Reserch Effects of Rice Strw Mngement on Sclerotium oryze Inoculum, Stem Rot Severity, nd Yield of Rice in Cliforni N. A. Cints nd R. K. Webster, Deprtment of Plnt Pthology, University of Cliforni, Dvis 95616 ABSTRACT Cints, N. A., nd Webster, R. K. 2001. Effects of rice strw mngement on Sclerotium oryze inoculum, stem rot severity, nd yield of rice in Cliforni. Plnt Dis. 85:1140-1144. Under continuous rice cropping, open field burning hs been the primry mens of rice residue disposl nd of minimizing the crryover inoculum of Sclerotium oryze, the cuse of stem rot of rice. The phse down of open field burning in Cliforni hs necessitted the development of lterntives to burning. In 1993, continuous-yer experiment ws initited in Colus County to evlute the effects of lterntive residue mngement strtegies on overwintering scleroti of S. oryze, stem rot incidence nd severity, nd yield. Tretments were rrnged in split-plot design with winter flooding nd winter nonflooding s the min plots, nd fll incorportion of the strw residue, rolling of the strw to enhnce soil contct, bling nd removl of residue, nd fll burning s the subplots. S. oryze inoculum nd disese severity were significntly lower nd yield ws significntly higher in 5 out of 6 yers in the winter-flooded min plots compred with the winter nonflooded plots. Over the durtion of the tril, S. oryze inoculum ws consistently lower in burn subplots when compred with ll other subplots. No consistent differences in disese incidence nd severity or yield occurred in the subplots, lthough verge yield over the 6 yers ws highest in burn subplots when compred with ll other subplots. The results suggest tht winter flooding is the best lterntive to burning for stem rot mngement. Additionl keywords: culturl control, Mgnporthe slvinii, Oryz stiv Stem rot, cused by the fungus Sclerotium oryze Cttneo, is serious disese of rice worldwide (18,24), including in Cliforni (13,15,31,32). The scleroti produced by the fungus serve s primry inoculum by floting on the wter nd infecting rice stems t the wterline (2,10,24,32). Scleroti form bundntly in infected tissues s the rice plnt ners mturity nd continue to form in crop debris (32). The most effective mens of mnging stem rot in Cliforni hs been by burning infested rice residue nd stubble tht is left in the field fter hrvest to minimize overwintering inoculum. Legisltion hs required reduction in griculturl burning in the Scrmento Vlley Air Bsin (3) so tht, by the yer 2000, permitted burns will be llowed on only 25% of rice crege Corresponding uthor: N. A. Cints E-mil: cints@slinis.rs.usd.gov Current ddress of N. A. Cints: USDA-ARS, PWA; 1636 E. Alisl St.; Slins, CA 93905. This reserch ws primrily funded by the Rice Reserch Bord of Cliforni nd in prt by Ducks Unlimited. Accepted for publiction 22 June 2001. Publiction no. D-2001-0820-03R 2001 The Americn Phytopthologicl Society plnted. A recent sty on the mndted phsedown ws pssed by the stte legislture to llow more time to find lterntives to burning rice strw for disese nd residue mngement (4). With the imminent loss of most residue nd stubble burning, lterntive mngement strtegies will be needed in the future. Likely lterntives to burning include rolling, incorportion, nd bling nd removing of residue, with nd without winter flooding. Rolling the residue increses its contct with the soil, which enhnces its ccess to microorgnisms for decomposition. Incorportion increses contct between residue nd soil even more, which in turn increses decomposition (22). Bling nd removl of rice strw is intended to remove residue, inoculum, nd substrte for further inoculum production. Winter flooding is ttrctive s mens of enhncing decomposition nd providing hbitt for wterfowl in rice pddies during the winter. Previous field trils hve shown tht mintining or fluctuting soil moisture results in reduction of sclerotil vibility (12). Strw incorportion by mens of moldbord plowing hs been shown to reduce the totl number of scleroti by burying them nd rendering them unvilble to flot to the surfce of flooded pddies in the spring nd cuse infection s the rice emerges from the flood wter (10,30). Strw incorportion, when compred with no-tillge, lso reduces sclerotil vibility (10), perhps due to dislodging scleroti from the residue, incresing their contct with the soil, nd enhnced microbil ctivity. S. oryze scleroti which re in contct with soil hve hlf-life of 1.9 yers (1). Bling nd removl of rice strw hs been reported to be nerly s effective s burning for reducing overwintering inoculum (2,29). In preliminry trils, winter flooding decresed the number of scleroti recovered from soil when compred with winter nonflooding. The objective of this reserch ws to determine potentil lterntives to open field burning of residue (with or without winter flooding) under continuous rice cropping. MATERIALS AND METHODS Field design. A field study ws estblished to determine nd compre the effects of lterntive tretments to burning rice strw residue on S. oryze inoculum levels, stem rot incidence nd severity, nd yield. A 74-cre field tril in Colus County ws estblished in the fll of 1993, followed by the first crop of rice grown in 1994. The tril ws rrnged in split-plot design. The min plots were winter flooding nd winter nonflooding, with subplots of fll burning, fll bling nd removing, fll incorportion of rice residue, or fll rolling to enhnce soil contct (Fig. 1). were 1.9 cres. Ech subplot prctice ws completed in the fll soon fter hrvest. All strw mngement tretments were completed prior to winter flooding except for the flood nd roll subplot, which ws cge rolled with 3 to 5 in. of stnding wter. Winter-flooded min plots were flooded in lte October or erly November nd drined in lte Februry or erly Mrch of ech yer. Ech subplot ws estblished with seprte irrigtion system to prevent movement of inoculum between subplots. Tretments were replicted four times nd cv. M-202 ws plnted ech seson. All other culturl prctices employed t the site were typicl of stndrd rice production systems used in Cliforni, including wter seeding (8). Soil smpling. Soil smples were collected from the prepred seedbeds in the spring just prior to flooding. The soil type ws Willows Cly Moderte Alkline (9). Due to the lrge size of the subplots, ech 1140 Plnt Disese / Vol. 85 No. 11
subplot ws split into three sections for smpling. Within ech section, W pttern ws wlked. Along this W 10 to 12 smples were tken with grden trowel from the upper 2 in. of the seedbed, where the inoculum tht cuses disese lies (26,27). Soil smples collected long the W were combined s composite smple nd, when necessry, ir dried before processing. All soil smples were ground in soil grinder (Iler Improved; The Fen Mchine Co., Clevelnd, OH) tht ws djusted for minimum clernce of 2 mm. Scleroti from the soil smples were then retrieved using the procedure developed by Kruse nd Webster (14) with the following modifictions: (i) the filtrte in the Buchner funnel ws surfce disinfestted with solution of 8 ml of commercil blech (5.25% sodium hypochlorite) nd 92 ml of wter (followed by three rinses with deionized wter), nd (ii) scleroti were plced on wter gr pltes mended with 100 ppm chlorotetrcycline. Plnt smpling. Plnt smples were collected nnully from 1994 to 1999. Smples were collected ner rice mturity, when the field ws being drined for hrvest. Tillers were cut below the wter line (nd bove the roots) from pproximtely the sme res where the soil smples were collected. In ll, 10 to 12 smples of t lest 10 tillers were collected long W pth within ech subplot section. For ech section of the subplot, over 100 tillers were collected for disese evlution. Plnt smples were rted for disese by the method developed by Kruse nd Webster (15) (i.e., helthy nd infected tillers were divided into five ctegories bsed on the mount of disese: 1 = helthy, no symptoms; 2 = slightly infected with symptoms nd scleroti on the outer lef sheths only; 3 = mildly infected with discolortion of the inner lef sheths, culm green nd helthy; 4 = modertely infected, slight to mild discolortion of the culm, interior of the culm helthy; nd 5 = severely infected, culms infected internlly, either collpsed or not). The disese index is then clculted s follows: disese severity or disese index = [1(H n ) + 2(L n ) + 3(M n ) + 4(M* n ) + 5(S n )]/totl number of tillers exmined, where H n = number of helthy tillers, L n = number of lightly infected tillers, M n = number of mildly infected tillers, M* n = number of modertely infected tillers, nd S n = number of severely infected tillers. A rting of 1 indictes smple of ll helthy tillers, nd rting of 5 indictes smple with ll severely infected tillers. Dt for ech subplot were verged before proceeding with nlyses. Pnicle weights. To determine the reltionship between the disese severity rting nd yield, over 10,000 pnicles were collected in both 1996 nd 1997. Individul tillers were rted for disese severity nd weight of the pnicle from ech tiller ws determined. After tillers were rted for disese severity, ll pnicles within ech severity clss were clipped from the stems nd dried individully in drying oven t 65 C until no weight loss from dy to dy ws mesurble. Pnicles then were weighed to determine their dry weight. Yield determintion. Yields of pddy rice, t 14% moisture, were determined for ech plot ech yer. Ech seson, the mjority of ech subplot ws hrvested (minus microplots within). In 1997, however, the yield ws determined from section 570 ft long by 32 ft wide (twice the width of the hrvester heder) hrvested in two strips through the center of ech subplot. Sttisticl nlysis. Split-plot nlysis of vrince ws performed using SAS (SAS Institute Inc., Cry, NC) to determine differences in sclerotil levels, disese levels, nd yield between min plots nd subplots nd the interctions between min nd subplot tretments. When significnt differences in mens were found with nlysis of vrince (ANOVA), lest significnt difference nlysis ws used for men seprtion. Correltion nd regression nlyses were performed using SAS REG nd CORR procedures to determine the reltionship between disese severity versus yield. For the pnicle weight per tiller dt sets, correltion nd regression nlyses were lso performed using SAS REG nd CORR procedures to determine the reltionship between disese severity nd yield. RESULTS Inoculum levels. In ech yer, except 1994, ANOVA reveled significnt differ- Fig. 1. Top: eril view of winter-flooded nd winter nonflooded min plots t the 74-cre Colus County site. Bottom: eril view of the bled, incorported, rolled, nd (the most obvious) drkened, burned subplots. Ech subplot hd seprte irrigtion system so tht no pssge of inoculum occurred between subplots. Plnt Disese / November 2001 1141
ences in mens between the min plot tretments. Men seprtion nlysis indicted tht, fter the initil yer of the tril, there were significntly fewer vible scleroti recovered per grm of soil ech yer in the winter-flooded plots compred with the winter-nonflooded plots (Tble 1). ANOVA reveled significnt differences in mens ech yer between the min plot tretments. Men seprtion nlysis indicted tht significntly fewer vible scleroti were recovered from the burned subplots ech yer of the tril compred with ll other subplots (Tble 1). Significntly greter vible scleroti were recovered from the rolled subplots ech yer compred with ll other subplots (Tble 1). After the initil yer of the tril, interctions between residue mngement nd winter flooding mngement were significnt in regrds to the number of vible scleroti per grm of soil (P 0.05) (Tble 1). Disese severity. In 5 out of 6 yers, there were significnt differences in disese severity between min plot tretments; men seprtion reveled significnt reduction in disese severity in the winter-flooded plots compred with the winter-nonflooded plots (Tble 2). There were significnt differences in stem rot severity between strw mngement subplots in 5 out of 6 yers determined by ANOVA, but men seprtion did not revel ny consistent differences in severity between subplots tretments (Tble 2). Interctions between residue mngement nd winter flooding mngement nd their effect on disese severity were significnt only in 1996 with Pr > F vlue of 0.0372 (P 0.05) Yield. In every yer except 1997, significnt differences in yield between min plot tretments were determined by ANOVA. Men seprtion nlysis reveled tht yield ws significntly higher in the winter-flooded plots compred with the winter-nonflooded plots (Tble 2). There were significnt differences in yield between strw mngement subplots over the 6 yers of the field tril, s determined by ANOVA, but men seprtion reveled no consistency to these differences (Tble 2). Burn subplot yields were highest in 3 of the 6 yers nd verged highest overll for the durtion of the tril. Interctions between residue mngement nd winter flooding mngement nd their effect on yield were not significnt. Disese severity versus individul pnicle weight. The Person correltion coefficient between stem rot severity nd pnicle weight in 1997 ws determined to be 0.60 (P = 0.0001, n = 135). As stem rot severity incresed from helthy through vrious rtings to severe, there ws significnt reduction in totl weight of the pnicles with significnt negtive correltion. The reltionship between disese severity levels mesured nd reduction in yield in 1997 is shown in Figure 2. The severity yield reltionship ws well described by liner regression model s indicted by highly significnt coefficient of determintion (r 2 ) of 0.36 (P = 0.0001, n = 135). Disese severity versus yield. The Person correltion coefficients between Tble 1. Effects of winter flooding nd vrious residue mngement tretments on Sclerotium oryze inoculum Vible scleroti per grm soil Plots b 1994 1995 1996 1997 1998 1999 Men Min plots Flood 1.35 0.98 0.94 0.78 0.77 0.51 0.88 Non flood 1.52 1.68 1.51 2.05 2.21 1.58 1.76 LSD NS 0.2999 0.2139 0.4571 0.3052 0.2935 Burn 1.03 0.63 0.62 0.52 0.60 0.33 0.62 Incorporte 1.48 1.15 1.33 1.36 1.42 0.93 1.27 Ble 1.30 1.20 1.23 1.35 1.68 0.96 1.12 Roll 1.91 1.86 1.73 2.43 2.26 1.97 2.02 LSD 0.4384 0.4241 0.3024 0.6465 0.4317 0.415 Min subplot 0.0761 0.2136 0.0024 * 0.0278 * 0.0027 * 0.0016 * Soil smples were collected from prepred seedbeds; NS = not significnt. b LSD = lest significnt difference t P 0.05. * = significnt t P 0.05, Pr > F vlues for min subplot interction. Tble 2. Effects of winter flooding nd vrious residue mngement tretments on stem rot disese severity nd yield of rice t the Colus site Plots 1994 1995 1996 1997 1998 1999 b Men Stem rot severity Min plots Flood 2.61 3.18 3.66 2.58 3.06 3.02 3.01 Nonflood 3.02 3.72 4.14 2.67 3.31 3.37 3.37 LSD 0.19 0.28 0.17 0.15 0.19 0.21 Burn 2.77 3.26 3.77 2.60 2.90 2.87 3.02 Incorporte 2.67 3.39 3.73 2.62 3.06 3.21 3.11 Ble 3.04 3.65 4.08 2.69 3.39 3.38 3.37 Roll 2.79 3.49 4.00 2.60 3.37 3.21 3.24 LSD 0.27 0.39 0.25 0.21 0.17 NS Yield pounds\cre t14% Min plots Flood 8,300 9,084 9,250 10,366 7,825 9,214 9,006 Nonflood 7,846 8,076 8,927 10,331 7,287 8,823 8,548 LSD 421 413 236 272 243 283 Burn 8,882 8,561 9,240 10,496 7,665 9,285 9,021 Incorporte 7,820 8,774 8,766 10,072 7,721 8,512 8,610 Ble 8,167 8,255 9,137 10,365 7,365 9,149 8,739 Roll 7,424 8,730 9,192 10,462 7,472 9,114 8,732 LSD 596 584 334 385 224 429 Stem rot severity is bsed on scle of 1 to 5, where 1 = helthy nd 5 = severely infected. LSD = lest significnt difference. b NA = not pplicble. 1142 Plnt Disese / Vol. 85 No. 11
stem rot severity nd hrvested yields were determined in 1994 to 1997. In 3 of 4 yers, the reltionship between severity nd yield ws strongly inversely proportionl, with 2 of the 3 yers hving highly significnt negtive correltion coefficients (Tble 3). DISCUSSION Fll incorportion followed by winter flooding resulted in decrese in stem rot severity s well s n increse in yield. These results suggest tht fll strw mngement followed by winter flooding is vible mngement strtegy for stem rot of rice to replce open field burning. The verge number of vible scleroti per grm of soil in the first yer of the tril ws 1.43. Burning reduced the number of vible scleroti per grm of soil to 0.33 by 1999. Pst reports hve shown burning mintins or decreses levels of vible scleroti per grm of soil, while unburned plots resulted in incresing numbers of vible scleroti (30,32). Significnt interctions between min plot nd subplot tretments for the number of vible scleroti suggest tht winter flooding of plots my hve influenced inoculum levels differently depending on the strw mngement tretment. Specificlly, burning rice residue effectively reduced inoculum whether the plots were flooded in the winter or not, but ll other strw mngement tretments resulted in significntly lower inoculum levels in plots tht hd lso been flooded in the winter (6). In pst studies of stem rot in Cliforni, inoculum levels nd disese severities vried. At Butte County rice field studied in the lte 1970s, inoculum levels rnging from 0.24 to 1.15 vible scleroti per grm of soil resulted in stem rot severities of 1.59 to 2.45 (31). At Sutter County site smpled in the erly 1990s, similr levels of 0.26 vible scleroti per grm of soil resulted in stem rot severities verging bout 2.7 (26). The cultivr grown in this study, M-202, hs yield tht is less ffected by stem rot thn mny commercil cultivrs grown in the pst. The sclerotil density t the Colus site exceeded 0.6 vible scleroti per grm. The resulting severities were, on verge, closer to rting of 3. When looking for rice lines resistnt to stem rot, the more resistnt cultivrs not only showed reduction in stem rot severity, but lso in sclerotil production (17). Sites with different sclerotil densities my hve similr levels of stem rot severities due to the tolernce of the cultivr grown. According to Kiem nd Webster, lternte wetting nd drying of S. oryze scleroti in rice soils reduced sclerotil vibility (12), which supports the results of our study where winter flooding reduced the number of vible scleroti per grm of soil from 1.35, in the initil yer of the tril, to 0.51 by 1999. It is possible tht the totl number of scleroti recovered ws lower in the winter-flooded tretments becuse the high soil wter content incresed the rte of decomposition of the scleroti. The rte of rice strw decomposition incresed when the soil ws modertely moist (60% moisture) compred with soils with low or high moisture, 30 or 150%, respectively (16,19). As temperture incresed, the rte of rice strw decomposition lso incresed (16,20). Thus, it is not surprising tht, s temperture nd moisture incresed, rice strw decomposition lso incresed (16,21). Winter flooding my id in the decomposition of scleroti becuse the winter-flooded pddies re usully wet longer in the fll nd fter drining in the spring s the temperture increses. It is lso possible tht sclerotil decomposition ws encourged by the microflor popultions in the winter-flooded tretments. Crtwright found differences in fungl microflor on stem rot scleroti between different rice residue mngement systems (5). Such microflor differences, which were both fungl nd bcteril, my explin some of the differences in sclerotil vibility observed. Also, the flooding of stubble in November my hve diminished the crbon vilbility in the residue for further sclerotil production. Shhjhn found significnt positive correltion between S. oryze inoculum nd stem rot severity (23). Yet, Webster et l. reported tht, when inoculum levels exceeded 0.6 vible scleroti per grm of soil, the liner correltion between inoculum nd disese severity ws lost (28). This conclusion supports our results tht strw mngement subplots most often showed no significnt differences in disese incidence nd severity despite significnt differences in inoculum levels. In lmost ll soil smples from the tril, levels of vible scleroti per grm of soil recovered exceeded 0.6. The lck of significnt difference in yield s result of disese occurring from high inoculum levels (>0.6 vible scleroti/g of soil) in some of the strw mngement tretments my be explined by the reltionship observed between disese severity nd pnicle weight (7). Pnicle weight ws known to decrese with incresed stem rot severity (7,28), nd this ws verified in the present study (Fig. 2). If prticulr plots showed finl disese severities of 2 nd 4, we would hve expected yield differences between these plots. Yet, most of the disese severity rtings verged bout 3, without much vribility. Inoculum in ll tretments ws lmost lwys greter thn 0.6; therefore, the disese severity rtings nd yield reductions due to disese would not be expected to vry significntly between tretments. Admittedly, in this study, the reltionship exmined between disese severity nd pnicle weight lost its linerity s severity pproched 5 (Fig. 2). This figure my bring to question the ccurcy of the rting system; more extensive rting system my hve given more precise description of the reltionship of yield to disese severity. However, the trend tht the figure Fig. 2. Incresed stem rot severity results in decresed pnicle weight. Pnicles were collected within ech stem rot severity clss (1 = helthy to 5 = severe infection) in 1997 from the Colus County site. Pnicles were collected from over 10,000 rted tillers. At ech point, the brs represent the stndrd devition. Tble 3. Correltion coefficients for stem rot severity nd yield Yer Severity versus yield 1994 0.33 1995 0.80** 1996 0.60** 1997 0.05 Stem rot severity is bsed on scle of 1 to 5, where 1 = helthy nd 5 = severely infected. Yield is mesured t 14% moisture; ** = F is significnt t P = 0.01. Plnt Disese / November 2001 1143
suggests is lso supported by the correltion nlyses between hrvested yield nd severity (Tble 3). The trend ws observed in the 1994 dt, then the trend continued significntly over the next two sesons. Pst reports show tht, s nitrogen fertiliztion is incresed, stem rot severity lso increses (11). Pst reports lso show tht, once the mount of nitrogen fertilizer pplied provides for mximum yield response, no further increse in yield ws obtined with further increses in nitrogen (25). On the other hnd, it hs been generlly observed tht stem rot severity is fvored by nitrogen fertiliztion bove tht required for mximum yield response (11,18,23). Preliminry results of other studies t the experimentl site hve shown tht ntive nitrogen levels hve incresed in the subplots where residue hs been incorported due to minerliztion of the strw incorported. All plots received the sme fertiliztion rte throughout the study; therefore, the incorported plots contin higher nitrogen levels vilble to the plnts thn those where strw nd stubble re burned or removed. Thus, disese severity could be higher in these plots due to nitrogen effects nd n bundnce of inoculum. In either event, it ppers tht disese severity in most of the plots is occurring t level typicl for this disese under inoculum levels nd culturl fctors tht re occurring in the different tretments. Ech yer, ech 1.9-cre subplot ws hrvested in its entirety, except in 1997, when only section through the center of ech plot ws hrvested, resulting in higher nd more uniform yields between tretments. This method of hrvest did not tke into ccount the effects of levees seprting the tretments tht cn often hve n impct on yield, prticulrly in 1.9-cre plots. Other exceptions likely contributing to the higher nd more uniform yields in 1997 were more ggressive use of herbicides due to the buildup of weeds in incorported nd rolled subplots. Also, rice blst disese ws found for the first time in Cliforni in 1996. Becuse of the lrge investment in time nd expense in the continuous-yer tril, two pplictions of Qudris fungicide were pplied during the 1997 seson to reduce the potentil impct of rice blst on the outcome of the study. The disese severity nd yield dt (Tble 1) for the 1997 seson suggest the fungicide pplictions contributed to lower stem rot severity during the 1997 seson. This could hve been expected becuse Qudris is lso considered effective in controlling stem rot s well s ggregte sheth spot of rice, which occurs t low level t the experimentl site. Similrly, the lck of correltion between severity nd yield in 1997 (Tble 3) my be due to reduced vribility of these fctors, result of the Qudris pplictions. In conclusion, fll incorportion of residue followed by winter flooding ppers to be the best lterntive to rice strw burning for disposl of residue nd mngement of stem rot. However, the cost of fll incorportion nd the wter for flooding is significntly higher thn tht of open field burning. Bling nd removl of strw ddresses the residue disposl problem in the field. However, the cost is significnt nd the lterntive uses for the strw hve been slow in development nd do not yet offset the bling nd removl costs. In ddition, when strw is removed, potssium becomes deficient on some Cliforni soils fter 3 yers (R. Mutters, personl communiction). Although this study nswered some importnt questions concerning the effect of potentil culturl control methods for stem rot of rice, these trils lso bring bout new questions worthy of further study, such s: by wht mechnism does winter flooding reduce overwintering scleroti of S. oryze? ACKNOWLEDGMENTS We thnk S. Scrdci for his oversight of the field tril mngement for the first 5 yers. LITERATURE CITED 1. Bockus, W. W., nd Webster, R. K. 1979. Decline in numbers nd inoculum potentil of Sclerotium oryze in field soil. Phytopthology 69:389-392. 2. Bockus, W. W., Webster, R. K., Wick, C. M., nd Jckson, L. F. 1979. Rice residue disposl influences overwintering inoculum level of Sclerotium oryze nd stem rot severity. Phytopthology 69:862-865. 3. Cliforni Stte Legislture. 1991. AB 1378 The Rice Strw Burning Reduction Act of 1991. 4. Cliforni Stte Legislture. 1997. SB 318 Air Pollution: Rice Strw Burning. 5. Crtwright, R. D. 1992. Biodecomposition of rice residue nd biocontrol of Sclerotium oryze in Cliforni. Ph.D. diss. University of Cliforni, Dvis. 6. Cints, N. A. 1998. Effects of different rice residue tretments on Sclerotium oryze. (Abstr.) Phytopthology 88:S106. 7. Cints, N. A. 1998. 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Phytopthology 64:1499-1502. 13. Kruse, R. A., nd Webster, R. K. 1972. The morphology, txonomy, nd sexulity of the rice stem rot fungus, Mgnporthe slvinii (Leptospheri slvinii). Mycologi 64:103-115. 14. Kruse, R. A., nd Webster, R. K. 1972. Sclerotil production, vibility determintion nd quntittive recovery of Sclerotium oryze from soil. Mycologi 64 (6):1333-1337. 15. Kruse, R. A., nd Webster, R. K. 1973. Stem rot of rice in Cliforni. Phytopthology 63:518-523. 16. Mikkelson, D. S., nd Brodbent, F. E. 1981. Rice strw disposl by soil incorportion. Pges 46-54 in: Agriculturl Residue Mngement A Focus on Rice Strw. J. E Hill, ed. University of Cliforni, Dvis. 17. Oster, J. J. 1992. Rection of resistnt breeding line nd susceptible Cliforni rice cultivrs to Sclerotium oryze. Plnt Dis. 76:740-744. 18. Ou, S. H. 1972. Rice Diseses. Commonwelth Mycologicl Institute. Kew, Surrey, Englnd. 19. Pl, D., nd Brodbent, F. E. 1975. Influence of moisture on rice strw decomposition in soils. Soil Sci. Soc. 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Hilgrdi 49(3):1-12. 1144 Plnt Disese / Vol. 85 No. 11