the evolution of viable strainsis probably a time-dependent process. There are, for ex­ample, three strains of Kennedya yellow mosaic virus(KYMV), a virus that has so far only been found in nativelegumes along the eastern seaboard of Australia, andthese three strains are geographically separated from oneanother (2, 3). There is a similar amount of variation amongisolates of white clover mosaic virus (WCMV) from easternAustralia. However WCMV is found throughout the world(4), and there is no consistent difference betweenAustralian isolates of the virus and those from elsewhere;they seem to be all one population. Furthermore, by con­trast with KYMV, there is no obvious correlation betweendifferences among WCMV isolates and the places of origin"of those isolates in eastern Australia (5). The simplest ex­planation for this difference between KYMV and WCMV isthat the former has been adaptively selected in Australiafor much longer than the latter; WCMV has probably beenrepeatedly introduced from many parts of the world, dur­ing the past two centuries. Other explanations are possibleand it is, of course, unwise to conclude much from com­parisons of two unrelated organisms, such as KYMV andWCMV, especially as we have, as yet, no idea of the rate ofspeciation of potexviruses or tymoviruses.

Another line of evidence relevant to White's idea wouldbe the presence of virus symptoms in the specimenscollected by the early European explorers of Australia. Asurvey of the area of Kurnell where Joseph Banks landedfrom the 'Endeavour' in April 1770, and from where hecollected specimens of Kennedia rubicunda, showed thatthe plant is still plentiful and about 70% infected withKYMV. Specimens of virus-free and KYMV-infected plantsair dried in the way that Banks preserved his specimensstill show clear symptoms (figure 1). Therefore all theAustralian legume specimens in the Herbarium of theBritish Museum (Natural History), London, where Banks'specimens are kept, were examined.

There are about fifty specimens of different Kennediaspecies in the collection. Banks' specimens are in verygood condition but showed no clear virus symptoms, ex­cept for a slight distortion of the tip leaves. However someother specimens showed clear virus symptoms (figure 2a,b, c). The earliest of these were two specimens collected byRobert Brown on the 1802-5 'Investigator' expedition; onewas of K. rubicunda collected in Port Jackson and showsmosaic and the other of K. coccinea from Swan Rivershowed mosaic and leaf distortion. Some later Kennediaspecimens showed symptoms, but a similar number ofspecimens of other Australian legumes appeared virus­free. Thus is seems that there were virus-infectedKennedia in the Swan River area before colonization byEuropeans, and in the Sydney area soon after colonization.

Perhaps the reason why so few speciments in the collec­tion show virus symptoms is that botanists instinctivelychoose healthy mature plants when collecting, and also, inthe particular case of Banks' specimens, that mature in­fected plants usually show few or no symptoms in autumn,which was the time of year that Banks collected hisspecimens.

Figure 1. Air-dr ied Kennedia rubieunda infected with KYMV andcollected from Kurnell , N.S.W. in late 1977; now in the Herbariumof the British Museum (Natural History), London.

Figure 2. Parts of specimens in the Herbarium of the Br it ishMuseum (Natural History), London.(a) K. rub icunda collected by Robert Brown in 1802-5 from Port

Jackson (Sydney).(b) K. rubicunda "collected within 125 miles of Sydney" in 1844-6

by Will iam Stephenson.(c) K. coceinea collected from the Swan River in 1839 by R. J.

Shuttleworth.

A Simple Method for Rearing Root-knotNematodes in the Laboratory

Mar ion A. FootPlant Diseases Division, D.S.I.R. ,

Auckland, New Zealand.

A method for rearing root-knot nematodes(Meloidogyne spp.) in standard laboratory incubators hasbeen developed. The method is an adaptation of that usedfor rearing potato cyst nematodes (G/obodera rostochien­sis and G. pallida) in which potato roots are grown in clos­ed containers in the absence of light (1).

Potato tu bers are su rface-steri Iised in sodi umhypochlorite (1% available chlorine) for 15-30 minutes,rinsed in sterile water, dried , and stored until sprout initia­tion. They are then planted in small clear plastic or glasscontainers (Figure 1) in sterilised sand of moisture contentapproximately 5% its over-dry weight (1). When rootdevelopment is observed, nematode eggs are inoculatedinto the container in a 1-2 ml water suspension. If accuratepopulation estimates are not required, several egg -massesor a short length of infested root may be placed on thesand surface as inoculum. A minimum effective containersize of 90 ml may be inoculated with a maximum of ap-

- proximately 5000 eggs. At 18°C the nematode life cycle iscompleted in 8-9 weeks. During this time the system mayrequire occasional add itions of liquid. If moisture dropletsare not visible on the container wall (Figure 1) several mlsofa 0.2% solution of the proprietary plant food 'l ush'(N:P:K = 32:12:12) are added.

Both Meloidogyne hapla and M. incognita have beenreared by this method. Heavily infested roots from a 90 mlcontainer (Figure 2) contained an average of 24 ± 12(95% Cl) female M. hapla per cm of root, each producing396 ± 265 eggs at 8 weeks of inoculation.

The kumara (Ipomoea batatas L.) has proven asuccessful alternative host in this method (Figure 1, lelt) ,and it is possible that other tuberous hosts may also be us­ed . The method has the advantages of saving space, nolight requirement, visibi lity of deve loping roots, and com­plete enclosure, which facilitate standardisation of rear ingconditions.

ACKNOWLEDGMENTSWe are very grateful to L. G. Adams (C.S.I.R.O. Division

of Plant Industry) for help with preparing K. rubicunda her­bar ium specimens, and to J . F. M. Cannon (Keeper ofBotany) and his staff at the Br itish Museum (NaturalHistory), london.

REFERENCES

(1) White, N. H. (1973). The geograph ic isolation of Australia inrelation to plant viruses. Australian PI. Path. Soc. Newsl. 2:32-3.

(2) Gibbs, A. J. (1978). Kennedya yellow mosaic virus.C.M././A.A.B. Descriptions of Plant Viruses No. 193.

(3) Boswell, K. F., Cooley, J., Fischer, M. and Gibbs , A. J. (1979).Stra in variation in Kennedya yellow mosaic virus. Inpreparation.

(4) Bercks , R. (1971). White clover mosa ic virus . C.M././A.A.B.Descriptions of Plan t Viruses No. 41.

(5) Guy , P. (1978). Stu dies on white clove r mosaic virus. Hons.thesis, Australian National University , Canberra , 142 pp ,

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Figure 1. Rearing method for root-kno t nematode: kum ara root(left), and potat o root (rig ht) in small contain ers inocul ated with M.hap la .