The Effects of Host Diversity and Other Management Components on Epidemics of Potato Late Blight in the Humid Highland Tropics

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1 Epidemiology The Effects of Host Diversity nd Other Mngement Components on Epidemics of Potto Lte Blight in the Humid Highlnd Tropics K. A. Grrett, R. J. Nelson, C. C. Mundt, G. Chcón, R. E. Jrmillo, nd G. A. Forbes First, fourth, fifth, nd sixth uthors: Centro Interncionl de l Pp, Csill , Quito, Ecudor; second uthor: Centro Interncionl de l Pp, Aprtdo 1558, Lim 12, Peru; nd third uthor: Deprtment of Botny nd Plnt Pthology, 2082 Cordley Hll, Oregon Stte University, Corvllis Current ddress of K. A. Grrett: Deprtment of Plnt Pthology, Knss Stte University, Mnhttn Accepted for publiction 8 June ABSTRACT Grrett, K. A., Nelson, R. J., Mundt, C. C., Chcón, G., Jrmillo, R. E., nd Forbes, G. A The effects of host diversity nd other mngement components on epidemics of potto lte blight in the humid highlnd tropics. Phytopthology 91: A field study t three highlnd sites ner Quito, Ecudor, ws conducted to determine whether host-diversity effects on potto lte blight would be s importnt s recently found in studies conducted in temperte res. We compred three potto mixtures nd use of mixtures in combintion with different plnting densities nd two fungicide regimes. Tretment comprisons were mde by bsolute nd reltive mesures of host-diversity effects nd incorporting truncted re under the disese progress curve s mens of stndrdizing comprisons cross sites. Potto-fb intercrops consisting of only 10% potto provided n estimte of the effects of dilution of susceptible host tissue. Host-diversity effects were very different cross study sites, with lrge host-diversity effect for reduced disese only t the site most distnt from commercil potto production. Plnting density hd little influence on host-diversity effects or on lte blight in single-genotype stnds. Fungicide use in combintion with potto mixtures enhnced host-diversity effect for reduced lte blight. Potto-fb intercrops produced only smll decrese in potto lte blight. Effects of host diversity on yield were vrible, with the gretest increse in yield for mixtures treted with fungicides t the site most distnt from commercil potto production. The effects of host diversity on lte blight severity my be less consistent in the tropicl highlnds thn in the temperte zone, but cn contribute to integrted disese mngement. Additionl keywords: cultivr mixtures, genotype diversity, intercropping, Phytophthor infestns, primry inoculum, Solnum tuberosum, Vici fb. There is evidence for potentilly importnt effect of host diversity on potto lte blight in temperte regions such s Frnce (D. Andrivon, personl communiction) nd the United Sttes (11). However, results from these studies cnnot be directly extrpolted to the humid highlnd tropics where the epidemiology of lte blight my be very different. One likely difference between tropicl nd temperte systems is the level of inoculum tht will rrive t newly plnted fields. Yer round potto production nd the continuous presence of lte blight hve been described by reserchers working in the highlnd tropics (13,19), nd fungicides re used regulrly throughout the yer (4). This is in contrst to Europe nd North Americ where blight epidemics must begin gin ech growing seson. Although inoculum dynmics for temperte nd tropicl systems hve not been directly compred, indirect evidence from other studies corrobortes the hypothesis tht the two systems re quite different. Studies of the miniml plot size required for mesuring lte blight resistnce in potto genotypes in Quito, Ecudor, suggests tht rows of five plnts re s effective s lrger plots isolted by nonhost (22). High levels of eril inoculum in the Quito re probbly mde inoculum supplied by neighboring plots reltively unimportnt. Lte blight is mjor constrint to potto production in the Anden highlnds. Although frmers sometimes employ moderte levels of host plnt resistnce, the primry control strtegy hs been to spry with fungicides. Often the disese is not mnged successfully nd losses re hevy. Use of more resistnt cultivrs nd better deployment of host resistnce could reduce losses nd Corresponding uthor: K. A. Grrett; E-mil ddress: kgrrett@ksu.edu Publiction no. P R 2001 The Americn Phytopthologicl Society dependence on chemicl control. Mixtures of potto cultivrs nd intercropping of pottoes with legumes re trditionl cropping systems in the Andes (21). There re mny motives for the use of mixtures nd intercropping, including n pprecition for the mintennce of genetic resources (P. Winters, personl communiction). We re unwre, however, of informtion bout the role tht cultivr mixtures or intercropping my ply in mnging lte blight in the highlnd tropics. Therefore, it would be very useful to quntify the effect of host diversity on lte blight epidemics s prt of n effort to optimize deployment of host resistnce. In the study reported here, we estimted the host-diversity effect for three potto genotype mixtures t three sites in the humid highlnd tropics of Ecudor. Potto-fb intercrops provided nother estimte of the effects of crop diversity. We lso exmined how two other fctors ffected lte blight nd intercted with the effect of host diversity. First, we mnipulted the plnting density of pottoes, both to determine how this influenced hostdiversity effects nd to mke our results more directly comprble to results obtined in the United Sttes (11), where higher plnting densities re typicl. The influence of host density on host-diversity effects in whet stripe rust ws importnt (12), nd we compre results for the two systems. Second, we pplied fungicides on two different schedules in potto mixtures nd in corresponding single-genotype plots. If fungicide pplictions re successful in slowing epidemics tht would hve led to the rpid destruction of plnts, the slower epidemics would include more time for more genertions of pthogen increse nd this would be expected to increse the effects of host diversity (10,17). Mintining this set of current nd potentil integrted pest mngement (IPM) components nd their combintions t three sites enbled us to mke generl comprison of their reltive contributions to potto lte blight mngement. Vol. 91, No. 10,

2 MATERIALS AND METHODS Experimentl sites nd plot design. Experiments were crried out t one site during the seson nd t three sites during the seson (Tble 1). The first site, t Estción Snt Ctlin (ESC), ws surrounded by other infected potto (Solnum tuberosum) fields, with some fields s ner s 500 m, in n re of intense potto production. The second site, t Instituto Andino Superior de Agropecuri (IASA), ws in humid vlley floor bout 5 km from the nerest pottoes, which grew on the slopes bove the vlley. The third site, t Centro Acdémico Docente Experimentl L Tol (CADET), ws locted in vlley tht ws considerbly drier nd wrmer thn the other sites nd bout 15 km from other potto fields. Plots were rrnged in rndomized complete block design with four blocks t ech site. The plots were 5 5 m 2 in re including 0.5 m deep perimeter of ots (Aven stiv) round ech plot. Plots of pottoes were lternted in checkerbord pttern with buffer plots (5 5 m 2 ) plnted only with ots. Ech plot consisted of four rows of pottoes, with rows spced 1.1 m prt (1.2 m prt t site IASA98) nd plnts 0.4 m prt within the row for totl of 40 plnts per plot in stndrd density plots. At ESC97, one plnt in the center squre meter of ech plot ws rndomly selected for inocultion with suspension of locl isolte of Phytophthor infestns. However, the bckground of infection, pprently from outside inoculum, ws so high tht the inoculted plnt could not be distinguished from others 1 week lter. The other sites were left to nturl infection. Cultivr mixtures, fb intercropping, density, nd fungicide tretments. Cultivrs selected for mixtures were chosen for mesurbly different levels of resistnce to lte blight nd were lso included in the study in single-genotype stnds (Tble 2). In , the positions of genotypes were rndomized; nd in , the positions were systemticlly determined, mximizing the distnce between individuls of the sme cultivr. Intercrop plots contined 1/10 (four plnts) potto nd 9/10 (36 plnts) of locl fb ben cultivr (Hugr in ; INIAP-Quitumbe in ). Only one potto cultivr ws present within ech intercrop plot: Snt Ctlin, Gbriel, Cecili, or Uvill (Tble 2, tretments 8 to 11). In , tretment with the 3/4 Snt Ctlin nd 1/4 Uvill mixture plnted t twice the stndrd density, or 80 plnts per plot, ws dded (tretment 14). For testing host-diversity effects t this higher density, dditionl tretments of single-genotype stnds of cvs. Snt Ctlin nd Uvill t high density were included (tretments 12 nd 13). In order to llow suitble cnopy size for study prior to the beginning of the lte blight epidemic, ll plots t ech site were treted with protectnt fungicide until shortly before disese evlution begn. To test for interctions between fungicide pplictions nd genotype mixing, fungicide tretments throughout the seson were dded for the 3/4 Snt Ctlin nd 1/4 Uvill mixture in , s outlined in Tble 2. Disese ssessment nd dt nlysis. The percentge of blighted lef nd stem tissue ws estimted visully on the dtes indicted in Tble 1. For intercrop plots, the percentge of blighted tissue ws estimted bsed only on the potto plnts, ignoring the lte blight-immune fbs. Clcultion of re under disese progress curve (AUDPC), reltive mixture response, nd truncted AUDPC. AUDPC, TABLE 1. Reserch sites ner Quito, Ecudor, for study of the effects of host diversity nd other integrted pest mngement components for mngement of potto lte blight cused by Phytophthor infestns Site Abbrevition Altitude Plnting dtes Number of disese evlutions (evlution period) Hrvest dtes Estción Snt Ctlin 1997 ESC97 3, nd 27 Nov (7 Feb 17 Mr 1997) Apr 1997 Estción Snt Ctlin 1998 ESC98 3, Jn (20 Feb 31 Mr 1998) Apr 1998 Instituto Andino Superior de Agropecuri IASA98 2, Feb (9 Apr 7 My 1998) Jun 1998 Centro Acdémico Docente Experimentl L Tol CADET98 2, Jn (6 Mr 17 Apr 1998) Jun 1998 Meters bove se level. TABLE 2. Tretments included in study of the effects of host diversity nd other disese mngement prctices on potto lte blight cused by Phytophthor infestns Tretment Density Fungicide b Genotype Expecttion under null hypothesis (H o ) c T1 Stndrd None Snt Ctlin (SC) (resistnt) T2 Stndrd None Gbriel (GA) (modertely resistnt) T3 Stndrd None Cecili (CE) (susceptible) T4 Stndrd None Uvill (UV) (susceptible) T5 Stndrd None 3/4 SC nd 1/4 UV 3/4 T1 + 1/4 T4 T6 Stndrd None 1/3 SC, 1/3 GA, nd 1/3 CE 1/3 T1 + 1/3 T2 + 1/3 T3 T7 Stndrd None 1/3 SC, 1/3 CE, nd 1/3 UV 1/3 T1 + 1/3 T3 + 1/3 T4 T8 Stndrd None 1/10 SC nd 9/10 fb bens T1 T9 Stndrd None 1/10 GA nd 9/10 fb bens T2 T10 Stndrd None 1/10 CE nd 9/10 fb bens T3 T11 Stndrd None 1/10 UV nd 9/10 fb bens T4 T12 High None SC T1 (H o : no density effect) T13 High None UV T4 (H o : no density effect) T14 High None 3/4 SC nd 1/4 UV 3/4 T12 + 1/4 T13 T15 Stndrd Biweekly SC T1 (H o : no fungicide effect) T16 Stndrd Biweekly UV T4 (H o : no fungicide effect) T17 Stndrd Biweekly 3/4 SC nd 1/4 UV 3/4 T15 + 1/4 T16 T18 Stndrd Weekly SC T1 (H o : no fungicide effect) T19 Stndrd Weekly UV T4 (H o : no fungicide effect) T20 Stndrd Weekly 3/4 SC nd 1/4 UV 3/4 T18 + 1/4 T19 Only tretments 1 to 4 nd 6 to 11 were pplied t site-yer ESC97. b Chlorothlonil pplied t stndrd lbeled rte. All tretments received fungicide pplictions t the beginning of the experiment to dely the beginning of the epidemic until plnts were lrger. c The null hypothesis being considered is tht there ws no host-diversity effect, unless otherwise indicted. 994 PHYTOPATHOLOGY

3 clculted by the midpoint rule method (2), ws used s one response vrible: n 1 k ) ) / 2] AUDPC ij = = [( t k tk )( yijk yij( k + where y ijk is the percent severity observed for the kth observtion dte of the ith tretment in the jth block, t k is the Julin dte of the observtion, nd observtions were mde on n dtes. Studies of the effects of host diversity on disese generlly involve comprisons of disese severity in host mixture to the men disese severity in the sme host genotypes growing in singlegenotype stnds. It is often useful to consider both the bsolute difference in severity between mixtures nd single-genotype stnds nd the reltive difference in severity. For the comprison of the reltive difference, we used wht we termed the reltive mixture response (RMR): AUDPC in mixture/weighted men AUDPC in single-genotype stnds. The RMR llows for comprisons between host-diversity effects in settings tht my hve very different overll levels of disese, such s different fungicide regimes. We lso used truncted formultion of the AUDPC, bsed on the following rtionle. Once one component of mixture pproches 100% disesed tissue, host-diversity effect for tht component is no longer possible. Even before 100% infection is reched, there my be tendency for disese progress to slow gretly s it pproches its mximum. If the host-diversity effect is expressed s n RMR, this rtio will be pushed towrd 1 if mesurements of percent disesed tissue continue beyond the point when disese severity of one component is nerly mximum. To llow for comprisons without this effect, truncted AUDPC (TAUDPC) ws lso clculted, bsed on the AUDPC only up to the point when the most susceptible component reched 90% infection in single-genotype stnds. For some mixtures this never occurred, therefore, the TAUDPC ws the sme s the complete AUDPC. Trunction to stndrdize AUDPCs for comprison. To produce the TAUDPC, ech mesure of percent severity ws djusted s follows. Let d mx = 90 be the cut-off disese level, the level of disese in the most susceptible component tht signls the time for trunction, t mj mx, in which we designte the most susceptible component s tretment m in block j. Becuse percent severity ws mesured only t pproximtely weekly intervls, d mx ws likely to be reched t time between observtion dtes. Just s typicl clcultions of n AUDPC rely on liner interpoltion between smpling dtes (2), t mj mx ws clculted by interpoltion between smpling dtes in the susceptible vriety. Suppose observtion dte is the lst observtion dte t which the percent severity in the most susceptible component is below d mx. We first clculted the slope for the most susceptible component, s mj = (y mj(+1) y mj )/(t +1 t ) nd then t mj mx = t + (d mx y mj )/s mj. The vlue of t mj mx clculted from the single-genotype stnd of the most susceptible component ws used for clculting the TAUDPC for ech component nd for the corresponding mixture, with clcultions done seprtely for ech block so tht observtions from replictes of given tretment were independent (6,12). The individul plot slope, s ij, between t nd t +1 ws clculted seprtely for ech component nd for the mixture: 1 i j = k = 1 [( tk + 1 tk )( yijk + yij( k + 1) ) / 2 ] mx mx {( t t )[2y + s ( t t )]}/ 2 TAUDPC mj ij For bsolute comprisons, TAUDPC ij ws stndrdized by division by t mj mx t 1 nd some tretments were nlyzed seprtely to void the issue of different trunction times for the sme singlegenotype plot when more thn one mixture contined tht genotype. Sttisticl nlysis of AUDPC, TAUDPC, nd RMR. An nlysis of vrince (ANOVA) ws performed in SAS (SAS Institute Inc., Cry, NC), tking ech tretment combintion s different level of single tretment to mke the formultion of tests clerer (Tble 2). The following liner model ws fit to the dt for ech site yer combintion: x ij = µ + τ i + β j + ε ij, where x ij is the AUDPC or TAUDPC for the ijth experimentl plot, τ i is the effect of the ith tretment (i = 1,,20), β j is the effect of the jth block (j = 1,,4), nd ε ij is the residul error. Plnned contrsts were used to test effects on the AUDPC nd TAUDPC s c τ, where τ is the vector of tretment effects in the order of Tble 2 nd c is the vector of coefficients for prticulr contrst. Similrly, for the RMR the following model ws fit: y ij = µ + τ i + ε ij, where y ij is the RMR for the ijth experimentl plot, τ i is the effect of the ith tretment (i = 5,,11,14,17,20), nd ε ij is the residul error. Blocks were not included in the RMR model becuse block term would represent n interction between host-diversity effect nd block. For nlyses of AUDPC, TAUDPC, nd RMR, the distribution of residuls from nlyses ws stisfctory for meeting the distributionl ssumptions of the ANOVA. Evlution of tretment effects. The liner contrsts used to test for tretment min effects re comprisons of observtions to the prediction under the null hypothesis of no tretment effect s outlined in the lst column of Tble 2. The liner contrsts used to test for tretment interctions re outlined in Tble 3. The RMR for intercrops ws lso considered for fb intercrops seprtely: y ij /y (i-7)j = µ + τ i + β j + ε ij, for i = 8,9,10,11. In this nlysis, if µ is significntly different from 1 this mens tht there is evidence for n overll effect of intercropping on lte blight severity. There is evidence for different effects of intercropping for the different potto cultivrs if the τ i re significntly different from 0. For the nlysis of the RMR, host-diversity effect could be detected if the RMR ws significntly different from 1. Note tht, for exmple, tests of host-diversity effect for mixtures in T5 through T8 re not independent from ech other becuse they ll contin estimtes from T1 in their denomintor. For tests of the influence of density nd fungicide use, there is no problem of dependence becuse seprte single-genotype stnds were included to correspond to ech density nd level of fungicide ppliction. ij mj + TABLE 3. Additionl plnned contrsts used to test hypotheses bout mngement components for potto lte blight cused by Phytophthor infestns Tests with AUDPC or TAUDPC s response vrible Contrst b H o : No overll liner prt of fungicide min effect τ 1 τ 4 + τ 18 + τ 19 H o : No overll qudrtic prt of fungicide min effect +τ 1 + τ 4 2τ 15 2τ 16 + τ 18 + τ 19 H o : No influence of plnting density on host-diversity effect (τ 14 3/4 τ 12 1/4 τ 13 ) (τ 5 3/4 τ 1 1/4 τ 4 ) H o : No influence of biweekly fungicide use on host-diversity effect (τ 17 3/4 τ 15 1/4 τ 16 ) (τ 5 3/4 τ 1 1/4 τ 4 ) H o : No influence of weekly fungicide use on host-diversity effect (τ 20 3/4 τ 18 1/4 τ 19 ) (τ 5 3/4 τ 1 1/4 τ 4 ) Tests with reltive mixture response s response vrible H o : No influence of host density on host-diversity effect τ 14 τ 5 H o : No liner prt of influence of fungicide use on host-diversity effect τ 5 + τ 20 H o : No qudrtic prt of influence of fungicide use on host-diversity effect +τ 5 2τ 17 + τ 20 AUDPC = re under disese progress curve; nd TAUDPC = truncted AUDPC. b Tretment numbers refer to the tretments s listed in Tble 2. Vol. 91, No. 10,

4 Yield. Fresh weights of tubers were recorded for ll rows of ech plot on the dtes indicted in Tble 1. Yield from fb intercrops ws not nlyzed becuse of the difficulties of interprettion given the smll number of potto plnts nd the poor stnds of fbs. The sme set of liner contrsts s used for disese ws used to compre tretment effects on yield. Becuse there ws tendency for the vrince of residuls to increse with incresed predicted vlues, the nlysis ws djusted to llow for different vrinces for different tretments. SAS Proc Mixed ws used to perform the nlysis, including the sttement, repeted/group = trt, s in Grrett nd Mundt (12), where trt indictes the tretments s listed in Tble 2. Anlysis of IPM components cross sites. Tretments from the three sites in tht llowed the most direct comprison were evluted to summrize the performnce of the different mngement techniques. Responses were expressed s the percent reduction in the AUDPC, TAUDPC, nd yield. We constructed comprisons of how well prticulr tretment performed compred with plots without the benefit of tht tretment (Tble 4). Becuse we only hd three dt points for ech nlysis t this sptil scle, we did not perform sttisticl nlysis. RESULTS Folir lte blight severity on site-by-site bsis: single-genotype stnds. The reltive levels of resistnce of potto cultivrs in single-genotype stnds were generlly s expected, though there ws some vrition from site to site. Gbriel hd intermedite levels of disese in ESC98 nd CADET98. At the other sites in , Gbriel performed similrly to highly susceptible cv. Uvill (Fig. 1). Cecili hd less disese thn Uvill t CADET98, lthough the two cultivrs were similr in disese expression for other site-yers (Fig. 1). Mixtures with vrying resistnce levels. A host-diversity effect for reduced AUDPC without fungicide use ws evident only t site CADET98 (Fig. 2). The lrgest estimted host-diversity effect ws for the 3/4 Snt Ctlin + 1/4 Uvill mixture with weekly fungicide pplictions. There ws trend towrd hostdiversity effects when considering ll the site yer combintions, nd this trend ws more striking for the nlysis with the TAUDPC (Fig. 2). Fb intercropping. Fb plnts hd poor emergence in the seson nd were generlly smller nd sprser thn potto plnts t ll sites in the seson s well. When cultivrs were nlyzed individully by liner contrsts, there ws only wek evidence for potto-fb intercropping effects (P > 0.11 in ll cses), lthough there ws cler trend towrd reduced lte blight in intercrops compred with single-genotype stnds (Fig. 3). Similrly, for the nlysis of the RMR in fb intercrops for ech cultivr seprtely, there ws no evidence for differences in effects for different cultivrs. When the overll effect of intercropping ws tested for ech site (combining cultivrs), the trend towrd reduced disese in intercrops ws reflected in some sttisticl evidence for this overll intercrop effect, though only for ESC98 (RMR bsed on AUDPC, P = 0.10; RMR bsed on TAUDPC, P < 0.01), IASA (RMR-AUDPC, P = 0.01; RMR- TAUDPC, P < 0.01), nd CADET98 (RMR-TAUDPC, P < 0.01). Density tretments. In single-genotype plots of cvs. Snt Ctlin nd Uvill, the AUDPC ws lower t the stndrd plnting density thn t the doubled plnting density t ech of the three sites (Fig. 4). However, this trend ws not sttisticlly significnt t ny of the three sites for liner contrsts on the AUDPC (P > 0.20 in ll cses). For mixtures t the higher plnting density, there ws wek evidence for host-diversity effect from liner contrsts on the AUDPC only t site CADET98 (P = 0.09) (Fig. 2). Fungicide tretments. Fungicide pplictions reduced lte blight severity in single-genotype stnds for both weekly nd biweekly tretments t ll sites (P < 0.01 for ll tests). In singlegenotype stnds, there ws lso strong trend towrd lower percent severity for weekly ppliction of fungicides compred with biweekly ppliction. There ws strong sttisticl evidence for this trend in liner contrsts on the AUDPC for both Snt Ctlin (P < 0.01) nd Uvill (P < 0.01) t IASA98, for neither cultivr t ESC98 (P > 0.15 for both), nd only for Uvill (P = 0.05) t CADET98. There ws lso strong evidence for incresed host-diversity effects with incresing fungicide ppliction levels. This effect ws highly significnt for the test of liner effect of fungicide ppliction on host-diversity effects on the RMR clculted from the AUDPC (P < 0.01 for ll sites). Results vried for other forms of tests of the interction between fungicides nd host-diversity effects. For liner contrsts on the AUDPC, there ws evidence supporting the liner prt of the fungicide effect on host-diversity effects only t ESC98 (P = 0.06), nd there ws little evidence supporting the qudrtic prt t ny site (P > 0.2). There ws evidence supporting the qudrtic effect of fungicide ppliction on host-diversity effects on the RMR clculted from the AUDPC only t IASA98 (P < 0.01). Results were similr for n RMR TABLE 4. Effectiveness of disese mngement prctices nd their combintions on potto lte blight cused by Phytophthor infestns bsed on responses for three reserch sites 996 PHYTOPATHOLOGY % Chnge in disese levels due to components Bsed on AUDPC b Bsed on TAUDPC b % Chnge in yield due to components Components Men Best site Worst site Men Best site Worst site Men Best site Worst site Resistnce c Fungicides d , Density e Intercropping NA f NA NA Host diversity g Resistnce + fungicides h , , Host diversity + fungicides i Host diversity + density j Tble entries re [(men observtion men prediction under null hypothesis of no effect)/(men prediction under null hypothesis of no effect)] 100. b AUDPC = re under disese progress curve; nd TAUDPC = truncted AUDPC. c Snt Ctlin (resistnt) versus Uvill (susceptible). d Uvill with weekly fungicide pplictions versus Uvill with no fungicide pplictions. e Uvill t stndrd density versus Uvill t high density. f Not vilble. Potto yield nlyses for the potto-fb intercrops were inpproprite becuse of the poor fb stnds. g Uvill nd Snt Ctlin in mixture versus weighted men of single-genotype stnds. h Snt Ctlin with weekly fungicide pplictions versus Uvill with no fungicide pplictions. i Uvill nd Snt Ctlin in mixture with weekly fungicide pplictions versus weighted men of single genotype stnds with no fungicide pplictions. j Uvill nd Snt Ctlin in mixtures t stndrd density versus weighted men of single-genotype stnds t high density.

5 bsed on the TAUDPC, but there ws evidence for qudrtic rther thn liner effect of fungicide ppliction on host-diversity effects t CADET98 (liner P = 0.35, qudrtic P = 0.01). Yield on site-by-site bsis. There ws evidence for hostdiversity effect on yield only for the 1/3 Snt Ctlin + 1/3 Gbriel + 1/3 Cecili mixture. This evidence ws for incresed yield t ESC97 (P = 0.09) nd CADET98 (P = 0.07), but there ws no evidence for n effect t ESC98 (P = 0.43) or IASA98 (P = 0.56). There ws little sttisticl support for n effect of density on yield in single-genotype stnds or for density effect on the host-diversity effect on yield (P > 0.25 for ll tests). Effects of weekly fungicide spry on yield of single-genotype stnds of Snt Ctlin were t lest mrginlly significnt t ll sites (P < 0.11). There ws less consistent evidence for n effect of biweekly fungicide regime. There ws no evidence tht the type or lck of fungicide regime influenced the mgnitude of hostdiversity effect on yield (P > 0.4 for ll tests). Effects on folir lte blight severity nd yield cross sites. The effect of high levels of host resistnce on folir blight severity in cv. Snt Ctlin compred with the level of resistnce in cv. Uvill ws similr to the effect of weekly fungicide regime (Tble 4). The effects of density nd intercropping were smll but consistent t ll three sites. The effects of host diversity were vrible, with n increse in disese in mixtures t one site. The combintion of host resistnce in cv. Snt Ctlin nd weekly fungicide regime ws quite successful t reducing disese severity, with consistent results t ll sites. Host diversity in combintion with fungicides lso provided substntil disese control, but the combintion of host diversity nd density mnipultion ws less effective. For yield, the effect of host resistnce in cv. Snt Ctlin in combintion with its other genetic differences from cv. Uvill provided firly consistent nd substntil benefits. The effect of fungicides on yield ws greter, with prticulrly lrge effect t CADET98. Incresing the stndrd plnting density tended to increse yield slightly. The effects of host diversity were vrible, with decrese in yield in mixtures t two sites. The effect of resistnt cv. Snt Ctlin s genetic differences from cv. Uvill in combintion with fungicide pplictions gretly incresed yield, prticulrly t the site with the lrgest fungicide effects. Host diversity in combintion with fungicide pplictions lso consistently incresed yield. Host diversity combined with the lower (stndrd) host density provided vrible results, tending towrd decrese in yield. DISCUSSION We found evidence for host-diversity effect on lte blight severity, but this effect vried over the three loctions of our study nd between yers (Fig. 2). For ESC, the site where we expected the most inoculum from djcent fields, host diversity did not reduce disese but, rther, there ws some evidence for hostdiversity effect for incresed severity. The lrgest host-diversity effect for decresed disese severity in mixtures ws for potto cv. Uvill t CADET98, the site most distnt from other potto fields. The contrsting results from these two sites re consistent with the ide tht the level of outside inoculum is n importnt fctor driving the mgnitude of host-diversity effects (23). However, the results for fb intercrops do not show the sme cler difference between sites. For these, the estimted effect of intercropping t the isolted site CADET98 ws essentilly the sme in mgnitude s for the other site-yers. The difference in results for fb intercrops nd potto mixtures might be explined by the following fctors. First, becuse of the generlly poor growth of fbs in this experiment, they my hve provided much less of physicl brrier between potto plnts of the sme genotype thn did pottoes in the potto mixtures. In ddition, pottoes in potto mixtures my hve been subject to more effects from plnt competition nd the wys in which competition vried from one site to nother. Likewise, there ws no possibility of compenstion Fig. 1. Disese progress curves for lte blight cused by Phytophthor infestns in single-genotype plots of ech of the four potto genotypes for the four site yer combintions. Vol. 91, No. 10,

6 in intercrops becuse severity ws mesured only on potto plnts; tht is, folige loss in the susceptible cultivr could not be replced by growth of the resistnt cultivr. Finlly, we nticipted greter vrince in our estimtes of disese severity in intercrop plots, becuse severity ws mesured for only four potto plnts per plot compred with the 40 plnts per plot in potto mixture. We might hve nticipted tht higher density plots would produce microclimte more conducive to lte blight (1). However, severl recent studies hve found either no effect of density or negtive ssocition between plnting density nd disese severity (5,7,12,20). One reson tht disese my show little response to density chnges is tht the relized density of folige my be similr over plnting densities (20). Trditionl potto cultivrs in the Andes, including those we used, re lte mturing nd produce lrge mounts of folige. It might be tht plnting density much lower thn stndrd would reduce lte blight, but yield might be reduced enough to mke such plnting density imprcticl. In generl, we might expect tht higher plnting density would produce greter host-diversity effect becuse susceptible tissue would pper in smller res (10). But, in this experiment, there ws no evidence for n influence of plnting density on hostdiversity effects, s might be expected becuse of the smll density effects in single-genotype stnds. This is in contrst to whet stripe rust, for which there ws lrge influence of plnting density on host-diversity effects (12). Differences in the influence of density in these two systems my be due in prt to differences in plnt rchitecture nd pthogen dispersl, but it would be useful to consider lower potto densities thn were included in this experiment to clrify the comprison. In generl, there ws greter host-diversity effect for plots tht received fungicide pplictions (Fig. 2). This interction ws especilly mrked t site CADET98. Greter host-diversity effects might be predicted becuse n epidemic tht lsts longer before plnts re destroyed by disese, whether becuse of fungicide pplictions or other fctors such s higher overll disese resistnce, would llow time for more pthogen genertions (17). This nlysis lso demonstrtes the utility of studying both the bsolute (in terms of liner contrsts) nd reltive (in terms of RMR) hostdiversity effects. For site CADET98, there ws little influence of fungicide regime on the bsolute host-diversity effect; the bsolute reduction in AUDPC due to host diversity ws pproximtely the sme regrdless of the fungicide regime (dt not shown). However, the reltive reduction ws much greter for plots with weekly fungicide ppliction (Fig. 2). The nlysis t CADET98 illustrtes the utility of the TAUDPC. We cn interpret the TAUDPC s mesure of the effects of host diversity while the susceptible component of mixture is still vilble for infection. If the greter number of pthogen genertions during the course of the epidemic resulted in greter hostdiversity effect for plots with fungicide tretments, we might expect tht the RMR bsed on the TAUDPC would lso show greter effect for fungicide tretments. This ws true for two sites, but t site CADET98, the RMR bsed on the TAUDPC ws essentilly the sme whether or not fungicide ws pplied (Fig. 2). Although the epidemic in the susceptible genotype ws still ctive, the host-diversity effect seemed to be just s gret with or without fungicide pplictions t this site. The difference under Fig. 2. The reltive mixture response (RMR; re under disese progress curve [AUDPC] in mixture per weighted men AUDPC in single-genotype stnds) for potto lte blight in three potto mixtures. An RMR less thn 1 indictes host-diversity effect for reduced disese. The mixture of 3/4 Snt Ctlin (resistnt) nd 1/4 Uvill (susceptible) ws studied t double the stndrd plnting density nd under two fungicide regimes. The sttisticl significnce of the host-diversity effect, bsed on liner contrsts, is indicted by the shding nd size of individul symbols. Symbols shded blck indicte P < 0.10; nd enlrged symbols indicte P < Results were lso clculted for the sme mixtures with RMR clculted using the truncted AUDPC (TAUDPC). Circle = site-yer ESC97; squre = site-yer ESC98; tringle = site IASA98; nd dimond = site CADET98. Fig. 3. The reltive mixture response (RMR; re under disese progress curve [AUDPC] in mixture per weighted men AUDPC in single-genotype stnds) for potto lte blight in four potto cultivrs in seprte intercrops with fb ben. An RMR less thn 1 indictes tht there ws n effect from intercropping for reducing disese. Becuse no effects were significnt in tests bsed on individul cultivrs, significnce levels for tests of n overll intercropping effect bsed on the RMR for ll fb intercrops t site re illustrted. Symbols shded blck indicte P < 0.10 for the overll intercropping test; nd enlrged symbols indicte P < 0.05 for the overll intercropping test. Results were lso clculted for the sme intercrops with the RMR clculted using the truncted AUDPC (TAUDPC). Circle = site-yer ESC97; squre = site-yer ESC98; tringle = site IASA98; nd dimond = site CADET PHYTOPATHOLOGY

7 fungicide regime ws tht the epidemic ws slowed so tht pprently the susceptible component could benefit from mixture with the resistnt component throughout the seson. It could lso be tht, while the host-diversity effect ws compounding in the fungicide-treted plots over time, the levels of outside inoculum nd inoculum from neighboring plots were lso incresing over time. Host-diversity effects on yield were gretest, on verge, t site CADET98, with the most importnt host-diversity effect being for the mixtures with weekly fungicide pplictions t tht site (Tble 4). This is consistent with the fct tht we observed the gretest host-diversity effect on disese severity for tht tretment. For tretments tht hd smll or no host-diversity effects on disese severity, there ws no reson to expect lrge host-diversity effect on yield. The potto cultivrs selected for this study were chosen bsed only on our knowledge of their levels of resistnce nd would not necessrily be comptible in other spects of their cultivtion. In compring the effect of the individul IPM components on lte blight severity, genetic resistnce nd fungicide pplictions were clerly the most importnt (Tble 4). Their effects were similr, though we should note tht fungicide pplictions were included t the beginning of the seson for ll tretments. The pproximte equlity in contribution between fungicides nd resistnce is similr to the results of Fry s (9) study of fungicides nd resistnce in the United Sttes. Both density mnipultion nd intercropping provided only smll effects, but their cumultive effects over mny sesons could be importnt. On verge, host diversity lone gve smll reduction in disese severity, but with vrible effects. The effects of host diversity mesured by the TAUDPC were generlly lrger. If we interpret the TAUDPC s giving results more similr to wht might be seen for successful mixture with higher overll levels of resistnce, this suggests tht host diversity could become more importnt mngement component s the overll level of resistnce for potentil mixture components increses. The combintion of resistnce nd fungicide pplictions reduced disese levels gretly (Tble 4). In the Quito re, where tuber blight is generlly not found (G. A. Forbes, personl observtion), 91% reduction in AUDPC my be dequte. The combintion of host diversity nd density mnipultion ws not effective enough for relible mngement without other mngement components. The combintion of host diversity nd fungicides ws more promising thn host diversity lone, though better direct tests for interctions between mngement components re given by the contrsts in Tble 3. With very few exceptions (14), this combintion of mngement techniques hs not been exmined. If the usefulness of this combintion is due t lest in prt to the fct tht more pthogen genertions cn occur during the longer epidemic produced by fungicides, then similr effect might be nticipted if the overll level of resistnce ws incresed within genotype mixture. It would be useful to compre the hostdiversity effect in two mixtures, both with the sme mgnitude of difference in resistnce between components, but with different overll levels of resistnce. We might nticipte tht the hostdiversity effect would be greter for the higher overll level of resistnce. In compring the effect of the different IPM components on yield, resistnce nd fungicide regimes were gin clerly the most importnt (Tble 4). Higher densities tended to produce slightly higher yields. Host diversity produced vrible responses; it might be expected tht its effect on yield would be even more vrible thn its effect on disese severity, becuse the mixture components were not selected to optimize yield. The combintion of resistnce nd fungicides ws very importnt for incresing yield, prticulrly t one site where susceptible cv. Uvill filed lmost completely in single-genotype stnds without fungicide pplictions. The combintion of fungicides nd host diversity showed some promise, though, gin, the mixture components would need to be selected on multiple criteri to boost the efficiency of this combintion. This study illustrtes the importnce of studying the effects of host diversity nd other potentil IPM components in rnge of environments; if ny one of these sites hd been the sole study site, very different conclusions might hve been drwn. Studies over longer period of time would lso llow considertion of how host diversity might influence pthogen evolution. Probbly the current use of potto genotype mixtures by smll frmers gives some cumultive benefit for mngement of lte blight over sesons nd locles, in ddition to generl bet-hedging for other gronomic fetures, lthough in ny given seson nd locle the effects my be vrible. To the extent tht high levels of outside inoculum my reduce the efficcy of host diversity for disese Fig. 4. Disese progress curves for lte blight in single-genotype plots of potto cvs. Snt Ctlin nd Uvill t stndrd nd high plnting densities. Vol. 91, No. 10,

8 mngement, this constrint might be removed s regionl mngement of lte blight improves; lndscpe-scle study exmining the role of specific infection sources, such s tht of Zwnkhuizen et l. (24), would help to clrify these interctions. We cn consider improvements in the efficcy of host diversity for disese mngement in terms of selecting components with better differentil resistnce nd lso in terms of selecting true potto seed prents for useful diversity in resistnce genes. One of the chllenges for developing good differentil mixtures for lte blight mngement in Ecudor is the pprent high degree of specific virulence in the bsence of selection (8). The cost of virulence in P. infestns my be low or negligible. However, if sexul reproduction of P. infestns occurs on lrge scle in the future, mixtures with mjor genes for resistnce might ply lrger role. Rce-nonspecific dpttion my be of use: Oyrzun et l. (19) found different levels of ggressiveness of isoltes from pottoes nd tomtoes when infecting potto. Rce-nonspecific dpttion to host genotype my be importnt in prtitioning pthogen popultions in host mixtures (3,15,16,18). To summrize, we found host-diversity effect on potto lte blight t one of three reserch sites ner Quito, Ecudor. This site ws the lowest (nd therefore wrmest), driest, nd we ssume most free from outside inoculum. In tht sense, this site is more similr to those in the temperte zone where host diversity hs hd more consistent suppressive effect on lte blight development (D. Andrivon, personl communiction; 11). At the other sites with higher ltitude, which re more typicl of potto production in the humid tropicl highlnds, there ws evidence, lthough wek, tht host diversity cn enhnce the disese suppressive effects of fungicides. Future reserch should explore the potentil use of host diversity in combintion with other prctices designed to slow epidemic rte, including fungicides, host resistnce, nd plnting t higher ltitudes where tempertures re lower. ACKNOWLEDGMENTS This reserch ws funded in prt by USAID linkge funds. We thnk C. Cñizres for mngement of the experiment t CADET nd the Forbes fmily for mking work t Quito such plesure. LITERATURE CITED 1. Burdon, J. J., nd Chilvers, G. A Host density s fctor in plnt disese ecology. Annu. Rev. Phytopthol. 20: Cmpbell, C. L., nd Mdden, L. V Introduction to Plnt Disese Epidemiology. Wiley-Interscience, New York. 3. Chin, K. M., nd Wolfe, M. S Selection on Erysiphe grminis in pure nd mixed stnds of brley. Plnt Pthol. 33: Crissmn, C. C., Espinos, P., Ducrot, C. E. H., Cole, D. C., nd Crpio, F The cse study site: Physicl helth nd potto frming systems in Crchi province. Pges in: Economic, Environmentl, nd Helth Trdeoffs in Agriculture: Pesticides nd the Sustinbility of Anden Potto Production. C. C. Crissmn, J. M. Antle, nd S. M. Cplbo, eds. Kluwer Acdemic, Dordrecht, the Netherlnds. 5. Dill-Mcky, R., nd Roelfs, A. P The effect of stnd density on the development of Puccini grminis f. sp. tritici in brley. Plnt Dis. 84: Federer, W. T Sttisticl Design nd Anlysis for Intercropping Experiments. Vol. 1. Two Crops. Springer-Verlg, New York. 7. Finckh, M. R., Gcek, E. S., Czembor, H. J., nd Wolfe, M. S Host frequency nd density effects on powdery mildew nd yield in mixtures of brley cultivrs. Plnt Pthol. 48: Forbes, G. A., Escobr, X. C., Ayl, C. C., Revelo, J., Ordoñez, M. E., Fry, B. A., Doucett, K., nd Fry, W. E Popultion genetic structure of Phytophthor infestns in Ecudor. Phytopthology 87: Fry, W. E Quntifiction of generl resistnce of potto cultivrs nd fungicide effects for integrted control of potto lte blight. Phytopthology 68: Grrett, K. A., nd Mundt, C. C Epidemiology in mixed host popultions. Phytopthology 89: Grrett, K. A., nd Mundt, C. C Host diversity cn reduce potto lte blight severity for focl nd generl ptterns of primry inoculum. Phytopthology 90: Grrett, K. A., nd Mundt, C. C Effects of plnting density nd the composition of whet cultivr mixtures on stripe rust: An nlysis tking into ccount limits to the repliction of controls. Phytopthology 90: Hijmns, R. J., Forbes, G. A., nd Wlker, T. S Estimting the globl severity of potto lte blight with GIS-linked disese forecster. Plnt Pthol. 49: Kousik, C. S., Snders, D. C., nd Ritchie, D. F Mixed genotypes combined with copper sprys to mnge bcteril spot of bell peppers. Phytopthology 86: Lnnou, C., nd Mundt, C. C Evolution of pthogen popultion in host mixtures: Simple rce-complex rce competition. Plnt Pthol. 45: Lnnou, C., nd Mundt, C. C Evolution of pthogen popultion in host mixtures: Rte of emergence of complex rces. Theor. Appl. Genet. 94: Leonrd, K. 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Fitoptologí 29: Wolfe, M. S The current sttus nd prospects of multiline cultivrs nd vriety mixtures for disese resistnce. Annu. Rev. Phytopthol. 23: Zwnkhuizen, M. J., Govers, F., nd Zdoks, J. C Development of potto lte blight epidemics: Disese foci, disese grdients, nd infection sources. Phytopthology 88: PHYTOPATHOLOGY