FIELD TOLERANCE OF SELECTED VARIETIES TO AND FUNGICIDE EFFICACY AGAINST ALTERNARIA BLIGHT OF SWEET POTATO ABSTRACT RÉSUMÉ

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1 Africn Crop Science Journl, Vol. 24, No. 3, pp ISSN /2016 $4.00 Printed in Ugnd. All rights reserved 2016, Africn Crop Science Society Africn Crop Science Journl by Africn Crop Science Society is licensed under Cretive Commons Attribution 3.0 Ugnd License. Bsed on work t nd DOI: FIELD TOLERANCE OF SELECTED VARIETIES TO AND FUNGICIDE EFFICACY AGAINST ALTERNARIA BLIGHT OF SWEET POTATO S.D. KANDOLO 1,5, A.H. THOMPSON 1, F.J. CALITZ 3, S.M. LAURIE 1, M. TRUTER 1,3, J.E. VAN DER WAALS 5 nd T.A.S. AVELING 5,6 1 Agriculturl Reserch Council, Vegetble nd Ornmentl Plnt Institute, Privte Bg X293, Pretori 0001, South Afric 3 Agriculturl Reserch Council, Biometry Division, 1134 Prk Street, Htfield, Pretori 0083, South Afric 4 Agriculturl Reserch Council, Plnt Protection Reserch Institute, Privte Bg X134, Pretori 0001, South Afric 5 Deprtment of Plnt nd Soil Sciences, University of Pretori, Pretori 0028, South Afric 6 Forestry nd Agriculturl Biotechnology Institute, University of Pretori, Pretori 0028, South Afric Corresponding uthor: terry.veling@fbi.up.c.z (Received 24 August, 2015; ccepted 12 August, 2016) ABSTRACT Alternri blight (AB) of sweet potto (Ipomoe btts L.), cused by Alternri spp., ws recently reported in South Afric, but is common in southern nd estern Afric. Elsewhere in the world, AB is controlled primrily using resistnt vrieties. Twenty-five sweet potto vrieties/breeding lines, from different origins were ssessed for tolernce to AB. The mterils were plnted in fields hving history of AB disese nd rted for tolernce bsed on Generl Disese Index (GDI), with the lowest scores representing tolernce, nd the higher scores representing susceptibility. Vriety hd the lowest GDI vlue, nd ws the most tolernt to AB; while W119 hd the highest GDI vlue nd ws the most susceptible to the disese. Other vrieties/breeding lines showed vrition in GDI vlues between most tolernt nd most susceptible. Among the fungicides tested under field conditions, the mixture zoxystrobin-difenoconzole ws the most effective in reducing AB intensity. Fungicides pyrclostrobin-bosclid, unizeb, zoxystrobin-chlorothlonil nd cymoxnil-mncozeb were lso effective ginst the disese. Key Words: Alternri btticol, Generl Disese Index, Ipomoe btts RÉSUMÉ L lternriose (AB) de l ptte douce (Ipomoe btts L.), mldie cusée pr Alternri spp., est une mldie nouvellement rencontrée en Afrique du Sud, mis très fréquemment rencontrée dns les pys u Sud et à l Est de l Afrique. Ailleurs dns le monde, l utilistion de vriété résistntes est l première mesure de lutte contre les dégâts cusés pr AB. Vingt cinq vriétés de ptte douce de diverses origines ont été évluées pour leur sensibilité à l lternriose. Le mtériel génétique vit été plnté dns des chmps ynt une fois infectés de AB et clssés tolérnts en fonction de leur index générl de l mldie (GDI). Les plus petits scores indiquent l tolernce, tndis que les scores les plus élevés indiquent l susceptibilité. L vriété vit l plus petite de GDI et donc étit le plus tolernt à AB, tndis que l vriété W119 présentit l vleur de GDI l plus élevée; pr conséquent étit l plus susceptible de toutes les vriétés. Les utres vriétés étient différentes en ce qui concerne leur GDI, les vleurs de GDI vriient du plus petit u plus grnd. Prmi les fongicides testés, le mélnge zoxystrobin-

2 332 S.D. KANDOLO et l. difenoconzole étit le plus efficce. Les fongicides pyrclostrobin-bosclid, unizeb, zoxystrobin-chlorothlonil et cymoxnil-mncozeb étient ussi efficce contre l lternriose. Mots Clés: Alternri btticol, index générl de l mldie, Ipomoe btts INTRODUCTION Sweet potto (Ipomoe btts L.) is the sixth most importnt food crop in the world, with 105 metric tonnes produced nnully. Of this, 95% is grown in developing countries (FAOSTAT, 2009). Ornge-fleshed sweet potto vrieties re prticulrly importnt for hving high betcrotene content, which helps to llevite vitmin A deficiency in children, especilly in rurl res of Keny (Lopes nd Boiteux, 1994; Mwng nd Ssemkul, 2001). Alternri blight (AB) is serious disese of sweet potto in Centrl, Estern nd Southern Afric (Vn Bruggen, 1984; Anginyh et l., 2001; Kping nd Crey, 2003; Osiru et l., 2007). In Ugnd, AB is rnked the most importnt constrint to sweet potto production (Anginyh et l., 2001). AB ws first reported from South Afric by Nrynin et l. (2010) nd hs loclised distribution (Thompson et l., 2011). Alternri btticol Ikt ex W. Ymmoto hs been observed to be the min cusl pthogen (Lopes nd Boiteux, 1994; Osiru et l., 2008) while A. lternt Fries (Anginyh et l., 2001) nd A. cpsici-nnui Svul. nd Sndu (Sivprksm et l.,1977) lso induce AB symptoms on sweet potto (Anginyh et l., 2001). Since the disese is fvoured by wet, humid conditions (Ames et l., 1997; Osiru et l., 2009), nd medin tempertures of 28 C (Osiru et l., 2007), it hs the potentil to develop in certin sweet potto production res of South Afric, nd should, therefore, be considered potentil thret to the sweet potto industry in the country. In most countries, control of AB of sweet potto is limited, with the disese being mnged primrily by plnting resistnt vrieties (Anginyh et l., 2001; Mwng et l., 2003; Osiru et l., 2009). In South Afric, preliminry glsshouse study showed vrition in Alternri blight levels mong sweet potto vrieties (Nrynin et l., 2010b). Use of pyrimethnil, cyprodinil, fludioxonil, zoxystrobin, difenoconzole nd potssium bicrbonte s control mesures hs been used with success in the USA (Olson et l., 2012). The objective of this study ws to evlute wide rnge of sweet potto vrieties/breeding lines from different origins under field conditions for tolernce to AB, nd screening for efficcy of selected fungicides ginst AB. MATERIALS AND METHODS Study loctions. The study ws conducted from , t two sites, nmely Roodeplt experimentl frm of the Agriculturl Reserch Council, Vegetble nd Ornmentl Plnt Institute (ARC-VOPI) (ltitude 25,60398; longitude 28,35429; ltitude 1168 m), Pretori, Guteng, South Afric, nd t the Owen Sithole College of Agriculture (OSCA), Empngeni (ltitude 28,72496; longitude 31,89813; ltitude 105 m), KwZulu-Ntl, South Afric. At ARC-VOPI only one vriety/breeding line ws ssessed ( seson) s very low disese intensity in the second growing seson ( ) mde the disese ssessment not possible. On the other hnd, the fungicide study ws conducted t Bronkhorstspruit, Guteng, South Afric nd ARC-VOPI from Vriety/breeding line field trils. Disese-free cuttings of 25 sweet potto vrieties/breeding lines, of different origin; were mnully plnted. A 9 m long ridge (plot) ws used, with betweenridge spcing of 1 m, nd in-ridge spcing of 30 cm. Ech plot ws replicted three times, in rndomised complete block design. The betweenplot ridges nd border ridges were plnted with mixture of vrieties A15 nd Excel, both reported to be susceptible to AB under greenhouse conditions (Nrynin et l., 2010b). The study t both ARC-VOPI nd OSCA ws conducted in fields hving history of AB disese. Plnts received overhed irrigtion for two hours in the evening ( hr), twice week, strting three months fter plnting to induce optiml conditions for development of AB.

3 Fields were fertilised with NPK of 18.5:0:18.5 t 500 kg h -1, pplied one dy before plnting; while limestone mmonium nitrte (LAN, 28%) ws pplied s top-dressing three weeks lter t 120 kg h -1. Weeds were removed mnully using hnd-hoes. Owing to sesonl differences of the loclities, experiments were conducted from December to April nd Februry to June t ARC- VOPI nd OSCA, respectively. Fungicide field evlution. A rndomised complete block design, with three replictes, ws used. Disese-free cuttings of vriety W 119, which is susceptible to AB (Nrynin et l., 2010b), were mnully plnted. Ech ridge (representing fungicide tretment plot) ws delimited by two border ridges. Detils concerning ll fungicides re shown in Tble 1. As mentioned bove, the ARC-VOPI field tril site hd history of AB; while the Bronkhorstspruit field tril site ws new field. Accordingly, plnts t Bronkhorstspruit were rtificilly inoculted in the evenings, by plcing AB disesed leves on top of the cnopy (three infected leves per spot) t three eqully distnt points (pprox m from ech other) per ridge, nd overhed irrigted in the evening(16:00-18:00 hr), twice week to promote nturl Alternri infection. Fertiliser ppliction nd field mintennce were similr to those used for the vriety field trils. Fungicides were pplied when AB symptoms first ppered, nd this ws repeted every 10 dys using knpsck spryer. The untreted Control of Alternri blight of sweet potto 333 control plnts received only sterile distilled wter. Fungicides were pplied four nd three times t the ARC-VOPI nd the Bronkhorstspruit trils, respectively. Plnts were not wtered for 24 hr, following ech fungicide ppliction to reduce wsh-off. Disese ssessment. Alternri infection ws too low during the first three months of the growing sesons, to ssess the re under the disese progress curve, which entils repeted disese ssessments (Jeger nd Viljnen-Rollinson, 2001). Thus, disese ssessment ws conducted once t the end of the growing seson when disese levels were highest. For ech plot, three eqully distnt (pprox. 2.3 m from ech other) smpling points were tken, with the two outer smpling points locted one metre from both ends. In the vriety study, both leves nd stems were ssessed; while in the fungicide tril plots, only leves were ssessed s AB symptoms developed poorly on the stems. An open, wooden frme (47 cm x 47 cm) ws used to ssess lef disese severity. The frme ws plced on top of the cnopy t ech smpling point. Fully developed leves inside the frme were rted using the Ordinl Disese Rting scle of 0-4 bsed on re of lef necrosis; where 0 = no disese; 1 = <25% covered by smll spots (~1mm dimeter, no yellowing, no bxil veinl necrosis; 2 = 25-50% covered by lrger spots (~2-3 mm dimeter), no yellowing, mild veinl necrosis; 3 = >50% covered, extensive smller TABLE 1. Fungicides used in field trils to test for efficcy ginst Alternri blight of sweet potto t two loctions in South Afric Active ingredients Dosge Type of fungicide Azoxystrobin-chlorothlonil 1.25 ml 100 ml -1 Systemic/preventive Azoxystrobin-difenoconzole 375 ml 100 ml -1 Systemic/contct Pyrclostrobin-bosclid 300 g 100 l -1 Systemic Cymoxnil-mncozeb 3 kg 100 l -1 Preventive Copper hydroxide 350 ml 100 ml -1 Preventive Copper hydro. Equiv 150 ml 100 l -1 Preventive Potssium phosphte 2 l 100 l -1 Systemic Mneb-zinc oxide 320 ml 100 ml -1 Preventive Mncozeb 200 g 100 l -1 Preventive Tp wter Copper hydro. Equiv = Copper hydroxide equivlent

4 334 spots; or few lrge spots with trget pttern, extensive veinl necrosis, prtil lef yellowing; 4 = complete yellowing, necrotic, leves dying nd flling off. For ech vriety, nine smpling points were tken, with ~225 individul leves ssessed nd totl of ~5625 individul leves ssessed for ll vrieties per field tril. At both fungicide study sites, twenty individul leves were ssessed per smpling point, with 1200 individul leves ssessed for the entire experiment. A wooden frme of 47 cm x 47 cm, subdivided into nine equl sub-squres with tut wire (0.7 mm dimeter), ws used to ssess stem disese severity. The frme ws plced t the sme smpling points used to ssess lef diseses. The sme ordinl rting scle described bove ws used to rte the percentge of infected stems. A single, ordinl rting ws given for ll the stems within ech sub-squre. For ech vriety, 81 subsqures were ssessed, with totl of ~2025 subsqures ssessed for ll vrieties per field tril. Sttisticl nlysis.the stem nd lef disese rtings were subjected to Generl Liner Model (GLM) technique, with logistic link function. The mximum likelihood estimtors (XBet s) were clculted s index vlues on n underlying scle (McCullgh nd Nelder, 1989). These estimtors (loction vlues) were on n intervl scle, from negtive to positive vlues, nd were dded together to produce the Generl Disese Index (GDI) mens, whose vlues expressed the level of susceptibility/tolernce of vrieties/ breeding lines to AB. Rnking of vrieties nd breeding lines ws bsed on GDI vlues, with the lowest being the most tolernt, nd the highest the most susceptible. The vrince of ech experiment ws tested for comprble mgnitude, using Levene s test (Levene, 1960), before the nlysis of combined dt ws performed. There ws strong evidence for homogeneity of vrinces between loclities (Tble 4); therefore, weighted nlysis of combined dt ws performed (John nd Quenouille, 1977). Fisher s t-lest Significnt Difference (LSD) ws clculted to compre mens of significnt effects t 5% significnce level (Snedecor nd Cochrn, 1967). Anlysis ws S.D. KANDOLO et l. performed using SAS/Stt Version 9.2 Sttisticl Softwre (SAS, 1999). RESULTS Field study. High AB disese levels were nturlly present t both field tril sites t the end of both growing sesons (Tble 3). GDI vlues for the different vrieties/breeding lines vried continuously from to Three groups emerged bsed on GDI vlues viz. tolernt, intermedite nd susceptible ws the most tolernt vriety/breeding line (GDI-7.229), followed by Muvhelo, nd Impilo. W119 ws the most susceptible vriety (GDI 4.555) (Tble 3). Vrieties/breeding lines with intermedite tolernce included Bophelo, , Purple Sunset, , Monte, Ndou, nd In ddition, there were significnt differences in disese levels between loclities nd between growing sesons (Tble 4). During the growing seson, the OSCA field tril recorded significntly higher disese severity thn ARC- VOPI field tril. In the growing seson, the infection level ws very low t ARC-VOPI field tril nd dt could not be nlysed sttisticlly. The disese severity ws significntly higher t OSCA field tril in 2011 thn in 2012 (Tble 4). Fungicide study. Tble 5 shows the performnce of selected fungicides ginst AB. The first symptoms of AB ppered 2 months fter plnting. Results from the three field trils showed tht zoxystrobin-difenoconzole ws the most effective fungicide tretment. Pyrclostrobinbosclid, mncozeb, zoxystrobin-chlorothlonil nd cymoxnil-mncozebshowed intermedite fungicide efficcy, while copper-hydroxidecopper equivlent ws the lest effective. DISCUSSION The Owen Sithole College of Agriculture tril site experienced men mximum reltive humidity of 95% nd the optimum temperture of 28 C in both sesons (Tble 2). It is likely tht these conditions were suitble to the spred of AB s

5 Control of Alternri blight of sweet potto 335 TABLE 2. Temperture, rinfll nd reltive humidity vlues t ARC-VOPI nd OSCA b from December to April nd Februry to June , respectively Prmeter ARC-VOPI OSCA b 2010/ / Men mximum temperture ( C) Men minimum temperture ( C) Totl rinfll (mm) Men mximum reltive humidity (%) Men minimum reltive humidity (%) ARC-VOPI - Agriculturl Reserch Council - Vegetble nd Ornmentl Plnt Institute, Pretori, Guteng, South Afric b OSCA -Owen Sithole College of Agriculture, Empngeni, KwZulu-Ntl, South Afric TABLE 3. Sweet potto vriety/breeding lines ssessed for Alternri blight disese severity ( ), expressed s Generl Disese Index mens using n intervl scle Vriety/ Origin Type Tuber colour GDI breeding line nd t-grouping W119 USA Vriety Ornge ARC-VOPI b Line Ornge ARC-VOPI Line Ornge Excel/A15 USA c /UKZN d Vrieties Ornge/Ornge Resisto USA Vriety Ornge Isondlo ARC-VOPI Vriety Ornge Hernndez USA Vriety Ornge ARC-VOPI Line Ornge Beuregrd USA Vriety Ornge ARC-VOPI Line Ornge b Blesbok ARC-VOPI Vriety Crem b ARC-VOPI Line Ornge 2.96 b ARC-VOPI Line Ornge b Bophelo ARC-VOPI Vriety Ornge cd ARC-VOPI Line Ornge cd Purple sunset ARC-VOPI Vriety Ornge cde ARC-VOPI Line Ornge cde Monte ARC-VOPI Vriety Crem -1.6 cde Ndou ARC-VOPI Vriety Crem cde Ugnd Line Ornge cde Ugnd Line Ornge cde Impilo ARC-VOPI Vriety Ornge -3.3 def Ugnd Line Ornge def Muvhelo ARC-VOPI Vriety Crem def CIP e Vriety Ornge def LSD (P = 0.05) 3.35 t-grouping = Vlues followed by the sme letter do not differ significntly ccording to Student s t-lest significnt difference test (P<0.05) GDI - Generl Disese Index; b ARC-VOPI - Agriculturl Reserch Council-Vegetble nd Ornmentl Plnt Institute, Pretori, South Afric; c USA - United Sttes of Americ; d UKZN - University of KwZulu-Ntl, Kwzulu-Ntl, South Afric; e CIP - Interntionl Potto Centre; LSD Lest Significnt Difference

6 336 this site recorded higher Alternri infection thn the ARC-VOPI tril site. Rotem et l. (1989) showed the role of high humidity in inducing Alternri infection. Similrly, Pleysier et l. (2006) reported tht humidity nd temperture significntly influenced the infection of A. lternt on Pulowni fortunei (Seem.) Hemsl. However, the disese did not develop well t ARC- VOPI in the second growing ( ). Severl fctors cn led to decrese in disese severity t given site including humidity, temperture, TABLE 4. Alternri blight of sweet potto disese severity expressed s Generl Disese Index (GDI) mens on n intervl scle t two loctions Loction GDI t-grouping OSCA b ARC-VOPI b 2010/ OSCA ARC-VOPI 2011/ LSD (P = 0.05) t-grouping = Vlues followed by the sme letter do not differ significntly ccording to Student s t-lest significnt test (P <0.05) OSCA = Owen Sithole College of Agriculture, Empngeni, Kwzulu-Ntl, South Afric b ARC - VOPI = Agriculturl Reserch Council, Vegetble nd Ornmentl Plnt Institute, Pretori, South Afric S.D. KANDOLO et l. rinfll, sowing dte or pthogen virulence (Kumr nd Srivstv, 2013). The results of this study showed the rection of sweet potto vrieties nd breeding lines during two growing sesons in different loclities. The GDI rnking of vrieties ginst AB showed wide rnge of tolernce. The vriety ws the most tolernt plnt mteril ginst the disese origintes from Peru nd hs been lso reported tolernt to AB in vrious sweet potto growing countries (Sunette Lurie, personl communiction). Vriety rnkings suggested tht Muvhelo, Monte, Ndou nd Impilo were tolernt to AB under field conditions. A similr study conducted by Nrynin et l. (2010b) reported these three vrieties s being tolernt to AB under glsshouse conditions. On the contrry, Beuregrd, Hernndez, Isondlo, Resisto nd Excel showed susceptibility to AB while W119 ws the most susceptble vriety. These findings re in greement with Nrynin et l. (2010b) who observed tht W119, Excel, Beuregrd nd Resisto were susceptible to AB under glsshouse conditions. W119 nd Resisto re originlly from the USA (Kping et l., 2010) nd re currently cultivted in mny Africn countries including Mozmbique, Mdgscr nd South Afric (Kping et l., 2010). The breeding lines , nd were tolernt to AB. Little hs been TABLE 5. Field rection of nine fungicides ginst Alternri blight of sweet potto Fungicides ARC-VOPI Bronkhorstspruit ARC-VOPI Azoxystrobin-difenoconzole d 0.84 Pyrclostrobin-bosclid Mncozeb Azoxystrobin-chlorothlonil Cymoxnil-mncozeb Mneb-zinc oxide Potssium phosphte Copper hydroxide Copper hydroxide-copper equivlent Control LSD ( 0.05) ARC - VOPI = Agriculturl Reserch Council, Vegetble nd Ornmentl Plnt Institute, Pretori, South Afric

7 published on the rection of these breeding lines ginst AB. Moreover, these tolernt mterils represent relible sources of resistnce, which cn be used in future breeding progrms to develop tolernt vrieties ginst AB. Other breeding lines such s , nd exhibited susceptibility to AB with being the most susceptible. Results from the breeding lines showed tht AB disese ws not tuber colour-dependent s previously suggested by Vn Bruggen (1984) nd Du Plooy et l. (2008) who reported tht sweet potto with red tubers were more susceptible to Alternri infection thn crem coloured vrieties. In this study ll tolernt nd susceptible vrieties/breeding line hd the ornge tuber colour. In the fungicide study the mixture zoxystrobin-difenoconzole ws the most effective in controlling AB. Both zoxystrobin nd difenoconzole hve been reported to ct s protectnt nd curtive fungicides, respectively ginst Alternri solni (Ellis & Mrtin) Soruer (Horsfield et l., 2010). The results of this experiment grees with reserch conducted by Reuveni et l. (2002) on A. solni nd A. lternt on red deciduous pple which showed tht zoxystrobin nd difenoconzole were both effective in controlling these two pthogens. A sprying progrm tht incorportes both protectnt nd curtive ctive ingredients hs proved to be effective in reducing plnt diseses. The results from this work showed tht t both ARC-VOPI nd Bronkhorstspruit, plnts spryed with mixture of pyrclostrobin-bosclid recorded low disese. This finding ws echoed erlier by Psche nd Gudmestd (2008) nd Gudmestd et l. (2013) in A. solni. However, the continuous use of pyrchlostrobin nd bosclid cn led to the rpid development of resistnce strins of Alternri s these compounds hve sitespecific mode of ction (Avenot nd Michilides, 2007; Avenot et l., 2008). In Austrli, frmers re discourged from pplying strobilurin fungicides, to which pyrchlostrobin belongs, consecutively nd their use should never exceed four pplictions per nnum (Infopest, 2003). Dt from this study reveled tht the mixture cymoxnil-mncozeb ws lso effective ginst AB becuse plnts treted with these compounds recorded low disese intensity. Similrly, Ctão Control of Alternri blight of sweet potto 337 et l. (2013) lso reported this mixture to be effective ginst A. tomtopil Simmons. Mncozeb is often used in combintion with other ctive ingredients s it hs mny sites of ction, therefore delying the occurrence of pthogen resistnt strins (Gullino et l., 2010). Copper hydroxide nd copper hydroxide-copper equivlent were the lest effective compounds in controlling AB mong ll the fungicides tested during ll the growing sesons nd t ll sites. Similr results were echoed by Rid nd Timmer (1999) on A. lternt. To our knowledge currently there is no registered fungicide for Alternri diseses of sweet potto in South (or southern) Afric. These results provide informtion for chemicl compnies indicting the most promising ctive ingredients tht control AB under field conditions. ACKNOWLEDGEMENT The uthors would like to thnk the South Africn Deprtment of Science nd Technology (DST) for funding this project. We re grteful for the technicl support provided by Ms. Mmpsek Mlebn, Mr. Themb Ngidi nd Mr. Andre vn den Berg. This work is prt of PhD reserch study by the first uthor. REFERENCES Ames, T., Smit, N.E.J.M., Brun, A.R., O Sullivn, J.N. nd Skoglund, L.G Sweet potto: Mjor pests, diseses, nd nutritionl disorders. Interntionl Potto Center (CIP), Lim, Peru. 81pp. Anginyh, T.J., Nrl, R.D., Crey, E.E. nd Njeru, R Etiology, effect of soil ph nd sweet potto vrietl rection to Alternri lef petiole nd stem blight in Keny. Africn Crop Science Journl 9: Avenot, H.F. nd Michilides, T.J Resistnce to bosclid fungicides in Alternri isoltes from pistchio in Cliforni. Plnt Disese 91: Avenot, H.F., Morgn, D.P. nd Michilides, T.J Resistnce to pyrchlostrobin, bosclid nd multiple resistnces to Pristine (pyrchlostrobin + bosclid) fungicide in Alternri lternt cusing Alternri blight

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