Biological control of the take-all fungus, Gaeumannomyces graminis, by Phialophora radicicola and similar fungi

BIOLOGICAL CONTROL OF THE TAKE-ALL FUNGUS, GAEUMANNOMYCES GRAMINIS. BY PHlALOPHORA RADZCZCOLA AND SIMILAR FUNGI J. W. DEACOK Department

of Microbiology.

School

of Agriculture.

West Mains

Road.

Edinburgh,

Scotland

S~rnmary-P~i~~~~~~~ru ~~ff~~ie~~uis an avirulent fungal root-parasite of grasses and cereals, with runner hyphae like those of G~~~~it~~~~~zo~~~~~ g~~~~i~~.~. Weakig and note-pathogenic varieties of these fungi control the pathogens, G. qratnitris vars. rritici and ~IUCIIUY. Biology of these fungi is considered and the evidence for biological control and possible mechanisms reviewed; control is probably widespread in natural plant communities, and host-medlated. perhaps by induction of plant resistance mechanisms. Prospects for application of biological control seem best for P. radicicola var. yruminicolu established on grass crops, as this is already exploited in British agriculture. New evidence is presented on the effects of grassland factors on this fungus. especially sward composition. age. mineral nutrition and management practices: its population might often be limited by the rate of new root production to replace those with cortices already colonircd. Prospects fol- control by seed inoculation with P. rdiciw/a var. rrrrlicicok~ and G. ortr~~ir~is var.
INTRODI’CTIOY

emphasis has been placed on biological control of plant pathogens; this is reviewed by Mitchell (1973), Papavizas (1973), Wilhelm (1973) and Baker and Cook (1974). This paper reviews evidence for a widespread natural biological control of the take-all fungus. G. ~~~~7177~~7~.s {Sacc.) Arx and Olivier, by P. r~(~~cj~~~~~l Cain var. ~~~~~~7~f7ic(~f~ Deacon. and prospects for application of biological control by this and similar dark mycelial fungi of cereal and grass roots. Recent work has shown the feasibility of this, but also the need for more information on the occurrence and activities of the control agents on a global scale. The wheat take-all fungus (G. grarninis var. tritici Walker) is an important pathogen of intensive cereals (Slope and Etheridge, 1971) in Britain and elsewhere. There are no resistant cultivars of wheat and barley: instead control often depends on crop rotation (Glynne. 1965) and judicious use of nitrogenous fcrtilizers (Garrett, 1948: Huber tir ul. 1968; Smilsy and Cook, 1973). G. gr~f~~~j~?~.s var. crre~ure (E. M. Turner) Dennis attacks oats and other cereals. and also causes ophiobolus patch disease of Agrostis turf (Schoevcrs. 1937; Smith. 1965). generally after soil fumigation for weed control. or liming to correct for excessive acidity, Perhaps more than most, thcreforc. these pathogens merit attention in biological control. and work on this has been concentrated on var. rritici. although var. armue presumably is similar in this respect. G. gmrvlinis is susceptible to antagonism by microIn recent

years

much

organisms present in most natural soils (Sanford and Broadfoot. IY31 ; VojinoviC. 1973; Shipton, 1975: Baker and Cook, 1974) and it competes poorly with the general soil microtlora for colonization of buried substrates (Garrett, 1970: Deacon, 1973a). However, direct use of these organisms in biological control is dificult; their large number suggests a non-specific mode of action and so there seems little point in encouraging particular ones at the expense of others. Nevertheless, this sensitivity of G. grcrinirlis to activities of the ycJjieY~//soii t~ricrojiorcl is important in limiting its activity (Henry, 1932) and also as a background against which spwific co~7rrd 1r~q-icv7f.s must oporate. These are present in only some soils or cropping sequences. They include parasites, like Dirl~n7rlh rri~itrlis (Mor) E. Muller (Siegle. 196 1) and P~hiirr~~ o/iqtrr7dwrn Drechsl. (Deacon. 1975); other antagonists (see. for example, Vojinovid, 1973): and perhaps competitors (Pope and Jackson, 1973). Some are considered in accompanying papers. but many have not been shown to be elective in disease control when introduced at realistic (preferably natural) inoculum levels into natural soils. P. mdicicolu and similar fungi meet this requirement and so offer good prospects for biological control of take-all in the near future. Finally. one or more specific control agent causes take-all decline (TAD) (Lester and Shipton, 1967; Gerlagh. 1968; and others-see Baker and Cook, 1974); however. P. mdicicolu cannot be implicated in this, as its population in cereal monoculture remains low except after grass (Deacon. 1973b).

ration on young lateral roots, with thinner cortices, but P. rtrtliciwlu var. grtrnzirzicola (non-pathogenic) both spccics show ‘curling-back’ of the marginal never does so. Presumably, vascular discoloration and blockage are predolninantly f?osr rc.spon.~e~~to preshyphae in agar culture and they produce similar dark ence of nnj’ of these fungi in the xylem, though perrunner hyphae on hosts. Their taxonomy is complex, haps mediated by fungal enzymes (Beckman, 1969a.b). and so not considered here, but details are available (Walker, 1972; Deacon. 197%; 1974a); instead Differences in pathogenicity between varieties of the fungi are thus based largely on ability or inability emphasis is placed here on features that affect their to invade the xylem from the root cortex. rbles as pathogens or biolbgical control agents. The (7) Ability to grow on wheat stem bases and couch different varieties are summarized in Table 1. or quack grass (,4~~~)p~~~j~rcpcns (L.) Beauv) rhiThree varieties of G. ~r~~~i~?i.~ are recognized zomes. Highly pathogenic G. ~~rf~/~j~zi.svarieties grow (Walker, 1972); (i) var. ri-itici; (ii) var. ctrr~cc~; (iii) var. well and penetrate deeply; G. ptmink var. gruminis qrur~i~i.s, a weak pathogen or rice and underground and P. ratlic~ird~~var. r~~/icic,o/n grow very well but stems of grasses, which forms distinctive lobed do not penetrate deeply; and P. radicicola var. puhyphopodia. All form perithecia. Two varictics of P. minicoh grows poorly, if at ail. These differences are raslicicola are recognized (Deacon. 1974a): (1) var. of major practical significance. but reasons for them ~dicic~i~, the species type, but now apparently changed in culture and of LlnkllowIl pathogenicity~ are unkno~vn. (2) var. ~ru~jl7i~~)i~. a non-pathogen abundant in Bri(3) Pigmented cells. fctrmed by all varieties when tish grasslands and distinguishable from all the others host-penetration ceases (Deacon. 1973b; I974a). by, among other things, its slower growth on agar These arc either (i) pseudoparenchyma-aggregates of (Table 1). Neither has yet been found to produce pcrpolygon-shaped ( < 1Ollrn dia) cells-for G. qurninis ithecia. Often, new isolates from different sources vars. ktici and LII’L?IIUP on stem bases and maize roots (crops or countries) do not exactly fit these taxa (which arc not complcteiy pcnctrated), but sometimes (chambers and Flentje. 1967: Nilsson. 1971: Deacon, formed by all varieties on stem bases; (ii) single large 1974a). Presumably they are adapted to particular cells. lobed or unlobed (the latter mean 28 x 1911m) hosts or local conditions and have diverged with profor G. qrtr~M.s var. 0rtrrnirli.s and P. I.LKCCOILIvar. longed isolation, As used here. tht” name. P. radicicoirr &ic~icola (lobed ones form predominantly on tissue var. rwciic~icoicr,refers to British isolates that resemble surf&es); (iii) groups of small (IO I5 /lrn), rounded but are not identical with G. pwninis var. gramir~is cells. for P. nrdicicol~~ var. pwmhimli~ in roots. A cau(weak pathogens with lobed hyphopodia) and that sal relationship between growth-cessation and formahave no known sexual stage; this does not imply full tion of these cells is not proven for these fungi. but acceptance of the taxonomic similarity of these and assumed from rcsuits for Psc~iriictc,ci~t,,sltctrriiti ~~~,~p,?~~ithe changed type culture of P. rtrrlicicoirt. Messiaen, dwirft~s (Fron) Deighton (Deacon. 1973~). Lafon and Molot (1959) apparently do the same for The groupings cover all British isolates examined French isolates, but others (Lemaire and Ponchet. so far and also some other dark mycelial fungi of 1963) do not. cereal and grass roots. But cxccpt for G. qrur~~iuis var. Biologically, the fungi fall into three groups (hrac~~~~~~~j~ii.~ (included in the above) too little is known keted in Table I), which Deacon (1974a) constructed of similar fungi from other countries to group them to try and circumvent taxonomic difficulties. The in this way. Thus fungus from turf (Pm pr~rtcrrsis L.) groups are based on the following criteria. in the U.S.A. (with curling back of hyphae and growth (1) Pathogenicity to wheat, as shown by ability to rate as for G. qrLl/tlilli,s var. tririci) was \V~YI!Y~J~ pathocause vascular discoloration of seedling roots in ungenic to wheat. did not grow on wheat stem bases, sterilised soil. Only G. graminis vars. rritici and nvrnac~ and formed groups of small rounded cells (Deacon. (high’: pathogenic) usually do so on root main axes. Lillp~blished)~ it therefore does not fit into this P. r~i~i~~~~~)~~/ var. ~~~~~i~~~#lfz and G. ~~rf1~~7~~1~.~ var. ~i‘cc- scheme. Simihrl y. ~~pfo.s~?~i~~~,~j~~ ~f~z~~~~u~j Walker and /PCS (weakly pathogenic) ma? cause vascular discoloSmith (1972) (DAR. 17497a and DAR. 20X06) and L. Table I. Some characteristics of varieties or (;. y?cl,llilri.sand P. rtrrlir~icoltr’

Production of perithecia

Pathogen&y

Clrowlh on stem hasIX

+

high

+

i-

high

+

-I-

low

0

low

0

0

Mean near-maximum Characteristic growth ratc ~~~~~t~l~~~ss~t~(~~ (mm, 24 h) structures ~~ 7.3 pseudoparcnchyma 7.3

pseudoparenchyma

++

9.0

lobed hpphopodia

++

9.0

lobed hpphopodiat

4.0

groups of small rounded cells

I

0

* Condensed and simplified from Deacon (1974al. Brackets delimit biological groupings discussed in the text i Single large uniobed cells within host tissues.

I

I

korrar (DAR, 13726) do not fit. These are dark mycelial fungi from grass stolons in Australia but they do not resemble G. yrumbris or P. rudjcicola in culture: they are weakly pathogenic in unsterile soil, grow well on stem bases, and form only pseudoparenchyma. On present evidence, therefore, the groupings may be too restrictive for use outside of Britain, but the criteria on which they are based are relevant to more fungi than was at first envisaged; sirSlur drtuils .sImdd he recorded irz all Jhurr work, especially if too little is known about the isolates used to permit valid taxonomic judgements.

EVIDENCE

FOR BIOLOGiCAL

CONTROL

There is much evidence for control of take-all by P. radicicoiu var. graminicolu, as first shown by Scott (1970). He grew wheat roots through inoculum of P. rudicicola into that of G. yrtminis and found less disease than hith G. qr’umir~isalone. Next, Balis (1970) found that control was enhanced by progressively longer exposures of the roots to P. r~licico/c~ before G. graminis. Moreover, inoculation of grasses with P. radicicola reduced infection by G. pwnini.s on a following wheat crop. But even more important. P. rtrdicicolu did not itself signi~cantly reduce wheat grain yields; in fact it tended to increase fresh weight of tops, though not grain. This established the potential for use of P. radicicotu in biological control. Deacon (1973b) sought evidence that natural inoculum levels of P. radicicoltr could control G. gruminis. He therefore made a survoy of the occurrence and population levels of these fungi in agricultural cropping and some natural plant communities, as shown by infection of a wheat assay crop. Both fungi were virtually absent from non-graminaceous crops. P. rudicicolu var. gran~inicolu was abundant in almost all British grasslands examined, but not in cereal crops unless these followed grass (when surprisingfy high levels were inaintained). By contrast, highly pathogenic G. ~~~i~lii?~.~ was present at only low population levels in grasslands, and at high levels in second and subsequent wheat crops: but when cereals followed grass-and therefore had high levels of P. vu& &o/u-the population of the take-all fungus was correspondingly reduced. Thus, their field distributions on grasses and cereals are complementary and this is suggestive, though not proof, of a causal relationship. P. rudicicola var. rudicicola seems uncommon in Britain, with no consistent pattern of distribution. although where it does occur it may become locally abundant. Moreover, I have not found G. yrclrrlirlis var. ~~~~~i~?~.~ here, but it seems common in warmer parts of the world (Walker. 1972. Effects of grassland soils with natural popuIat~ons of P. rud~~~~~luvar. gruF?z~ni~o~u were examined experimentally by growing wheat roots through them into a lower layer of G. y~ar~i~ais. Rate of spread of the pathogen (as vascular discoloration, because their runner hyphae are similar) or plant yield were then compared with controls (Deacon. 1973~). Inverted turves and soils from beneath grass leys significantly reduced disease. This effect disappeared when grassland soil was sterilized and subsequently allowed to become rccoloniz.ed from fallow soil. Moreover, effec-

tiveness of different turves was directly proportional to their resident population levels of P. rudici~olu (I. = 0.85; P = 0.02). but not to turf age. In one experiment. inoculum of P. radicicolu added at an unnaturally high level markedly increased control by turf, and when added to both turf and soil both then gave excellent control. This demonstrates firstly, the importance of using natural inoculum levels, but secondly, that it is reasonable to ascribe the controlling effect of grassland soils predominantly to P. rudicicoh To support this premise. fallow soil (without P. mdicico/u) was supplemented with a natural level of the fungus (to give the same degree of establishment as on roots grown through turf) and compared in a series of experiments with the same soil non-supptemented (Deacon, 1973h c, d). Good disease control was obtained against G. ~tzrrnir~is var. trifici on wheat and var. rrvcrule on grass, but not on oats (see later), thereby confirming the suggested r61e of P. rudicicola in grassland soils. In the field, Deacon (1973d) obtained strong circumstantial evidence that the normal population of P. rudi~icoh var. graminicolu in turf prevents ophiobolus patch disease. Thus. liming of turf to correct for excessive acidity is often a pre-requisite for the disease in Britain. and this is consistent with the increased growth rate of G. gra~j~~s on roots at higher pH’s (Garrett. 193?: Smiley and Cook, 1973). But P. ~ufi~ci~o~~i var. g~~~nzjni~~~u also grows better on roots at higher pH’s (Balis, 1970) and since it is usually abundant in turf, liming would expectedly increase corltroi of the disease. This paradox is explained by the intolerance of P. radicicoh to extreme acidity; it is virtually absent from turf (< pH 4.5) that needs liming to restore its vigour. and so G. gruminis can grow unimpeded from its low resident population level (or perhaps from airborne ascospores). Hence, there are two predisposing factors for development of this disease in Britain: liming, and a previously low pH. Soil fumigation. as widely practiced in the U.S.A., removes resident an~gonists (P. ~ud~~~~~~u?) presumably without subs~ntialIy altering pH, which is normally high enough in turf to permit adequate growth of G. yr~ni/lis. Further experiments (Deacon 197413) compared establishment of most varieties of G. gramirh and P. uudicicrolu on roots when inocula of the fungi were mixed into soil singly and in various combinations. Interactions occurred between all combinations of the fungi tested, but the net observed effect depended on (i) relative inoculum levels; (ii) host species (and hence relative effectiveness of various inoculum potentials, as explained in the original paper); probdbly (iii) type of host organ (root or stem); and (iv) environmental conditions (t~rn~r~~ture, pH. water, etc). These results provided the first evidence for potential &es of P. s~~~l~~i~~~l~4 var. ia&cicolu and G. g~u~~~i~li~~ var. gruf~~~~is in biological control. But. G. yrumirzis var. trihci itself reduced growth of its potential competitors under some conditions, and Deacon (I 974b) used these data to explain why grasses sown directly after infected cereals partially maintain a population of G. gruminis var. rritici. Generally. @2ctizlr corltrol of the purhogal will he uchirwl orll_v if its pup~lution lecel is low and tl7ur qf’fhe ~‘oufrd trgeuf high, unless factors as mentioned above al-t‘ect the fungi themselves or host resist-

ante to them diil’erentially: this has marked conscin wheat seminal roots at I5 C died within 3 weeks quences in practice. as discussed later. of being formed); hence perhaps they have only a Wong (1975) has provided further evidence for biosmall direct effect on the fungal interactions. logical control of take-all by similar fungi other than In general terms. s~lcccss of a fungus in penetrating P. dicicola var. gmf77inicolrr. Using agricultural soils the stele (and hcncc its pathogenicity) depends on in pots he obtained good control of G. qm777il7i.s var. its ability to reach the endodermis n,ith .xrficiicirnt inrririci by var. grcrmir7i.s and consequently a markedly ocd1rr77 po/~11ti~7/ to penetrate in whatever state of increased wheat yield. This is the first report from suberization it finds this. Prc&~r.s or prwr7f ottr777pted cxpcrimental work of protection by these fungi operir11usior7 01 roots hy ot7e of tl7csc firr7cqiwill ujjfkt thr ating throughout the life of the host plant and it is ,sucu~.~sof oth~~~.s;this is a concise statement of the extremely encouraging. Interestingly. in earlier cxperpresent hypothesis to explain biological control by iments Wong used a sand-soil mixture. but in later one of these fungi of infection bq another. experiments he achieved good (better?) disease control Similar disease control mechanisms. using less in soil without sand. Most work on P. dicicoh has virulent organisms to induce host resistance. are used sand-soil mixtures to favour growth of both the reported for other fungi and bacteria (Weber and take-all fungus (Garrett. 1936. 1937) and P. 7m/icicoh Stahmann. 1966: Littlefield. 1969; Kelman and (Balis, 1970). so perhaps better control by P. I.LI&~- Scqueira. 1972, among others). This general method c,o/cr also might be possible in undiluted soils. Wong of control may achicvc widespread application: deterfound that var. yra777i77is also effectively controls var. minants of microbial pathogcnicity to animals appar(IV~~~IUC’. but a P/~ia/opho,a-like fungus from grass roots ently are still incompletely understood (Smith. 1972) (weakly pathogenic. growth rate as for G. qwl7i77is. but this has not prevented widespread use of vaccinanot forming lobed hyphopodia [P, T. Wang. personal tion in control of animal disease. communication]) was not as efective as var. qrrr777it7i.s. PKOSPE(‘TS FOR

APPLI(‘I‘ION

OF RIOLOGICAL

CORTROL

MECHAI\ISM OF BIOLO(;IC41. (‘(I\ I’KOL Control of pathogenic G. grrrmi77k by similar but weakly- or non-pathogenic fungi is probably hostmediated. by one or more relatively non-specific mechanisms. Evidence for this is firstly. that when opposed on agar, varieties of P. rrrclicicoltrand G. qrtrr77ini.s stop growing when their colonies meet, but with no apparent adverse elrects on the hyphae (Deacon, unpublished); hence. there is no direct interaction. Secondly. the outcome of their interactions depends on type of host or host organ (Deacon. 1974b); interactions are therefore probably host-mediated. Thirdly. the control agents have an effect on the pathogens additional to that of the normal soil microflora: hence, there is some specificity. Fourthly. several closely related but nevertheless difterent fungi control pathogenic G. gw777i77i.s(Deacon. 1974b: Wong. 1975). Finally. control is exercised against these by G. ~:I’L!n7ini.s var. tritici. itself (Deacon, 1974b). Interactions between these fungi might occur on the root surface by competition for infection courts. modification of the rhizosphere microflora. etc-but there is no evidence for this: more probably these fungi induce host resistance to each other. J. Holden (personal communication) found both similarities and differences between (a) the infection processes of these fungi and (b) host resistance to them, but none that readily explains all known interactions. Host resistance to G. grL~n1ini.sresides initially in the root cortex (see review by Nilsson. 1969). Resistance to P. dicicoh is apparently similar (Holden. personal communication) but the endodermis often seems to form an efTective barrier to this and to G. qw77ir7is var. qrtrr77ir7i.s after the root cortex has been colom/ed (Deacon, 1974a). It might therefore be valuable to cxaminc secondat-> suberization and associated changes of the endodcrmis in relation to coloniration of the root cortcv bJ each of these fungi. This also follows from Holden’s (1975) demonstration that most cells of the cereal root cortex live only a short time (65”,, of cortical cells

Biological control of G. grrrr,linis by P. rurlicicoh var. qrvrr77inicoltris perhaps already exploited in agriculture. because in Britain cereals generally yield more and habe less take-all after grass leys than other crops (Rosser and Chadburn. 196X: Deacon, 1973b; Prew. 1974). Of course, other factors~~~especially nitrogen (Williams. 1967t both directly affect yields and interact with disease in such cropping sequences: but now that one probable basis of disease control by leys is known it can be exploited more fully. There is no evidence that other varieties of these fungi exert biological control in current agricultural practice; thus although G. g7mrir1i.s var. tritici might reduce disease by var. LIIXVXIC 017 outs when these are grown in cereal crop rotations. Weste and Thrower (1970) found only slight interaction between these fungi 017 1~11ctrtin glasshouse experiments. As well as effecting direct control, these fungi might have the same hyperparasites or induce similar changes in the soil microflora for example. take-all decline (TAD). if so. then prospects for control by indirect means are greatly increased. But Hornby (lY72. cited in Rawlinson, Hornby t’f crl., 1973) failed to find virus-like particles in four isolates of P. dici~>/a. and Gerlagh (196X) could not induce TAD with an avirulent culture of G. pwni77is var. tritid (which presumably did not even grow on roots); therefore. to date there is no supporting evidence for these suggestions. Regardless of how they, are used in control. weaklyand non-pathogenic varieties of these fungi do not build-up naturally to high levels in British cereal crops. They must therefore be introduced-either on other crops or by direct inoculation~~-as discussed below. E.sttrldi,~/zn76wt 077 other

c'rop5

Grass but not wheat crops in Britain often permit establishment of high populations of P. r~trtlicicolavar. qrcrminicolo (Deacon. 1973b). In the same paper, Dea-

Biological

control

con showed that wheat sown at a high seed rate in the glasshouse was as good as a grass mixture in permitting establishment of this fungus from a low natural level; he therefore ascribed differences between the effects of these crops in the field to their different rooting densities. Rooting density of field-grown cereals might be increased by mineral nutrition and, at least early in the season, by higher seed rates (though this unfortunately increases risk of eyespot lodging [Glynne, 19511); its effects on both natural establishment of this fungus on cereals and its persistence after grass merit further study. We know little of the ecology of other, similar fungi, but suitable crops for their establishment should now be sought and root cortex thickness may be important in this respect. Thus, P. rudicicolu var. grnminicolu seems ideally suited to grasses as its hosts, because it apparently colonizes most of the root cortex but not the stele; more highly infective fungi might be adapted to hosts with thicker root cortices, or underground stem tissues if they colonize these. Like rooting density, root cortex thickness also might affect prrsistence of the control agents in cereal crops (see later); but further discussion of cstuhlisk~mwt on other crops is restricted to P. mdicicolu var. yra~ninicoh, and new evidence on factors that affect its population in grasslands is summarized below. Sprcies composition. Most grass species in Britain favour the build-up of P. rndicicolu. but few quantitative data arc available. At the Grassland Research Institute (G.R.I.). Berkshire. IJ.K.. cores 2.54 cm dia x 5.08 cm deep were taken from replicated plots 01’ /.o/;lf,n I)?II’CMf1(1 L. (S23): I;c,\t1,cir tri’l&Jltrc~cY1 Schreb (SI 70): Dtrr,r r/is ~\/oIIIzI.L/~~~L. (S37): and P/~/elr,rl /JI’LIICVI.SL’ L. (S48).* Each was sown with four wheat seeds and after 3 weeks at 20” the wheat roots were washed and scored for P. rudicicola on a @4 rating by visual assessment at x 20 magnification. Results (5’:, LSD = 0.36) showed a higher infection potential of P. radicicola under L. perrnnr (1.49) and F. ururdinuceu (1.44) than under D. ylor~~rmtu (1.08) and this in turn more than under P. pruteme (0.72); this work should be repeated with more grass cultivars, but those sampled are commonly used in Britain. Non-grass species neither build-up nor perpetuate P. rudicicolu; turf with many dicotyledonous weeds thus contains less of this fungus than does weed-free turf (Deacon, 1973b) and grass-legume leys presumably less than grass leys. Rhizomatous/stoloniferous grasses deserve special comment: P. rutlicicoh var. grmhicola does not colonize stem tissues and so a high proportion of these in pasture may increase the capacity of the pasture to harbour the take-all fungus by providing a niche free from P. rudicicola. This may * Experiment H.1312: all plots were sown 26 July 1972 and sampled ~~ebruary 1974: \ecd rates were IO. 20. 20 and 70 kg ha. respcctivelq : tlq were cut to 4- 5 cm every 4 weeks. TExperiment H861 of Dr. A. Smith at Grassland Research Institute; all plots sown July/August, and sampled February 1974. Since the start of the experiment they had been ploughed and resown annually prior to coming into phase: all received N. P and K at 375, 21 and 55 kg/ha and were cut for hay four times annum- I, respectively, each year.

of take-all

279

explain widespread observations (S. D. Garrett. personal communication) that serious outbreaks of takeall that have been traced to grass carriers have often implicated perennial rhizomatous grasses, such as species of Agropyon. Holcus and Agrostis. Grusslund uge. Evidence on this is conflicting. presumably because of interaction with other factors (mineral nutrition. management. etc). Deacon (1973b) found a substantial population (infection potential) of P. radicicolu under 6-week-old grass on previously fallowed soil, and in his Table 3 this compares well with populations in much older turves. But the 6-week-old turf was established under ideal conditions from a high seed rate. Moreover. the assessment categories used--none. ‘spot’ and ‘extensive’--- -did not adequately differentiate between moderate and high population levels (‘extensive’ = two or more ‘spots’ or the whole root covered with runner hyphae). Field observations suggest that at least one year is needed for near-maximum P. rodicicolu levels to develop in most Icys. but perhaps less time is needed in mown amenit) turf (lawns. sports turf, etc). A combination of high seed rate and seed-inoculation might give most rapid establishment of the fungus. but this has not been tried. Populations of P. rudicicolu var. grurninicolu in grasslands may enter an untimely decline. Thus. at Jealott’s Hill Farm (ICI Ltd.), Bracknell, Berkshire, U.K., the population declined so much after the first year of leys that soils no longer controlled C;. graminis var. tritici in laboratory experiments (Deacon, 1973~). In contrast, decline does not seem to occur in mown turf (when grass clippings are returned), perhaps because mineral nutrients are recycled. Further evidence on grassland age is now presented. Sample cores were taken from replicated plots of perennial ryegrass < l-6 years old, all established after ploughing of previous leys. The treatments were thus different ages of swardt. not different years of grass cropping per SC. Mean infection scores for 144 roots (5% LSD = 0.57) showed that the infection potential of P. radicicolu increased from 0.51 in the first year to a near-maximum (1.56)in the second year and then remained more or less undiminished (1.86; 1.27; 1.71 ; 1.54). Two points are of practical significance: (1) with adequate nutrition and management long leys can maintain a high population of P. rudicicola: (2) 2-year-old ones are as good. These results seem to discount the possibility of a TAD-type phenomenon for P. rudicicola in grasslands, but a microbial factor could have persisted from the previous (grass) cropping at G.R.I. and depressed the infection potential of the population throughout the experiment. More information on this is needed, and also on whether or not P. radicicoka is affected by TAD soils themselves. Mirzerul rmtritiofl and m~~agermwt. Effects of nitrogen--nutrition and management (cutting versus gra7inp by sheep) were examined in two experiments. (i) A 1’ ’ yr old perennial ryegrass sward was sampled (Experiment H861 at Grassland Research Institute). P and K had been supplied at constant rates of 21 and 55 kg/ha annum-‘, respectively, and N (as Nitroxhalk) at 187.5, 375 and 750 kg/ha annum-‘. Mean infection scores on wheat roots grown in cores from these respective N-treatments were 1.27, 1.56 and 1.21 for cut plots, and 2.12 for grazed plots at

180

J. W.

D~ACUX

375 k&N (5”,, LSD = 0.44). Thus, nitrogen had little effect on the population of P. rudicicola in this experiment. but grazing significantly increased the population. (ii) Experiment H1255 at the Grassland Research Institute was sampled. The 21,2 yr old perennial ryegrass sward had received 48 kgP and 200 kgK/ha annum-‘. Infection indices on wheat roots grown in cores from plots receiving 100 and 400 kgN.‘ha annum- ’ were 0.60 and 1.85 respectively (difference significant at P = 0.01 level). The limited data from these two experiments therefore suggest that c 200 KgN!ha annum-’ is sufficient to maintain a high infection potential of P. rarlicicolu under grass. (;~r.s.&r!lt/ /&ors-~ irlterpretcltiorz. The population of P. rcctlicico/cr, as of any plant parasite, is both directly affected by environmental factors (Balis, 1970) and indirectly affected through crop growth. Since it is a rootlamust often bc limited by the rate of supply of fresh cortices for colonization; hence, in this case indirect effects of environmental factors may predominate. A similar hypothesis was suggested by Eggieton (1938) to explain effects of various factors on the bacterial population of grasslands. More information is needed on the effects of mineral nutrition and management practices on root production in grasslands; Garwood (1967a. b. 1968) examined this, but points out that root weight is often used in these studies and this merely reflects the balance of new root production, growth of existing ones and decay of old ones. Also. roots with rotted cortices can still function in transport from actively growing white tips (Garwood, 1967a). and so information needed by agronomists--numbers of living roots-~-is of little relevance here. Waid (1957) recognised this in a study of fungi decomposing ryegrass roots, and also seems to be the first to record the abundance of P. rau’icicola var. graminicola (as runner hyphae) in British grasslands. though he did not identify the fungus as such. His recent review (Waid. 1974) presents much information of potential relevance to P. ndicicola in grasslands.

dispersed. much the same is probably true of P. radicicola and similar fungi in cereal crops. However. unlike Prniophoru they cannot be applied directly to a large area of plant tissue. nor can they oust the pathogen once it is established, as Peniophorct does (Ikediugwu c’t trl.. 1970). It remains to be seen if P. tudicico/u and similar fungi can become well enough established from a relatively small inoculum to achieve control; but even if so. they would be effective only against a low inoculum level of the pathogen. Thus, when winter wheat was seed-inoculated with P. ratlicico/u var. ~udicico/cr (Plb) and sown in pots of soil taken from a field after two successive cereal crops (Deacon, unpublished). the control agent became well established. but the overwhelming pathogen population resulted in almost total failure of the crop. In practice. seed inoculation may prole most advantageous for grasses and perhaps spring-sown cereals, and the chief r61e of these fungi may be to t/c/q estuhlislztmw~ of the pathogens early in a cereal monoculture, rather than to reduce disease levels later. when the pathogen population is high. A major drawback to use of P. ~ulicicola var. grunzinicolu as a seed inoculum is its virtual inability to colonize stem bases and so grow onto crown (adventitious) roots. By contrast. P. rutlicicdu var. rarlicicolu and G. gmnini.s var. grrlrninis can grow well over the stem base from roots and vice-versa (Deacon. 1974a) and so seem ideally suited for use as seed inocula. However. whereas P. rat/icico/a var. rudicicolu always grew vigorously. G. grurnirlis var. yrurninis often grew poorly on wheat roots from an agar disc placed under the seed. Deacon (1974a) therefore suggested that it is essentially a stem-base colonizer in natural environments. and this seemed consistent with its intolerance of low partial pressures of oxygen (Smith and Noble, 1972). But Deacon (1974b) and Wong (1975) have since reported good disease control by this fungus dispersed in soil. and J. Holden (personal communication) ascribes these contrasting results (which he also has experienced) to the different methods of inoculation. Clearly. more work is needed on factors affecting the establishment and growth of these fungi from limited inocula before they can be widely used in biological control.

Inoculation of field soil with these fungi does not seem feasible in agriculture, but might be for amenit) turf (perhaps by ‘spiking-in’ rotovated turf with a natural population): it is not considered further. However, seed inoculation may be possible, as with numerous bacteria (Brown. 1974) and some fungi (Wood and Tveit, 1955: Chang and Kommedahl. 1968); moreover, it offers prospects for rapid establishment of the fungi in cereal crops without the need to grow less profitable or prdctkibk crops. Rishbeth’s (1963) stump-inoculation treatment with Prr~ioplzorw (-ligmta~ (Fr.) Messee to control Fo,lzes unn~s~~s Fr. in conifer stands provides a valuable practical example of direct introduction of an organism in biological control. As Garrett (1970) pointed out, it is successful because P. ~igmtru colonizes stumps naturally. but seldom sufficiently regularly to give adequate protection. because its airspora levels are usually too low. Although they may not be aerially-

Little attention has been given to this, but it clearly affects the economic viability of a control programme. Once a population of P. rtrtlicico/u var. grunlinicolu is established on grasses it can persist for several years on subsequent cereals. though at progressively diminishing levels (Deacon, 1973b). This has never been explained, but may be due partly to the high inoculum potential of the fungus provided by a grass crop. Thus. P. JYJdkkOkJ var. (launlir~ico/u normally does not fully exploit the cereal root cortex, judging by the positions of its growthxessation structures. But it can do so if it is present on young roots at a high inoculum potential (this is usually seen immediately below the inoculum disc in conventional pathogenicity tests (Garrett, 1936)). More highly infective fungi may persist better than P. radicicob var. qra~~~ir~icolu on cereals. especially after establishment from a low inoculum level. but there are no data for this.

Biological

control

For all these fungi, other factors that merit attention include (i) depth of ploughing and possible benefits of direct drilling (Brooks and Dawson. 1968; Wong, 1975); (ii) survival (Balis. 1970; Garrett, 1974) and the feasibility of minimizing the fallow period between successive crops in the field: (iii) possible means of ‘boosting’ the populations of the control agents in intensive cereals. perhaps by undersown grasses. IMMEDIATE

CONSIDERATIOKS

Prevention of ophiobolus patch disease of turf should be possible by (a) preserving the natural population of P. radicicolu and (b) introducing it where it is either naturally absent or killed by management practices, like fumigation (but after first establishing suitable conditions for its growth). Once the disease is established, its control is more problematical as inoculum of P. rudicicola must be introduced at a very high level to combat the resident pathogen. Perhaps the population of the pathogen could first be reduced by fungicides (Woolhouse. 1972) and P. ~&icicol~l then established to prevent its recolonization: it seems unlikely that fungicides now in use would have differential effects on these fungi, but this might also be examined. Prevention and control of take-all in cereals present more difficulties in practice and these must be resolved in the field. In take-all free soils in Britain a .cereal monoculture probably is best preceded by a ley. Preliminary results (Prew, 1974) suggest that a 2-year grass/legume ley both increases yields and delays appearance of severe take-all in a subsequent cereal sequence, but these benefits are unlikely to persist indefinitely. When take-all builds up. the bami: cropping sequence usually cannot be repeated without first growing a non-graminaceous crop, to prevent carry-over of the pathogen. Hence. unless TAD is to be exploited. it might be best to revert to grass before the disease becomes serious, as is done at Jealott’s Hill (Deacon, 1973~). Perhaps P. rarlicicol~r could be used as suggested above to delay onset of severe takeall until TAD is fully established and these biological controls thereby combined, but this will require more work on both mechanisms before it is practicable. if even possible. When take-all is already established a 2-year break of non-graminaceous and grass crops in r/m .seqzrenc~ might be a suitable introduction to a run of cereals in Britain. This is now being examined at Rosemaund Experimental Husbandry Farm, Hereford. U.K., with swedes as the first break crop: winter oil-seed rape might also be used for this purpose after winter barley (Dench, 1974). These suggestions arise from work in Britain; their relevance to other countries depends both on the feasibility of grass cropping and on the nature of the indigenous dark mycelial fungi of grass roots. However, when assessing the potential value of grass crops in disease control, an important distinction should be made between (i) old pastures. and leys sown in take-all infested soil, after both of which cereals may suffer severe disease; (ii) short-term leys in uninfested soil. which are beneficial in Britain. as described above.

‘XI

of take-all

Two final points are relevant to use of P. rtrrliciand similar fungi in biological control. regardless of how this is achieved. Firstly, where they are uncommon in a region. the abilities of these fungi to form hcterokaryons or to recombine sexually with pathogenic varieties should be studied before their widespread introduction. There seems little danger of introducing unwanted characteristics with P. rcdicicolrr var. grcuuinico/rr. hut even so until more is known about all thcsc control agents they should be selected from within local populations. This offers the best prospects for succexsful control. through adaptation to local conditions. Secondly. P. ratlicicolr~ var. (~~~rrt~i~~ico/rr increases plant yields even in the absence of pathogenic G. (/IXu1irli.s (and so it may help to pay for itself!). Thus. Balis (1970) found increased fresh weights of ccrcal tops on plants inoculated with P. dicidrr bar. qu,mirzicola alone, and Deacon (1973b. and unpublished) found a significant increase in dry weight of grass tops. These cffccts, though always small. Lverc greatest when plants were (a) crowded or (b) spaced but with inadequate mineral nutrition. Grasses in natural communities must often experience these rigours. P. rtrtlicicoltr var. grcminicolu thus joins Ihe growing ranks of control agents. like strain Al3 of Boc,i//us ,sd~ti/is (reviewed in Baker and Cook. 1974) and some mycorrhizal fungi (Marx. 1972), that are benelicial in their own rights and hcncc may enJoy a sclcctivc advantage in nature. The widespread occurrcncc of P. rarlicicolrr var. (-lrc~minico/u in Britain. and prrsumably of similar fungi elsewhere. might explain the plant breeder’s chagrin-the apparent scarcity of effectice plant resistance to G. qrtrruirh. co/u

;I~k~~o~~lrt/y~~?~~~~~r.s~~I wish to thank Dr. J. Holden. Botany School. Cambridge. for permission to use unpuhllshcd information, and both him and Professor S. D. Garrett for valuable discussion. I am grateful to Mr. A. J. Corrall, Dr. A. J. Heard and Dr. A. Smith. of the Grassland Research Instikutc. for kind permisslon lo sample from their experiments. and especially to Dr. A. J. Heard and Mr. G. C. Lewis for help with the sampling and their interest in this work. Finally. 1 gratefully acknowledge recclpt of a grant El-om the Perry Foundation. in support of the most rcccnt work described here.

REFERENCES

BAI
Biological

control

SI on D. B. and EI-BEKII)GE J. (1971) Grain yield and incidcncc of take-all (~~/~j~~b~~~~sqr~nrinis Sacc.) in wheat grown in different crop sequences. AIIU. appl. Bid. 67. 13 ‘7. Shrifts R. W. and COOK R. J. (1973) Relationship between take-all of wheat and rhizosphere pH in soils ferttlized with ammonium vs nitrate-nitrogen. Pf~~r~?pf~~~7~~ff~~ 63, X82--890.

The Sports Turf Research Institute. VOJIIXOVI~2. D. (1973) The influence

..

- _

of micro-organisms

W~to J. S. (1957) Distribution of fungi within the decomposing tissues of ryegrass roots. Trcrns. Br. m~co/. Sot. 40. 39 I-406.

of take-all

2x3

Pugh. G. J. F. Eds.) pp. 175521 1. Academic Press. London. WALKER J. (1972) Type studtes on G. yrc~ni~ris and related fungi. Trims. Br. nrrcol. Sot. 58, 427 457. WALKI-R J. and SOUTH A. M. (1972) Lcptosplxrrrict nnrnzclri and t. koriue spp. nav., two long-spored pathogens of grasses in Australia. 7‘r7~ns. Bri. m~coi. Sot. 58. 459-466. WI.B~R D. J. and STAHMANK M. A. (1966) Induced immunity to Ceratocystis infection in sweetpotato root ttssue. PIt!~tqxrrhoioq,t 56, 1066 1070. WI-ST1 G. and TIIR~W~K L. B. (1970) Comp~tltioil for wheat roots between O~?itiohoius grxrtrinis and Oplrioho/~aqrcmir1i.s var. U~UU~. Pk!;top,cn(lt. Z. 68. 106. 209. WILHI I-M S. (1973) Princiales of biolorrical control of aoilborne plant diseases. &ii Bid. Bio&m. 5, 729.-737. WILLIAMS T. E. (1967) The influence of sown grass~clover swards on soil fertility. ilrm. R~J. Gr~ssitf. Rrs. IIIV~.1966. 63 71. Wohci P. T. W. (1975) Cross-protection against the wheat and oat take-all fungi by Guclrrtlur~/tct/r?!res grc~rtinis var. ~~rfl~~i;~zi.~. Soil Bid. ~i~~~z~~~~. 7. 189- 194. Wooi> R. K. S. and Tvi--ri M. (i955) Control of plant diseases by use of antagonistic organisms. Bat. Krr. 21, 441-492. WooLtrovst. A. R. (1972) Fungicide trials, 1972. J. Sports Trrrf Ru. insi. 48, X--35.