RHIZOCTONIA ON SUGARBEET FOLLOWING ROTATION CROPS. Carol E. Windels and Jason R. Brantner

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1 RHIZOCTONIA ON SUGARBEET FOLLOWING ROTATION CROPS Crol E. Windels nd Json R. Brntner Professor nd Reserh Fellow, respetively University of Minnesot, Northwest Reserh nd Outreh Center, Crookston Rhizotoni solni (= R. solni) is ommon, soilorne fungl pthogen of rops grown throughout the world. The fungus is omposed of different popultions lled nstomosis groups or AGs, whih ttk ertin rops nd plnt prts (6,9). Rhizotoni root nd rown rot (RRCR) of sugreet is used y R. solni AG 2-2, whih is further divided into two supopultions (lled intrspeifi groups or ISGs) nmed AG 2-2 IIIB nd AG 2-2 IV. The ISGs re differentited y their growth t 95 F; the AG 2-2 IIIB popultion grows t 95 F, while AG 2-2 IV does not. The ISGs lso differ in tht R. solni AG 2-2 IIIB is more ggressive in ttking sugreet thn AG 2-2 IV (7,12). In the Red River Vlley (RRV) nd southern Minnesot, RRCR on sugreet hs een inresing in prevlene nd severity in the lst dede. Infetions our from 54 to 95 F (optiml tempertures re from 68 to 86 F) nd seldom our elow 59 F (6). Build-up of inoulum in the region is ttriuted to inresed prodution of soyen nd edile en rops, whih re suseptile to stem nd root rot used y R. solni AG 2-2 IIIB (2,7). Cerel rops (e.g., whet, rley, orn) re onsidered non-hosts for R. solni AG 2-2 nd thus, re reommended for rottion with rodlef rops (sugreet, soyen, sunflower) in the upper Midwest. Rottion with erel rops dereses popultions of R. solni. Reports from Europe, however, indite R. solni AG 2-2 IIIB uses root nd stlk rot of orn nd lso is the primry use of Rhizotoni root nd rown rot of sugreet (3). In the southestern U.S.A., R. solni AG 2-2 IIIB uses root nd re rot on orn (1,11) ut this disese hs not een reported in the upper Midwest. Although R. solni AG 2-2 IV is the primry use of root nd rown rot on sugreet (6), the reent report from Europe tht AG 2-2 IIIB is ttking sugreet rises onerns out whether inresed prodution of orn nd en rops in Minnesot nd North Dkot is inresing R. solni AG 2-2 IIIB. Presene nd distriution of the two ISGs in the sugreet growing regions of the RRV nd southern Minnesot re unknown. Our lortory hs identified oth ISGs of R. solni (AG 2-2 IIIB nd AG 2-2 IV) from sugreet in the RRV nd southern Minnesot. Preliminry pthogeniity tests onfirm R. solni AG 2-2 IIIB s very ggressive in ttking soyen, edile en, nd sugreet ompred to R. solni AG 2-2 IV, whih uses moderte root rot on these rops (12). We lso hve found tht R. solni AG 2-2 IIIB uses lesions on orn roots with no ove ground symptoms or dereses in yield. With inresing rege plnted to soyen nd orn in the RRV (nd oth of these rops ommonly grown in southern Minnesot), it is essentil to understnd the influene of rops rotted with sugreet on uild-up of the pthogen, so rops n e rotted to minimize RRCR. OBJECTIVES Our ojetives were to ondut field trils to determine pthogeniity of R. solni AG-2-2 IIIB nd AG 2-2 IV on 1.) rottion rops (orn, whet, soyen) grown in 26 nd 2.) the following, 27 sugreet rop. This report summrizes repet of the tril onduted in 25 nd 26 (12). MATERIALS AND METHODS Rottion rops. Field trils were estlished in 25 nd 26 t the University of Minnesot, Northwest Reserh nd Outreh Center, Crookston in split-plot tril of four replites. Eh yer, min plots were inoulted in mid My with R. solni AG 2-2 IIIB, R. solni AG 2-2 IV, nd not inoulted. Inoulum of R. solni ws grown on sterile rley grin for 3 weeks, ir-dried, sprinkled over eh min plot (11.3 ounes per 33 x 3 ft), nd inorported with Melroe multiweeder to out 2-inh depth. Rottion rops then were sown s suplots of min plots nd inluded spring whet Knudson, soyen GoldCountry 923RR, nd orn Pioneer 39D81 (9 l, 6 l, nd 3, seed per re, respetively). Eh rop ws fertilized, treted with pestiides, nd mintined following

2 reommended prties. Results for root rot ssessment, isoltion of R. solni from roots, nd yields hve een pulished (12). Sugreet rops. Two sugreet vrieties (Bet 131R nd Bet 135R, tolernt nd suseptile to RRCR, respetively) were sown on My 19, 26 nd April 27, 27 s su-suplots within the previous yers rottion rop. Sugreet seed of eh vriety ws sown t 2.6 inhes in rows 3 feet long nd 22-inhes prt (3-row plots per vriety). Plots were fertilized t reommended rtes nd the insetiide Counter (1. l.i. per re) ws pplied over the row t plnting. Roundup (1 qurt per re) ws pplied pre-emergent nd Selet + MSO (12 oune + 1 pint per re, respetively) out 2 weeks fter plnting. Mirortes of the heriides Betmix + UpBeet + Stinger + Selet + MSO (.5-.7 pt oz + 35 ml ml pt per re, respetively) t four intervls, five to six dys prt. Heriides were pplied with trtor-mounted spryer nd TeeJet 83 flt fn nozzles t 3 psi. Stnds were thinned, where needed, to the equivlent of 2 plnts per 1 feet of row out 5 weeks fter plnting. Plots were ultivted in lte June or erly July. Cerospor lef spot ws ontrolled y pplitions of SuperTin, Eminent, or Hedline (5, 13 nd 9 oz per re, respetively), s needed. Stnds were ounted t regulr intervls fter emergene until plots were thinned. Four middle rows of eh plot were hrvested on Septemer 27, 26 nd Septemer 28, 27. Mrketle roots were ounted nd 2 were rndomly seleted from eh su-suplot nd rted for RRCR with to 7 sle, where = helthy nd 7 = root ompletely rotted nd folige ded. Ten roots lso were rndomly seleted nd nlyzed for yield nd qulity y Amerin Crystl Sugr Compny Qulity Lortory, Est Grnd Forks, MN. Wether dt. Dily mximum ir tempertures, soil tempertures t 4-inhes, nd preipittion were olleted from plnting to hrvest of sugreet trils in 26 nd 27. Dt were reorded y n utomti wether sttion t the University of Minnesot, Northwest Reserh nd Outreh Center, Crookston West Frm. Dt nlysis. Dt were sujeted to nlysis of vrine nd if signifint (P =.5), mens were seprted y Lest Signifint Differene (LSD). RESULTS Wether dt. In 26, mximum ir tempertures (Fig. 1A) typilly were in rnge fvorle for R. solni AG 2-2 to infet sugreet plnts throughout the seson; even soil tempertures t the 4-inh depth were ove 59 F within few dys fter plnting (Fig. 1B). During the 26 tril, preipittion totled 8.25 inhes, with.88 inhes within 1 month fter plnting (dt not shown). In 27, mximum ir tempertures within the first month fter plnting (Fig. 1A) flututed widely from fvorle to unfvorle for R. solni AG 2-2 to infet sugreet; during this period, soil tempertures t the 4-inh depth were elow 59 F (Fig. 1B). Preipittion during the 27 tril totled 16.9 inhes, with 2.66 inhes flling within 1 month fter plnting (dt not shown). Sugreet stnd estlishment. Rhizotoni dmping-off ws more tive in 26 thn in 27 euse of more fvorle tempertures within the first month fter plnting in 26. The sme trends, however, ourred for sugreet seedling stnds in 27 s reported for the 26 tril (see 26 Sugreet Reserh nd Extension Reports, 37: ). Thus, results re reported in this setion for the 27 sugreet rop. In 27, Rhizotoni dmping-off of sugreet seedlings strted out 3 weeks fter plnting in plots inoulted with R. solni AG 2-2 on My, 26 ompred to the non-inoulted ontrol, whih mintined exellent stnds (Fig. 2). Overll, sugreet stnds (verged ross sugreet vriety nd previous rottion rop) were signifintly higher in the non-inoulted ontrol thn in oth Rhizotoni-inoulted plots (Fig. 2A). Sugreet stnds were signifintly lower in plots inoulted with R. solni AG 2-2 IIIB ompred to R. solni AG 2-2 IV. Rhizotoni dmping-off ourred in oth sugreet vrieties (vlues verged ross soil inoultions nd previous rottion rops) (Fig. 2B). Stnds were signifintly higher for the Rhizotoni-tolernt vriety (Bet 131R) thn for the suseptile vriety (Bet 135R) for stnd ounts through 32 dys fter plnting, when plots were thinned.

3 Mx ir temperture ( F) A Plnted April 27, 27 Plnted My 18, /27 5/11 5/25 6/8 6/22 7/6 7/2 8/3 8/17 8/31 9/14 9/28 Four-inh soil temperture ( F) B Plnted April 27, 27 Plnted My 18, /27 5/11 5/25 6/8 6/22 7/6 7/2 8/3 8/17 8/31 9/14 9/28 Dte Fig. 1. Tempertures reorded t the University of Minnesot, Northwest Reserh nd Outreh Center, Crookston during the 26 nd 27 sugreet experiments for A.) mximum ir temperture nd B) soil temperture t 4-inh depth; infetions seldom our elow 59 F (noted y dshed line). ================================ There were no losses in sugreet stnds in the non-inoulted ontrol plots previously sown to whet, soyen, nd orn (Fig. 3). Rhizotoni dmping-off of sugreet ws most severe in plots inoulted with R. solni AG 2-2 IIIB the previous yer nd losses were signifintly greter nd equl when following orn nd soyen ompred to whet. There were intermedite losses of sugreet stnds in plots inoulted with R. solni AG 2-2 IV the previous yer nd were unffeted y previous rop. Sugreet hrvest dt. In 26, RRCR ws more severe thn in 27, whih ffeted sugreet yields nd qulity, so hrvest dt re presented for oth yers in Tle 1. For most vriles mesured in eh yer, there were no signifint intertions etween Rhizotoni-inoulted nd non-inoulted soil tretments, sugreet vriety, or previous rop, so dt re presented seprtely for eh of these min tretments.

4 No. sugreet plnts/6 ft row No. sugreet plnts/6 ft row A Sugreet sown April 27, B Sugreet sown April 27, 27 Control AG 2-2 IV AG 2-2 IIIB Dys fter plnting Bet 131R Bet 135R Fig. 2. Sugreet seedling stnds in plots sown April 27, 27 tht were previously soil-inoulted on My 18, 26 with Rhizotoni solni AG 2-2 IIIB, R. solni AG 2-2 IV (11.3 ounes per 99 ft 2 ), or not inoulted (ontrol); these min plots were rrnged in rndomized lok design (replited four times) nd then sown with whet, soyen, nd orn s suplots; A.) stnds in Rhizotoni-inoulted nd ontrol plots verged ross two sugreet vrieties (Bet 131R nd 135R, tolernt nd suseptile to R. solni, respetively) nd previous rottion rop (eh vlue is n verge of 24 plots); B.) stnds of eh sugreet vriety verged ross Rhizotoni-inoulted nd ontrol plots nd rottion rops (eh vlue is n verge of 36 plots). For eh figure nd dte, vlues followed y the sme letter re not signifintly different (P =.5). ===================================== In 26 nd 27, RRCR rtings were low nd signifintly less in the non-inoulted ontrol thn in plots where soil hd een inoulted the previous yer with R. solni AG 2-2 IV nd AG 2-2 IIIB (vlues were verged ross sugreet vriety nd previous rop). Disese rtings of 1.4 nd 1.6 in non-inoulted plots in 26 nd 27, respetively, indited roots were nerly symptom-free nd helthy. For oth yers, roots hrvested from plots inoulted with R. solni AG 2-2 IIIB the previous yer hd signifintly more disese thn in plots inoulted with R. solni AG 2-2 IV. Overll sugreet yields nd qulity were higher in 27 thn in 26 euse of less RRCR nd longer growing seson in 27 (plnted 3 weeks erlier thn in 26). In oth yers, numers of hrvestle roots nd pounds of surose per ton followed the sme trends with signifint differenes mong the soil tretments; vlues were highest in the non-inoulted ontrol, lowest in the R. solni AG 2-2 IIIB-inoulted plots; nd intermedite in the R. solni AG IV-inoulted plots. In 26 there lso were signifint differenes mong soil tretments for tons of root per re, perent surose, pounds of reoverle surose per re, nd gross return per

5 No. sugreet plnts/6 ft row de d e Sugreet sown April 27, Dys fter plnting 26 Rottion rop Soyen Whet Soyen Whet Whet Soyen 26 Soil inoulum Control AG 2-2 IV AG 2-2 IIIB Fig. 3. Sugreet stnds in 27 verged ross two vrieties Bet 131R nd 135R (tolernt nd suseptile to Rhizotoni solni, respetively) sown in plots previously soil-inoulted on My 18, 26 with R. solni AG 2-2 IIIB, R. solni AG 2-2 IV (11.3 ounes per 99 ft 2 ), nd non-inoulted ontrol nd then sown with whet, soyen, nd orn s suplots (rrnged in split plot design nd replited four times). Eh vlue is sed on n verge of eight plots. For eh dte, vlues followed y the sme letter re not signifintly different, (P =.5). ============================== re; est results were in the non-inoulted ontrol, lowest in plots previously inoulted with R. solni 2-2 IIIB, nd were intermedite for R. solni AG 2-2 IV. In 27, results were somewht higher ut sttistilly the sme in the non-inoulted ontrol nd plots previously inoulted with R. solni AG 2-2 IV for tons of roots per re, perent surose, pounds of reoverle surose per re, nd gross return per re nd oth tretments were signifintly higher ompred to plots inoulted with R. solni AG 2-2 IIIB. In 26, RRCR rtings were the sme for oth the tolernt nd suseptile sugreet vrieties (vlues were verged ross soil inoulum nd previous rops) euse disese ws so severe (Tle 1). In 27, disese ws less severe thn in 26 nd verge RRCR rtings were signifintly lower in the tolernt thn in the suseptile vriety. Yield nd qulity differed etween the two vrieties, espeilly in 27. Overll, the tolernt vriety (Bet 131R) tended to hve lower perent surose nd pounds of surose per ton nd higher tons of roots nd pounds of reoverle surose per re in oth yers. Gross return of dollrs per re tended to e highest for the tolernt vriety thn the suseptile vriety, ut differenes were not sttistilly different in 26 or 27. The effet of growing sugreet fter whet, soyen, nd orn ws not signifintly different in 26 nd 27 for RRCR rtings, numers of roots t hrvest, perent surose, nd pounds of surose per ton (Tle 1). In 26, root yields nd pounds of reoverle surose per re were signifintly higher following whet ompred to soyen nd orn, whih where sttistilly the sme; in 27 these vrile were not signifintly ffeted y previous rop. In 26, gross return per re ws signifintly higher following whet ompred to orn nd intermedite following soyen. In 27, gross return per re ws sttistilly the sme for sugreet following the three rottion rops, ut ws highest following soyen nd lowest following orn.

6 Tle 1. Rtings for Rhizotoni, yield, nd qulity of two sugreet vrieties grown in 26 nd 27 in plots tht in the previous yer (25 nd 26, respetively) were inoulted with R. solni AG 2-2 IV, R. solni AG IIIB, or not inoulted nd then sown with full-seson rop of spring whet, soyen, nd orn. Rhizotoni Z root & rown rot (-7 sle) No. roots/6 ft hrvest Yield (T/A) % Surose Surose (l/t) Reov. su (l/a) Gross return ($/A) Min tretment U Inoulum V Non-inoulted R. solni AG 2-2 IV R. solni AG 2-2 III B LSD (P =.5) Y Sugreet vriety W 131R (resistnt) R (suseptile) LSD (P =.5) Y NS.3 NS 5 NS NS 428 NS NS Previous rop X Whet Soyen LSD (P =.5) Y NS NS NS NS 2 NS NS NS NS NS 666 NS 87 NS U V W X Y Z For the 26 sugreet rop, soil hd een inoulted with R. solni AG 2-2 IV, R. solni AG 2-2 IIIB (11.3 oz per 99 ft 2 ) or not inoulted on My 17, 25) nd then sown with whet, soyen, nd orn. For the 27 sugreet rop, soil hd een inoulted nd non-inoulted s ove, on My 18, 26 nd then sown with the three sme rottion rops. Eh vlue is verged ross previous rop nd sugreet vriety. Eh vlue is verged ross soil inoultion nd previous rop; 131R is resistnt to R. solni AG 2-2 while 135R is suseptile. Eh vlue verged ross soil inoulum nd oth sugreet vrieties. For eh olumn, vlues followed y different letter re signifintly different (P =.5); NS = not signifintly different. Rhizotoni root nd rown rot rtings sed on to 7 sle, where = root nd folige helthy nd 7 = root ompletely rotted nd folige ded.

7 Root & rown rot (-7 sle) A Whet Soyen Non-inoulted AG 2-2 IV AG 2-2 IIIB d d d Yield (Ton/A) B d Whet Soyen Non-inoulted AG 2-2 IV AG 2-2 IIIB d d d Reoverle surose (l/a) C Whet Soyen d d d Non-inoulted AG 2-2 IV AG 2-2 IIIB Gross return ($/A) D d Whet Soyen Non-inoulted AG 2-2 IV AG 2-2 IIIB e de de Rhizotoni solni Rhizotoni solni Fig. 4. Sugreet hrvest dt in 27 presented y min tretment (soil inoulted on My 18, 26 with Rhizotoni solni AG 2-2 IIIB, R. solni AG 2-2 IV [11.3 ounes per 99 ft 2 ], or not inoulted) nd y previous rop (sown on My 18, 26 with whet, soyen, orn) for: A.) Rhizotoni root nd rown rot rtings (-7 sle, = helthy, 7 = root ompletely rotted nd folige ded), B.) yield (tons of roots per re), C.) pounds of reoverle surose per re, nd D.) gross eonomi return per re. Eh r is sed on n verge of eight replites (verged ross two sugreet vrieties). For eh grph, rs with the sme letter re not signifintly different, P =.5). =================================== Effets of the 26 rottion rop grown in Rhizotoni-inoulted nd non-inoulted plots on the susequent, 27 sugreet rop re illustrted in Figure 4. Very little RRCR ourred in the non-inoulted ontrol nd rtings were signifintly lowest when sugreet followed soyen ompred to orn, nd intermedite fter whet (Fig. 4A). Disese lso ws low on sugreet roots in plots inoulted with R. solni AG 2-2 IV nd plnted to whet nd soyen, ut RRCR ws signifintly higher following orn. Rtings for RRCR were eqully severe in plots previously inoulted with R. solni AG IIIB, regrdless of rottion rop. Yields (tons per re) of sugreet were similr in the non-inoulted ontrol nd plots previously inoulted with R. solni AG 2-2 IV nd sown with whet, soyen nd orn ut were lower in plots previously inoulted with R. solni AG 2-2 IIIB, regrdless of rottion rop (Fig. 4B). Pounds of reoverle surose per re were similr nd signifintly higher for the non-inoulted ontrol nd plots previously inoulted with R. solni AG 2-2 IV ompred to plots inoulted with R. solni AG 2-2 IIIB; rottion rop did not signifintly ffet sugreet qulity (Fig. 4C). Gross return of dollrs per re ws highest in the non-inoulted ontrol nd plots inoulted with R. solni AG 2-2 IV, espeilly when sown to whet or soyen (Fig. 4D). Eonomi enefits were lowest when sugreet ws sown in plots inoulted the previous yer with R. solni AG 2-2 IIIB, regrdless of rottion rop (Fig. 4D).

8 DISCUSSION This field reserh illustrtes tht R. solni AG 2-2 IIIB is more ggressive in using dmping-off nd RRCR on sugreet thn R. solni AG 2-2 IV. Pnell (8) lso reported R. solni AG 2-2 IIIB s more ggressive in using RRCR on sugreet ompred to R. solni AG 2-2 IV in greenhouse trils. The role of whet, soyen, nd orn in ltering popultions of R. solni AG 2-2 IIIB nd AG 2-2 IV on susequent sugreet rop ws somewht inonsistent in 26 nd 27. When these rottion rops were grown in Rhizotoni-inoulted nd non-inoulted plots in 25 nd 26 trils, yields of eh rop were unffeted nd there were no ove-ground symptoms of Rhizotoni diseses (12). In oth yers, there ws n inrese in root rot lesions only on orn grown in plots inoulted with R. solni AG 2-2 IIIB. This ws the first report of R. solni AG 2-2 IIIB using lesions on orn in the Midwest, lthough the disese hs previously een reported in the southestern United Sttes (1,11) nd in Europe (3). Furthermore, isoltion of R. solni AG 2-2 IIIB nd 2-2 IV from whet roots verged 5 nd 2%, respetively; from soyen verged 16 nd 12%, respetively; nd from orn verged 33 nd 5%, respetively (12). These results indite whet is non-host for R. solni AG 2-2; soyen inreses popultion levels of R. solni AG 2-2 IIIB nd 2-2 IV (even when roots were symptomless); nd orn inreses popultions of R. solni AG 2-2 IIIB. Effets of rottion rops likely re mgnified over time, or s these rops re ontinully grown in rottion with sugreet. In 26, sugreet grown fter whet in plots with R. solni AG 2-2 IV the previous seson hd signifintly less disese nd improved yield nd qulity ompred to previous rop of soyen or orn, whih were similr. In 27, there ws less disese pressure nd results were not s lerut. In 26, sugreet sown in plots inoulted in 25 with R. solni AG 2-2 IIIB suffered severe disese nd losses in yield nd qulity, regrdless of rottion rop. Yield nd qulity were signifintly higher (ut not eonomilly eptle) following whet ompred to orn, nd intermedite following soyen. In 27, disese ws less severe on sugreet grown in plots previously inoulted with R. solni AG 2-2 IIIB nd previous rop hd no ffet. These outomes suggests when popultions of R. solni AG 2-2 IIIB re high, whet ( rop likely to e non-host) needs to e grown for more thn one seson to signifintly derese popultions of the pthogen to sfe levels. Tht is, the influene of rottion rops in deresing or inresing popultions of R. solni AG 2-2 IIIB (nd less so for R. solni AG 2-2 IV) my tke severl growing sesons. Our Rhizotoni Nursery is mnged so tht inoulted plots re tken out of sugreet prodution t the end of n experiment nd sown to spring whet for three growing sesons they then re sfe to plnt to sugreet nd hve minimum of RRCR. Experiments urrently re eing designed to test the long-term effet of rop rottions when popultion levels of oth ISGs of R. solni AG 2-2 re t high levels. Severity of Rhizotoni diseses on sugreet in our trils ws onsiderly higher in 26 thn in 27. A mjor differene etween the two yers ws plnting dte My 18, 26 nd April 27, 27. The erly plnting dte in 27 ment tht ir nd soil tempertures were too low for R. solni AG 2-2 to use muh dmge (even in soil with high popultion levels of the pthogen) ompred to more fvorle ir nd soil tempertures enountered when plnting mid My of 26 (Fig. 1). Josen et l. (4) nd Khn et l. (5) reported tht ontrol of R. solni AG 2-2 is most effetive when zoxystroin (Qudris) is pplied when soil tempertures re 6 to 65 F t 4-inhes, presumly to prevent infetion. In 26, these onditions ourred shortly fter plnting. Erly plnting of sugreet in 27 likely delyed infetions - nd on older roots, infetions re less severe (1). Erly plnting dte is ftor in reduing/mnging RRCR nd my explin why the disese is not mjor pthogen in the Southern Minnesot Beet Sugr Coopertive where sugreet hs een grown in rottion with orn nd soyen sine the oopertive ws formed in the mid 197s. To dte, over 7 ultures of R. solni AG 2-2 hve een olleted from sugreet with RRCR throughout the RRV nd southern Minnesot. Field histories lso re eing olleted for eh smple (typilly whet, soyen, or orn). Identifitions re in progress nd sed on temperture differentil ssy where AG 2-2 IIIB grows t 95 F ut AG 2-2 IV does not (nd supplemented y moleulr nlysis). Crop rottion is n effetive prtie for mnging plnt diseses nd improving rop prodution. Benefits of rop rottions re omplited nd vry mong regions (nd fields) euse of mny ftors inluding whether R. solni AG 2-2 IIIB or R. solni AG 2-2 IV re present, popultion densities, plnting dte, nd vriety. Surveys will ontinue to identify the distriution nd prevlene of R. solni AG 2-2 IIIB nd AG 2-2 IV in the RRV nd southern Minnesot. This informtion is ritil in dopting rop rottion prties tht void or dely uild-up of inoulum nd to mnge disese in fields where R. solni is estlished.

9 CONCLUSIONS 1. Rhizotoni diseses of sugreet re signifintly more severe when soil is infested with R. solni AG 2-2 IIIB thn with R. solni AG 2-2 IV. 2. Rottion rops ffet popultion levels of R. solni AG 2-2 IIIB nd AG 2-2 IV ut my tke more thn one seson to signifintly inrese or derese the pthogen. 3. roots re infeted y R. solni AG 2-2 IIIB nd my uild-up popultion levels in soil. 4. When popultion levels of R. solni AG 2-2 IIIB re high, one yer of nonhost rop (whet) is indequte to redue pthogen popultion nd dmge to susequent sugreet rop. ACKNOWLEDGEMENTS We thnk the Sugreet Reserh nd Edution Bord of Minnesot nd North Dkot for prtil funding of this reserh; Betseed for providing sugreet seed; Jeff Nielsen nd Todd Cymluk, University of Minnesot, Northwest Reserh nd Outreh Center, Crookston for plnting, mintining, nd hrvesting plots; Mry Johnshoy for isolting nd mintining ultures of R. solni; Jeff Nielsen for performing sttistil nlysis of dt; nd Amerin Crystl Sugr Compny Qulity Lortory, Est Grnd Forks, MN for onduting sugreet yield nd qulity nlyses. LITERATURE CITED 1. Engelkes, C.A., nd C.E. Windels Reltionship of plnt ge, ultivr, nd isolte of Rhizotoni solni AG 2-2 to sugr eet root nd rown rot. Plnt Dis. 78: Engelkes, C.A., nd C.E. Windels, Suseptiility of sugr eet nd ens to Rhizotoni solni AG-2-2 IIIB nd AG-2-2 IV. Plnt Dis. 8: Ithurrrt, M.E., G. Büttner, nd J. Petersen. 24. Rhizotoni root rot in sugr eet (Bet vulgris ssp. ltissim) Epidemiologil spets in reltion to mize (Ze mys) s host plnt. J. Plnt Disese Protetion 111: Josen, B.J., N.K. Zidk, M. Johnston, A.T. Dyer, K. Kephrt, nd J. Ashley. 26. Studies on optiml timing of zoxystroin pplition for Rhizotoni rown nd root rot ontrol. 25 Sugreet Res. Ext. Rept. 36: Khn, M.F.R., R. Nelsen, C.A. Brdley, nd J. Khn. 26. Developing mngement strtegy for ontrolling Rhizotoni root nd rown rot in sugreet. 25 Sugreet Res. Ext. Rept. 36: Leh, L.D Seedling diseses. Pges 4-8Compendium of Beet Diseses nd Insets. E.D. Whitney nd J.E. Duffus, eds. APS Press, The Amerin Phytopthologil Soiety, St. Pul, MN. 76pp. 7. Ogoshi, A Eology nd pthogeniity of nstomosis nd intrspeifi groups of Rhizotoni solni Kuhn. Annu. Rev. Phytopthol. 25: Pnell, L. 25. Pthogeniity of different Anstomosis Groups nd sugroups of Rhizotoni solni on sugreet. (Astr.) J. Sugr Beet Res. 42: Sneh, B., L. Burpee, nd A. Ogoshi Identifition of Rhizotoni speies. Amerin Phytopthologil Soiety, APS Press, St. Pul, MN. 133 pp. 1. Sumner, D.R Rhizotoni rown nd re root rot. Pges in: Compendium of Diseses, 3 rd edition. D.G. White, ed. Amerin Phytopthologil Soiety, APS Press, St. Pul. 11. Sumner, D.R., nd D.K. Bell Root diseses indued in orn y Rhizotoni solni nd Rhizotoni zee. Phytopthology 72: Windels, C.E., nd J.R. Brntner. 27. Rhizotoni inoulum nd rottion effets on following sugreet rop. 26 Sugreet Res. Ext. Rept. 37: