Trans. Br. mycol. Soc. 73 (1) 27-33 (1979) Printedin Great Britain

EFFECTS OF THE HERBICIDES GRAMOXONE W AND ROUNDUPON SEPTORIA NODORUM

By D. HARRIS AND E. GROSSBARD

School of Natural Sciences, The Hatfield Polytechnic, P.O. Box 109, College Lane,Hatfield, Hertfordshire AL10 9AB, England

The action of Gramoxone W (a.i. paraquat) and Roundup (a.i. glyphosphate) on vegetativegrowth, spore formation, germination and pathogenicity of spores of Septoria nodorumBerk, to detached wheat leaf segments, was examined. Both herbicides had inhibitory effects,but Roundup was the more active. At 80 ppm it reduced the growth rates of four isolates by50-55 %, whilst Gramoxone W at 80 ppm produced a 25-40 % reduction. Roundup at80 ppm reduced spore formation by 90 % but Gramoxone W was ineffective. Pre-treatmentwith Roundup reduced spore germination by 60 % even when the herbicide wasabsent from the germination medium. However, with Gramoxone W this inhibition occurredonly when the herbicide was incorporated in the germination medium. A potential carry-overof the active ingredient of Gramoxone W was indicated by the presence of14C in washed sporesuspensions prepared from cultures grown on media containing 14C-labelled paraquat,and by the greater severity of lesions on detached leaves infected by spores from culturesgrown with Gramoxone W, when compared to lesions produced by untreated spores. Round-up pre-treated spores produced lesions less severe than control, except at high inoculumdensity, where in some experiments lesion severity was enhanced late in the incubationperiod.

Techniques of reduced cultivation and directdrilling are used in agriculture to an increasingextent. There is considerable concern that thesemethods may favour the survival and proliferationof those cereal pathogens that are known to over-winter on straw remains. While in conventionalploughing the straw residues and stubbles areburied to considerable depths, in reduced culti-vation and direct drilling they remain near or onthe soil surface where they may serve as anincreased supply of substrate for the pathogen anda potential source of infection for the followingcrop. Septaria nodorum Berk. (Leptosphaerianodorum Muller) is a classic example of sucha fungus since its survival on stubbles has beendescribed by several workers (Weber, 1922; VonWechmar, 1966; Harrower, 1974). The work ofHolmes & Colhoun (1975) on production of viablepycnidia on straw, and the subsequent infectionof wheat seedlings in the field has provided furtherexperimental evidence that straw and stubbles mayserve as inoculum sources.

Infected straw residues may be exposed in thefield to either Gramoxone W or Roundup, herbi-cides used for weed control in reduced cultivationand direct drilling. It has been shown by GarethJones & Williams (1971) that Gramoxone Winhibited growth and sporulation of S. nodorum in

pure culture. This was confirmed by Grossbard &Harris (1976) who also demonstrated a markedinhibitory action of Roundup on this pathogen.These workers showed that washed spores, pro-duced on media containing Gramoxone W orRoundup, caused more severe infection of detachedwheat leaves than did untreated spores when theinoculum was large. They suggested that residuesof herbicide, retained even by washed spores, wereresponsible for the enhancement of infection. Inpreliminary experiments, when small inocula wereused, the infection produced by Roundup-treatedspores was less severe than in controls.

This paper describes a more detailed study of theeffects of the two herbicides on growth, sporulationand spore germination of S. nodorum. The reten-tion of herbicide residues by washed spores hasbeen investigated using 14C-labelled paraquat. Apotential interaction between herbicide treatmentand inoculum size which may modify the patho-genicity of this fungus, has also been examined.

METHODS AND MATERIALS

Pure culture studies

Isolates. S. nodorum R1, supplied by RHMResearch Limited, Lincoln Street, High Wycombe,

0007-1536/79/2828-5100 $01.00 © 1979 The British Mycological Society

RESULTS

Vegetative growthThe growth rates of four isolates of S. nodorum onmedia with a range of concentrations of Gram-oxone Wand Roundup are shown in Table 1.

Tests of pathogenicity

Leaf segments (5 em) were cut from the centre ofthe second leaf of wheat seedlings cv. Capelle-Desprez grown in pots in a growth room witha 16 h daylength and a light intensity of 5500 lxat 20°. The leaf segments were inserted into 1 %agar containing 80 ppm benzimidazole (Jones &Hayes, 1971) and inoculated in the centre witha drop of spore suspension from an Astell pipette(0'02 ml), containing 5 x 106, 1 X 106, 5 X 104, or1 x 104 spores/ml and incubated at 18° in a growthroom. In some experiments spores grown withoutherbicide were suspended in solutions of Gram-oxone W (0'25, 0'1, 0'01 ppm) immediately beforeinoculation. The inoculum drops were removedafter 3 days and the lesions scored daily accordingto size and intensity on an arbitrary scale of 0-10.0, no lesion; 1-2, chlorosis; 3-5, chlorosis centrelight brown; 6-8, chlorosis centre dark brown orbleached, some dessication; 9-10, extensive chlor-osis centre dark brown or bleached, extensivedessication. After 23 days incubation the numbersof pycnidia per leaf were counted.

28 Herbicides and Septoria nodorum

was used throughout this work. In addition the onto the surface of the plates and incubated forvegetative growth rates of the following isolates 12 h at 18°. The spores were then killed withwere tested: (1) LJ from Dr Gareth Jones, formaldehyde and the plates examined usingUniversity of Aberystwyth, (2) AVI from Dr A. a x 20 phase contrast objective. From each plateHall, Hatfield Polytechnic, (3) E1 isolated from five samples of 100 spores were chosen at randominfected wheat seedlings cv, Capelle-Desprez and the percentage that had germinated wasobtained from Dr M. Richardson, Department of recorded.Agriculture and Fisheries for Scotland, East Estimation of radioactivity in spore suspensions.Craigs, Edinburgh, Scotland. Spores harvested from cultures grown with or

Herbicides. Stock solutions of Gramoxone W without 14C-Iabelled paraquat formulated as(Gramoxone UK formulated product of paraquat, Gramoxone W, were washed six times. At each1,1'-dimethyl-4-4'-bipyridilium dichloride) and washing 1 ml samples of spore suspension andRoundup (formulated product of glyphosphate, supernatant were removed for counting. TheN-(phosphonomethyl) glycine (isopropylamine volume of water used to re-suspend the sporessalt)) were sterilized by filtration through sintered after each washing was adjusted such that theglass filters, diluted with sterile water and added spore density remained constant. The samplesasceptically to molten agar media. In experiments, were mixed with 10 ml of dioxan scintillator ofdesigned to test the retention of paraquat in spore the composition: dioxan, 500 ml; ethoxy-ethanol,suspensions, labelled paraquat chloride (methyl- 100 ml; PPO (2,5-diphenoxazole), 6 g; POPOP14C), specific activity 167 pCi/mg, was added to (1,4-bis-(5 phenoxazol-z-yljbenzene), 0'3 g andGramoxone W to give an activity of 0'04 pCi/ml napthalene 30 g. The disintegrations were countedin the final medium. All herbicide concentrations on an ICN Tracerlab Conumatic 2700 liquid scin-are expressed as those of the active ingredient. tillation counter and the results calculated as dpm,

Cultural conditions. Cultures for measurement using a quench correction curve.of vegetative growth were inoculated with 5 mmplugs in the centre of plates of Czapek-Dox V8agar containing a range of concentrations ofherbicide. Agar without herbicide was used asa control. Incubation was at 18 °C in the dark toprevent sporulation. The colony diameters weremeasured daily for 11 days. Lawn cultures(Holmes & Colhoun, 1971) for spore productionwere prepared by spreading 2 ml of a sporesuspension (1 x 106 spores/rnl) onto the surface ofCzapek-Dox V8 agar or PDA plates. Sporulationwas induced by incubation at 18 °C with constantNUV illumination (Cooke & Jones, 1970) .

Harvest of spores and preparation of spore sus-pension. For estimates of total spore production,the surface layer of agar from each plate washomogenized in 40 ml of water. The spores werecounted on a haemocytorneter without filtration orwashing. Spores for germination tests or forinoculation onto detached leaves were harvestedby scraping the surface of 7-day-old cultures witha scalpel, the scrapings were suspended in water,gently homogenized in a glass homogenizer andfiltered through two layers of muslin. The sporeswere then washed six times by centrifugation, thespore density determined by haemocytometercounts and the suspension diluted as required.

Spore germination. Water agar plates, withoutadditive, or with 160 ppm of either Gramoxone Wor Roundup, were used to test the germination ofspores. One millilitre of washed spore suspension(1 x 106 sporesjrnl), prepared from cultures grownon medium with or without herbicide, was spread

D. Ham's and E. Grossbard 29

Table 1. Effect of Gramoxone W and Roundup on the vegetative growth rates offour isolatesof S. nodorum

Growth rate (mmlday)*

Isolate , .. Rl ±S,E. LJ ±S,E, El ±S,E. AVI ±S,E,Concentration

Treatment (ppm)

No herbicide 0 5'73 0'29 5'40 0'08 5,06 0'06 5'02 0'15Gramoxone W 40 5'12 0'15 4'80 0'16 4'89 0'08 4'70 0'10

80 3'53 0'11 3'17 0'22 3'76 0'11 3'25 0'12160 2'15 0'09 2'15 0'05 2'44 0'09 2'21 0'06

Roundup 40 3'55 0'08 3'76 0'03 4'06 0'17 3'55 0'0880 2'58 0'16 2'73 0'08 2'42 0'07 2'36 0'03

160 1'85 0'03 1'78 0'05 1'42 0'03 1'67 0'09* Means of four replicate plates ± S,E, of mean growth rate,

5'1

4'3

12'5

2'5

31***32***

16No herbicide

No herbicide

No herbicideGramoxone W

160 ppmRoundup 160 ppm 34***

Growth medium

Table 3, Effect of Gramoxone Wand Roundup ongermination of washed spores of S. nodorum

%Germination germinationt

medium ±S.E.

~

80 3'9

No herbicide

No herbicide(control)

Gramoxone W160 ppm

Roundup 160 ppmNo herbicide

Table 2. Effect of Gramoxone Wand Roundup onspore production by S, nodorum on two differentmedia

Herbi- Total spores/plate x 106t

cideconcen- Czapektration Dox

Treatment (ppm) V8 agar ±S.E. PDA ±S.E.

No Herbicide 0 14'2 0'34 3'1 0,61(control)

Gramoxone W 80 14'6 0'69 2'8 0'48160 12·8 0,61 2'1 0'29

Roundup 80 1'3*** 0'42 0'5*** 0'11160 0'9*** 0'31 0'3*** 0,06

t Means of haemocytometer counts on four samplesfrom each of three replicate plates, ±S.E. of betweenplate means calculated from log transformations oftotal spore numbers.

*** Significantly different from control atP = 0'001.

t Means of five samples of 100 spores on each ofthree plates, ±S.E. of between plates means calculatedfrom angular transformations of percent germination.

*** Significantly different from control atP = 0'001.

Both herbicides inhibited the growth of the fourisolates, all were more sensitive to Roundup thanto Gramoxone W, The differences between isolateswere smaller than those between herbicides.

Spore production

The total number of spores produced per plateby S, nodorum in lawn culture was greater onCzapek-Dox V8 agar than on PDA in all treat-ments. Roundup at 80 and 160 ppm markedlyreduced spore formation. Gramoxone W had nosignificant effect on the number of spores pro-duced on either medium (Table 2),

Spore germination

When spores, produced on media contammg160 ppm of Gramoxone W or Roundup, were

washed and transferred to water agar plates withoutherbicide, the percentage of spores which hadgerminated after 12 h incubation was reduced byRoundup but was unaffected by Gramoxone W.The germination of spores grown without herbi-cide, when washed and transferred to water agarcontaining 160 ppm of Gramoxone W or Roundup,was reduced to a similar extent by both herbicides(Table 3),

Estimation of radioactivity in spore suspensions

Spore suspensions were prepared from culturesgrown on media containing Gramoxone W withHC-Iabelled paraquat. Counts of disintegrationsin samples of spore suspensions and supernatantsfrom each of the six washings were made. A rapiddecline in the HC content of the supernatants

Herbicides and Septoria nodorum

occurred until the fourth washing, after which thecount was equal to background. The radioactivityof the spore suspensions, after an initial decline,remained constant after the third washing ata level above that of control (Fig. 1). The retainedHe was equivalent to a paraquat concentration of0'43 ppm in a spore suspension of 1'67 x 107

spores/rnl. A corresponding suspension of 1 x 106

spores/rnl would contain 0'025 ppm paraquat.

Injection of detached leaves

In a preliminary experiment using two inoculumdensities (1 x 10 6 and 1 x 104 spores/rnl) Gram-oxone W-treated spores produced lesions more

65432oOL.--L.---.L.----"----'---~-----'-----'--

5

2

4

Number of washes

Fig. 1. Counts of dpm (means of counts on four samples from each of three replicate suspensions) inspore suspensions of S. nodorum, prepared from cultures grown on media containing 14C-labelledparaquat, after each of six washings. (A--.A) Spore suspension, (e--e) supernatant.

1 x 106 sporesl ml 1 x 10' spores/rnl

150

140:::- 1308;:: 120ou 110!l"-'-' 100e8 90'"g 80.v; 70.5

60

150

~1 40

130

120

110

t-~--t--r-t--r-t--~----

7 8 9 10 I 1 12 13 90

Days 80

70

6050

40

30

20

Fig. 2. Mean scores (means of nine replicates), as per cent control, of lesions produced on detachedwheat leaves, after inoculation, at two inoculum densities, with washed spores from cultures grown onmedia containing Gramoxone W or Roundup. (e--e) Gramoxone W, (.--.) Roundup.

Table 4. Frequency of scoresof lesions on detached wheat leaves produced by spores of S. nodorum from cultures grown on mediawithout herbicide, or with Gramoxone W or Roundup

Frequencies of scores, ,

Inoculum No herbicide GramoxoneW RoundupDays of density , -, , --" , ,

incubation (spores/ml) 0* 1 2 3 4 5 6 7 8 0* 1 2 3 4 5 6 7 8 0* 1 2 3 4 5 6 7 8

5 5 x 10· 0 0 0 1 4 5 2 2 0 0 0 13 3 0 0 0 0 0 0 0 0 1 7 6 3 0 0

1 X 10· 0 1 5 3 4 4 0 0 0 0 12 4 1 0 0 0 0 0 0 0 1 2 7 3 2 0 1

5 x 10' 0 3 12 2 0 0 0 0 0 0 14 0 0 0 0 0 0 0 0 3 7 3 1 0 0 0 0

1 X 10' 0 7 9 0 0 0 0 0 0 5 6 2 0 0 0 0 0 0 0 7 5 1 0 0 0 0 0 t:l7 5 x 10· 0 0 0 0 0 1 5 7 1 0 0 0 0 0 1 5 10 1 0 0 1 2 7 5 1 0 0 .

1 X 10· 0 0 1 1 5 1 3 5 1 0 0 0 1 0 4 8 2 1 0 7 4 3 3 0 0 1 0 ~5 x 10' 0 1 10 4 2 0 0 0 0 0 1 3 2 7 2 0 0 0 0 12 2 0 0 0 0 0 0~

1 X 10' 0 4 9 2 1 0 0 0 0 0 2 7 3 1 0 0 0 0 3 6 2 2 0 0 0 0 0 ~.

8 5 x 10· 0 0 0 0 0 0 1 8 5 0 0 0 0 0 0 1 14 2 0 0 0 0 1 7 5 3 0 $:l

1 X 10· 0 1 0 1 6 1 3 5 1 0 0 0 0 0 0 10 4 2 0 1 2 7 0 4 2 0 1 ~5 X 10' 0 1 9 4 3 0 0 0 0 0 1 2 2 2 5 2 0 0 0 7 5 1 1 0 0 0 0 ~1 X 10' 0 6 5 2 2 1 0 0 0 0 1 4 2 5 1 0 0 0 5 4 1 1 1 1 0 0 0

f9 5 x 10· 0 0 0 0 0 0 2 8 4 0 0 0 0 0 0 0 17 0 0 0 0 0 1 4 3 3 51 X 10· 0 0 0 1 2 2 7 5 0 0 0 0 0 0 5 3 1 0 0 1 0 7 1 1 6 1 0

5 x 10' 0 1 6 3 4 3 0 0 0 1 0 1 4 4 2 4 0 0 2 1 1 4 0 0 0 0 0 [1 X 10' 0 3 8 1 1 1 2 0 0 2 2 3 2 3 1 0 0 0 9 2 0 0 2 0 0 0 0

10 5 x 10· 0 0 0 0 0 0 3 7 4 0 0 0 0 0 0 4 11 6 0 0 0 0 0 4 5 6 0

1 X 10· 0 0 1 1 2 2 6 5 0 0 0 0 0 0 0 6 10 4 0 1 0 5 2 2 1 5 0

5 x 10' 1 3 4 6 2 1 0 0 0 1 0 2 1 2 4 4 0 0 2 4 2 5 1 0 0 0 0

1 X 10' 0 8 4 0 2 2 0 0 0 1 2 2 3 3 2 0 0 0 6 3 2 0 1 0 1 0 0

11 5 x 10· 0 0 0 0 0 0 2 5 5 0 0 0 0 0 1 4 7 5 0 0 0 0 2 3 4 7 0

1 X 10· 0 1 1 0 2 2 7 6 0 0 0 0 0 0 4 6 6 0 0 1 1 1 5 2 5 1 1

5 x 10' 1 5 3 2 3 2 1 0 0 1 1 1 0 2 5 4 0 0 4 0 2 0 3 5 0 0 0

1 X 10' 1 7 4 0 4 0 0 0 0 2 3 1 1 5 1 0 0 0 7 3 0 0 0 1 1 1 0

* Score categories 0-8. Categories 9 and 10, though included in the analysis, are omitted from this table. Only one lesion, produced by Roundup-treatedspores at 1 x 10· spores/ml on day 10, was recorded in category 9, and none in category 10.

w...

32 Herbicides and Septoria nodorum

5 X 108 ± S.E. 1 X 108± S.E. 5 X 104± S.E. 1 X 104± S.E.

7'4 1'99 6'1 1'91 12'2 5'89 12'6 5'422·6 0'84 1'9 0'79 9'8 3'71 7'1 1'662'1 0'88 4'3 1,62 10'3 5'36 8'8 0'93

o160160

Herbicideconcentration

(ppm)

Table 5. Effect of Gramoxone Wand Roundup on pycnidial production by S. nodorum ondetached wheat leaves after 23 days incubation at 18°

Mean no's of pycnidia/leaf'[Inoculum density (spores/ml)

TreatmentNo herbicide (control)Roundup***Gramoxone W***

t Means of 18 replicates, ±S.E. of mean number of pycnidia per leaf.*** Herbicide treatment significantly different from control at P = 0'001.

severe than control at both spore concentrations.The dense inoculum of Roundup-treated sporesinitially gave rise to lesions less severe than thoseproduced by untreated spores but after 7 days thescores for these lesions exceeded those of controls.At the low inoculum density lesion severity wasless than control throughout the incubation period(Fig. 2).

The results of a later experiment using fourinoculum densities (5 x 106, 1 X 106, 5 X 104, 1 X 104

sporesjml) were analysed by contingency tables.The effects of inoculum density and herbicidepre-treatment of the spores were examined, ateach day of assessment, by fitting a log-linearmodel to the frequency table of the scores in eachcategory (Table 4). Overall category differenceswere fitted first, and the effect of inoculum density,for example, was assessed from the inoculumdensity-category interaction. This was done byexamining the effect of inoculum density on thedistribution of scores in each category. Theanalysis showed that both inoculum density andherbicide treatment separately affected the dis-tribution of lesion scores at each assessment.However, no interaction between inoculum densityand herbicide treatment was detected.

A decrease in lesion score at all inoculum levelswas recorded for Roundup-treated spores, whencompared to controls from cultures grown withoutherbicide. The effect was particularly marked inthe early stages of infection and at the lowerinoculum densities. The scores for lesions pro-duced by Gramoxone W-treated spores weregreater than controls at the three lower inoculumdensities, but not at the highest. Differences fromcontrol were greatest in the early stages of infectionwhen the inoculum density was 1 x 106 sporesjmlbut at the lower inoculum densities the contrastwas greater in the later stages.

Lesions produced by herbicide-treated spores,formed significantly fewer pycnidia than did thoseproduced by untreated spores. The numbers ofpycnidia produced at the high inoculum densities(5 and 1 x 106 sporesjml) were smaller than at the

low inoculum densities (5 and 1 x 104 sporesjml)in all treatments (Table 5),

Previously untreated spores, suspended insolutions of Gramoxone W before inoculation ontodetached leaves, produced lesions of greaterseverity than did spore suspensions withoutherbicide. The addition of 0'01 ppm GramoxoneW to the spore suspension sufficed to enhancethe severity of the resulting lesions. These lesions,which had bleached centres, were similar inappearance to those characteristically produced byspores from cultures grown on media containingGramoxone W. Herbicide solutions of 0'01 ppmalone produced no visible lesions on the leaves.

DISCUSSION

The important result of this study is the clearantifungal activity of both herbicides, especiallythat of Roundup, Gramoxone W, though alsoantifungal was more selective in its inhibitoryaction than was Roundup. While reducing veg-etative growth under all conditions, it failed toaffect spore formation in lawn cultures. Thisherbicide has, however, been shown to reducespore formation when growth originated froma central inoculum plug (Gareth Jones & Williams,1971; Grossbard & Harris, 1976). This may bedue to the reduction by Gramoxone W of the areaof vegetative growth available for spore formation.That spore production was markedly greater onCzapek-Dox V8 agar than on PDA, agrees withthe results of Cooke & Jones (1970).

A transfer of phytotoxic amounts of paraquat bythe inoculum is suggested. First, by the enhance-ment of lesion formation by suspensions of washedspores originating from cultures grown on mediacontaining Gramoxone W.Secondly, by the simi-larity of such lesions to those produced by a combi-nation of previously untreated spores and dilutesolutions of Gramoxone W. The retention of HCsupplied as labelled paraquat, after repeated wash-ings of the spores, also indicates a retention ofresidual herbicide. The possibility that the transfer

D. Harris and E. Grossbard 33may be by debris from the spore harvest, ratherthan by the spores themselves, mustnot be excluded.

Transfer of residual herbicide may also occur inRoundup-treated spores since germination wasimpaired even on agar that did not contain theherbicide. Furthermore, in some pathogenicitytests at high inoculum density, lesions of greaterseverity than control were produced by Roundup-treated spores in the later stages of infection.Roundup acts more slowly on plants than doesGramoxone Wand the useful life of detachedleaves may be too short for the consistent expressionof the phytotoxicity of any transferred herbicide.Thus in the early stages of infection and at lowinoculum density, the antifungal action of Round-up may outweigh the phytotoxic effect of anytransferred herbicide. With the system of lesionscoring used in the tests of infection of detachedleaves, it proved to be difficult to differentiatebetween very severe lesions. That the scores oflesions produced by untreated and Gramoxone W-treated spores at the highest inoculum level weresimilar, is probably due to this difficulty. Bothherbicides were similarly effective in reducingthe numbers of pycnidia formed on detachedleaves. This is at variance with previous resultswhere an increase in numbers of pycnidia wasbrought about by both herbicides, particularly byGramoxone W (Grossbard & Harris, 1976). Inthis latter case pycnidia appeared before the leaveshad died, but in the present work pycnidia did notnormally appear before the leaves had becomemoribund. The condition of the host may wellaffect the formation of pycnidia. The relationshipbetween inoculum size, herbicide effect and thenumber ofpycnidia formed requires further study.

We wish to thank the Agricultural ResearchCouncil for financial support of this work,Mr M. G. Stokes for his help with the statistical

analysis of the data, Mr R. M. Payne for theanalysis of the scoring system used in this work,Dr D. Hornby for useful discussions, Dr R. N.Smith for constructive criticism of the manuscriptand Miss Fiona Checkley for excellent technicalassistance.

REFERENCES

COOKE, B. M. & JONES, D. G. (1970). The effect ofnear ultraviolet irradiation and agar medium on thesporulation of Septaria nodorum and Septaria tritici,Transactions of the British Mycological Society 54,221-226.

GARETH JONES, D. & WILLIAMS, J. R. (1971). Effect ofparaquat on growth and sporulation of Septarianodorum and Septoria tritici, Transactions of theBritish Mycological Society 57, 351-357.

GROSSBARD, E. & HARRIS, D. (1976). The action ofGramoxone W and Roundup on cereal pathogens.Mededlingen van de Faculteit Landbouunuetenschap-pen Rijksuniversiteit Gent 41,693-702.

HARROWER, K. M. (1974). Survival and regenerationof Leptosphaeria nodorum in wheat debris. Trans-actions of the British Mycological Society 63, 527-533.

HOLMES, S. J. I. & COLHOUN, J. (1971). Infection ofwheat seedlings by Septoria nodorum in relation toenvironmental factors. Transactions of the BritishMycological Society 57, 493-500.

HOLMES, S. J. I. & COLHOUN, J. (1975). Straw-borneinoculum of Septaria nodorumand S. tritici in relationto incidence of disease on wheat plants. PlantPathology 24, 63-66.

JONES, I. J. & HAYES, J. D. (1971). The effect of sowingdate on adult plant resistance to Erysiphe graminisf.sp. auenae in oats. Annals of Applied Biology 68,31-39.

VON WECHMAR, M. B. (1966). Investigation on thesurvival of Septaria nodorum Berk. on crop residues.South African Journal of Agricultural Science 9,93-100.

WEBER, G. F. (1922). Septoria diseases of wheat.Phytopathology 12, 537-585.

(Accepted for publication 30 August 1978)

Jlye 73