JUNE 1983

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minutes in 4% sodium hypochlorite), on moist sterilepaper in petri dishes. Plants without fungal discs acted ascontrols. Pathogenicity was assessed after four days onthe basis of root discolouration. Two Rhizoctonia isolates,A and S, were selected for further tests. Inoculum wasprepared by growing both isolates on autoclaved wheatgra in for three weeks at room temperature. Colonisedwheat inoculum was mixed with a pasteurised potting mix(loam-sand 67:33 w/w mix , steamed for 60 minutes at 520Kpa) at four rates , namely 0 , 0.5% w/w, 2% w/w and 5%w/w. Seven white clover (cv. New Zealand) seeds wereplaced on top of the inocu lum/soil mix in a 7 cm diameterpot , and covered with 10 g of pasteurised soil. For eachinoculum treatment half of the pots were watered dally to65 per cent of water holding capacity (WHC) withdeionised water and half were watered to 65 per cent WHCbut allowed to dry to 40 per cent WHC before being againwatered to 65 per cent WHC. Temperature was heldconstant at 15°C in a water bath. Plant survival wasrecorded six weeks after sowing and the surviving plantswere removed from the soil , washed and rated for diseaseseverity on the tap root. Ratings were as follows - 0 =healthy root; 1 = girdle of tap root <, 50% rotted; 2 = girdleof tap root >50% but not completely rotted; 3 = girdle oftap root completely rotted. Root and shoot dry weights perplant were recorded. Isolations were made from 32diseased roots to dete rmine whether Rhizoctonia was thecause of the root rot observed .

All but one of the 21 Rhizoctonia isolates werepathogenic on white clover, causing a characteristic browndiscolouration of the ent ire tap root. Results from the potexperiment are shown in Table 1. Plant survival was noteffected by adding Rhizoctonia inoculum to the soil.Increasing the level of inoculum above 0.5% w/w did notresult in an increase in disease sever ity. This agrees withthe results of Hendrix and Campbell (2) who showed thatthe relationsh ip between propagules per unit of soil anddisease severity is largely undefined , the most importantfacto r being that a minimum inoculum level necessary fordisease development be present. Stovold (5) working withPythium irregulare Buisman on subterranean clover alsoshowed that a minimum inoculum level in the soil isimportant for the development of disease and that anyincrease in inoculum above this minimum causes noincrease in disease. Root disease was more severe underfluctuating soil moisture condit ions than when the soli wasmaintained at 65 per cent WHC.

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NUMBER 2

R.D. Maughan and M.J. BarbettiDepartment of Agr iculture, Bunbury,

Western Australia , 6230.and

Department of Agriculture, Jarrah Road,South Perth, Western Australia, 6151, respectively.

White clover (Trifolium repens L.) is a useful pasturelegume in the wetter coastal areas of south westernWestern Australia. In recent years there has been amarked decline in density and production of white cloverin the summer months in these irrigation areas. In somepastures nearly all white clover may be dead by lateJanuary. Surviving plants show rott ing and stripping of allfeeder roots, and severe rotting of the tap roots andstolons. Previous investigat ions (M.J. Barbetti ,unpublished data) indicated that Rhizoctonia spp. may beinvolved. In this note we present the results of aninvestigation to determine whether Rhizoctonia spp. are apr imary cause of the problem. Emphasis was put oninvestigating the effects of soil moisture and inoculumlevel on disease severity.

Plant samples were removed from an irr igated field atHarvey, 150 km south of Perth , in which white clover wasseverely affected by root and stolon rot in the previoussummer. Ten samples, each of five plants, were taken at20 m intervals in a transect across the 3 ha field . Pieces ofwashed roots and stolons (3mm) from each plant wererinsed three times in sterile dist illed water and blotted dry .One hundred diseased root and stolon pieces were platedon water agar + aureomycin hydrochloride (25 ppm) andincubated at 27°C. All fungal isolates which grew from thepieces in 3 days were subcultured on potato Marmitedextrose agar plus 25 ppm of aureomycin hydrochloride(PMDA+ A) for identification. In addition to Rhizoctoniaspp. a range of normally saprophytic fungi were isolated,in part icular Fusarium oxysporum schlecht. and Mucorspp. The pathogenicity of each of the 21 Rhizoctoniaisolates obtained was determined by placing a 6 mmRhizoctonia fungal disc , from a culture growing onPMDA+A, between the roots of two 3 day old white cloverseedlings, growing from surface sterilized seeds (4

Rhizoctonia root rot of white clover

RESEARCH NOTES

VOL. 12

Table 1. The effect of Rhizoctonia inoculum level , water regime and isolate upon plant survival , per plant topdry weight, and root rot score.

Plant survival Top dry weight Average(Av . number per plant (mg) root rot

plants per pot score

Inoculum level 0 5.700 5.173 0.032

Inoculum level 0.5% w/w 5.800 4.488 1.698

Inoculum level 2.0% w/w 6.025 4.901 1.748

Inoculum level 5.0% w/w 5.601 4.034 1.808

Significance NS * * * * * *

LSD (p 0.05) 0.554 0.131

Water regime - fluctuating

moisture 5.138 3.450 2.002

Water regime - constant 65% WHC 6.425 5.849 0.641

Significance * * * * * * * * *

Rhizoctonia isolate A 5.747 4.087 1.257

Rhizoctonia isolate B 5.815 5.211 1.386

Significance NS * * * * * *

These results agree with those found by Crowder andCraigm iles (1) who showed that the incidence of diseaseon wh ite clover roots, caused by Rhizoctonia, was greaterwhen the soil was relatively dry compared with when It wasunder irrigation. They concluded that the saprophyticact ivity of Rhizoctonia on white clover was greater at lowersoi l moisture levels. Papavizas and Davey (4) showed thatat lower soil moisture levels (20 to 50 per cent WHC) thecompetitive saprophytic activity of Rhizoctonia wassign ificantly greater than at high soil mo isture levels (70 to90 per cent WHC). Kovoor (3) showed that at a moisturelevel of 30 per cent saturation there was no bacterial actionaga inst Rhizoctonia hyphae. However, at high soilmo isture levels (80 per cent saturation) complete lysis ofmycelium by bacteria occurred.

Since 1975, the water allocated to farmers in the Harveyarea for ir rigation has decreased from 9.2 thousand cubicmetres (TCM's) per hectare, to 5.3 TCM 's per hectare (K.Bartley, Irrigation Commiss ion of Western Australia;personal communication). This has resulted in pasturesbeing watered less frequently and consequently soilmo isture levels can be expected to have fluctuated morebetween waterings. It is likely that the recent problem ofdeath of white clover in the Harvey irrigation pasture isassociated with an increase in Rhizoctonia spp. in driersoil , wh ich will have accentuated any effects of droughtstress between waterings.

ACKNOWLEDGEMENTS

The authors wish to thank Dr. J. Henstridge, Universityof Western Australia for discussions and advice on dataana lysis, Miss F. Bunny and Mr. G.D. Adam for techn icalass istance, and Dr. K. Sivas ithamparam for helpfuldiscussions and advice.

14

REFERENCES

(1) Crowder, L.V. and Cra igmiles , J.P. (1960) - The effect of soiltem per atu re, so il mo isture, and flowering on thepers istency and for age production of white clover stands.Agronomy Journal 52 : 382.

(2) Hendrix, F.F. Jr and Campbell , W.A. (1973) - Pyth iums asplant pathogens. Annual Review of Phytopathology 11 :77

(3) Kovoor, A.T.A. (1954) - Some factors affecting the growth ofRhizoctonia bataticola in the soil - Journal of the MadrasUniversity 24 : 47.

(4) Papavizas, G.C. and Davey , C.B. (1962) - Activity ofRhizoctonia in soil as affected by carbon dioxide.Phytopathology 51 : 759.

(5) Stovold , G.E. (1974) - Root rot caused by Pyth iumirregulare Buisman, an important factor in the decl ine ofestablished subt erranean clover pastures. AustralianJournal of Agricultural Research 25 : 537.

Pleospora Leaf Spot of Jojoba

T. Woodcock,Plant Research Institute,Burnley, Victoria, 3121

A severe leaf spot disease of jojoba was observed inJune 1981 at Ouyen , in the Mallee region of N.W. Victoria,where it affected a 25 ha plantation of 9 month-old jojobaseedl ings. The plants were grown from seed importedfrom Israel and the United States. Disease occurredthroughout the crop canopy on leaves of all ages andappeared to be respons ible for premature defoliation ofplants.