J. Phytopathology 136, 73—81 (1992)© 1992 Paul Parey Scientific Publishers, Berlin and Hamburg

ISSN 0931-1785

Department of Plant Pathology^ International Rice Research Institute,Los Banos, Philippines

Effects of Seedbed Management on Blast Developmentin Susceptible and Partially Resistant Rice Cultivars

D. N. SAH and J. M. BONMAN

Authors' address: D. N. SAH, Institut fiir Pflanzenpathologie und Pflanzenschutz, Grisebachstrafie 6,3400 Gottingen-Weende, F. R. Germany; J. M. BONMAN, Department of Plant Pathology, Interna-

tional Rice Research Institute, P.O. Box 933, Manila, Philippines.

Vifith one figure

Received October 14, 1991; accepted November 29, 1991

Abstract

Four rice cultivars, susceptible or partially resistant to Pyricularia grisea were evaluated for theirapparent infection rates (r) and for terminal severity values in seedlings grown in seedbeds. Suscepti-bility of partially resistant cultivars decreased when seeded in wet seedbed compared to those grown inraised or upland seedbed. The degree of blast reduction due to flooding varied with rice genotype,seeding rate, and environmental conditions. Flooding was effective in managing seedling blast ofpartially resistant cultivars but not of susceptible cultivar. Use of a lower seed rate in comparison to ahigher seed rate also led to reductions in r-values and terminal disease severities. However, in case ofthe susceptible cultivar IR50, effect of seed rate was more pronounced, when low amount and shonduration of rainfall occurred. Flooding of wet beds with low seed rate further reduced the blastincidence. Results of this study suggest that with appropriate use of water, seed rate, and partial hostresistance there are prospects for effective control of leaf blast in the tropics.

Zusammenfassung

Der Einflufi des Saatbeetmanagements auf die Blast-Entwicklungin anfailigen und teilresistenten Reissorten

Die Samlinge von vier Reissorten, anfallig oder teilresistent gegenuber Pyricularia grisea,wurden auf ihre sichtbaren Infektionsraten (r) und Endbefallsstarkewerte in Saatbeeten beurteilt. DieAnfalligkeit der teilresistenten Sorten verringerte sich nach Aussaat in nassen Saatbeeten vergliehenmit Pflanzen, die in erhohten Saatbeeten wuchsen. Die Verringerung des Blast-Befalls durchUberflutung war vom Reisgenotyp, der Aussaatstarke und den Umweltbedingungen abhangig.Uberflutung war effektiv bei der Kontrolle von Samlingsblast bei teilresistenten aber nicht anfalligenSorten. Niedrigere Aussaatstarkcn fiihrten zu Vernngerungcn m den r-Werten und Endbefallsstarken.Bei der anfalligen Sorte IR50 jedoch war der Einfluft der Aussaatstarke am starksten ausgepragt, wenn

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SAH and BONMAN

nicdrii;c und kurz andnucrnde Regenfalle hcrrschten. Eine Oberflutung von nassen Saatbeeten,f;ckoppch mil nicdrigon Aiiss.i.ustiirkcn. verursachte cine weitere Reduktion des Blastvorkommens.Die i;ri;cbnis,sc dicscr Arbcii dcuicn d.irauf hin, dal̂ cs mit einer angemessenen Wasserversorgung,Auss.i.uM.ukc und der 'reilrcsisten/ des Wirtes moglich ist, Reisblast in den Tropen wirksam zu

Blast, caused by Pyricularia grisea Sacc, is one of the most destructivediseases of rice (Oryza sativa L.), (Ou 1985). In Neapel, India, and Bangladesh,seedhngs ot popular cultivars raised in upland seedbeds are seriously damaged byblast when weather favors disease development. Often, farmers have difficulty ofsaving enough seedlings to transplant. Alternative management practices areneeded to reduce damage caused by blast, because the use of fungicides is notusually possible for resource-poor farmers in the tropics. Factors contributing tooutbreaks of blast in farmer's seedbeds include the use of susceptible cultivars andhigh seed rates, and the occurence of water deficit.

Partially resistant cultivars, which show fewer lesions than susceptiblecultivars, could be one component of a system for managing blast in seedbeds. Inthe lowland tropics, partial resistance is associated with durable blast resistance insome cultivars, such as IR36 (YEH and BONMAN 1986). However, the level ofpartial resistance that is effective under irrigated conditions probably would notprove useful in the more blast-prone upland areas (BONMAN and MACKILL 1988).Thus, partial resistance alone might not be sufficient in upland seedbeds.

Seed rate might influence the microclimate of the seedbed and hence blastdevelopment. In transplanted fields, close plant spacing caused longer periods ofleaf wetness from dew and higher blast disease incidence (EL REFAEI 1977). Highseed rate might have similar effects in seedbeds.

In the past, management of irrigation water was used as one tactic forintegrated control of blast in Japan (KOZAKA 1965). Seedlings grown in uplandnurseries are more susceptible to blast than those grown in flooded nurseries(OTANI 1952). Results from greenhouse studies have also indicated that thesusceptibility of nee decreases with mcreasing soil water content (HEMME and ABE1932, HEMMI 1933, SUZUKI 1935 a, b, KAHN and LIBBY 1958, TOKUNAGA and OTA1959, GILL and BONMAN 1988). Recently, flooding was found to effectivelycontrol leaf blast in direct seeded rice in Louisiana, USA (KiM 1986, KiM et al1988). Based on greenhouse studies in Taiwan, HASHIOKA (1950) demonstratedthat the resistance of tropical rice cultivars to blast was not influenced by soilmoisture.

However, there have been no previous field studies on seedbed managementand blast in the tropics. Therefore, this study was made to assess the effects ofpartial resistance, seed rate, and flooding on blast development in seedbeds.

Materials and Methods

Three experiments were conducted at the experimental farm of the International Rice ResearchInstitute (IRRl) in the Philippines in 1988 and 1989. Wet seedbeds were surrounded by 15 cm earthenlevees. In raised (upland) seedbeds, the soil surface was 15 cm above the flood-water level.

Effects of Seedbed Management on Blast Development 75

Tensiometers were placed in unflooded treatments to monitor soil moisture tension. Tempera-ture, leaf wetness, and relative humidity of upland and flooded seedbeds were continuously moni-tored using thermohygrographs (G. Lufft, GmbH & Co.). Data on rainfall, wind speed, and hours ofsunshine were obtained from the meteorological station located at the IRRI farm.

The experiment I used a randomized complete block design with a spin plot treatmentarrangement and four replications. Plots were 1.0 X 1.5 m and were 2,0 m apart. Water managementtreatments were assigned to main plots and the two cultivars IR50 (susceptible) and IR36 {partiallyresistant) to subplots. The five water management treatments were: raised seedbeds receiving rainfallonly; unflooded seedbeds that were periodically irrigated, preflooded seedbeds that received 1—2 cmflood until the first appearance of blast lesions, postflooded seedbeds that received 1—2 cm water onlyafter the appearance of lesions, and flooded seedbeds that had 1 to 2 cm standmg water from the sixthday after seeding until the last day of observation. Pregerminated seeds were broadcasted at the rate of200 g per square meter of seedbed area. Nitrogen (N) (200 kg N/ha) was applied as ammonium sulfateat the time of seeding.

Each plot was surrounded by a single row of IR50 seedlings that was inoculated with a virulentisolate of P. grisea to allow the natural dispersal of moculum into the test plots. Lesion counts weremade every second day begmning 14 days after seedmg. Four readmg sites were randomly markedwithin each subplot and typical spindle-shaped blast lesions were counted on 10 seedhngs at each site.The number of lesions per seedlmg was converted into percent diseased leaf area by the proceduredescribed earlier (ViLLAREAL et al. 1980).

As disease progressed and lesion counts became impractical, the percent of disease leaf area perseedling was estimated directly (KiM et al. 1988). Disease severity proportions were transformed tologits and were regressed on days to calculate the apparent infection rate (VANDEKPLANK 1963).Terminal disease severity values were analyzed after arcsin transformation.

In experiment 2, treatments were as in experiment 1, but with the following additions: raisedseedbed regularly watered to avoid water stress, raised seedbed with a seed rate of 100 g seeds/m", andflooded seedbed with a seed rate of 100 g seeds/m^. Plots were 1.5 x 1.5 m spaced 1.5 m apart. Plotswere surrounded by one row of sorghum (Sorghum vulgare) to minimize interplot interference.Twenty-five infected seedlings of IR50 were transplanted in the windward corner of each seedbed asan inoculum source. Plots were covered with polyethylene sheets supported by wooden frames forthree consecutive nights per week until blast lesions appeared on test plants. All other procedures werethe same as in experiment 1.

In experiment 3, four cultivars (IR36, IR50, IR58, and IR64), two water managements (uplandand flooded), and two seed rates (100 g/m' and 200 g/m^) were used as treatment factors in arandomized complete block design with three replications. The soil in upland seedbeds was allowed todry completely before seeding. Plots were 1.0 X 1.0 m, and 1.5 m apart. All other procedures werethe same as in experiment 2.

Results

The rate of blast disease progress was significantly lower in the partiallyresistant cultivar IR36, compared to the susceptible cultivar IR50 (Table 1).Disease progress was faster on seedlings grown in raised seedbeds regardless ofcultivar. Seedbed X cultivar interaction was detected only for disease severity andnot for r-values.

Terminal disease severities were significantly less on seedlings of IR36 grownin wet, unflooded seedbeds than those grown in raised-beds (Table 1). Floodingfurther reduced terminal disease severity in IR36. However, IR50 was heavilydiseased and flooding had little effect on terminal severity. Lesions on IR36 weresmaller than those on IR50, but no difference in lesion size due to type of seedbedand flooding was detected.

SAH and BONMAN

Disease progress of IR50 was slower at the low seed rate (100 g/m^) than atthe high rate (200 g/nr') (Table 2). A significant interaction between cultivar andseodbcd was observed for both terminal disease severities and r-values, indicating

Table IApparent infection rates (r) and terminal disease severity (%) for blast disease progress on ricecultivars IR36 (partially resistant) and IR50 (susceptible) grown in seedbed at 200 g seeds/m'' seedbed

area using five water management practices in 1988 (experiment 1)

Seedbed management

RaisedWet, unfloodedWet, preflooded-'Wet, postflooded^Wet, flooded

r

IR36

0.57 d̂ '

0.52 d0.42 e0.39 e0.41 e

(unit/day)"

1R50

0.83 a0.68 c0.74 be

0.71 be0.78 ab

Terminal disease

IR36

80.7 b53.6 c22.5 d27.2 d17.8 d

seventy (%)"

IR50

99.6 a93.1 ab91.4 ab92.9 ab93.0 ab

Values followed by the same letters are not significantly different within terminal disease severity orr-values at p < 0.05 according to Turkey's test.Determined as linear regression coefficient of logit values for disease proportion against time.Data were analyzed after arcsin transformation.Flooded until appearance oi mitial blast lesions.Flooded only after appearance of blast lesions.

Table 2Apparent infection rates (r) and percent terminal disease severity (%) for blast disease progress on ricecultivars IR36 (partially resistant) and IR50 (susceptible) grown in seedbeds using eight seedbed

management practices in 1988—1989 (experiment 2)

Seedbed

200 g seed/m^ area

RaisedRaised, wateredWet, unfloodedWet, preflooded^Wet, postflooded^Wet, flooded

100 g seed/m^ area

Wet, flooded

Raised

I-l

IR36

0.22 b0.24 b0.23 b0.23 b0.20 b0.22 b

0.16 b0.17 b

(unit/day)"^

IR50

0.5 a''0.45 a0.48 a0.47 a0.51 a0.50 a

0.29 b0.23 b

Terminal

IR36

28.6 b24.4 be22.1 be18.2 be18.2 be8.6 be

6.1 c12.4 be

disease severity (%)"

IR50

95.3 a

85.6 a83.2 a86.6 a87.2 a83.5 a

28.8 b29.5 b

Means followed by the same letter within r-values or terminal disease severity (%) are notsignificantly different at p < 0.05 according to Turkey's test.Determined as linear regression coefficient of logit values for disease proportion against time.Data were analyzed after arcsin transformation.Flooded until appearance of initial blast lesions.Flooded only ^^ter appearance of blast lesions.

Effects of Seedbed Management on Blast Development

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78 SAH and BONMAN

that flooding was more effective in reducing disease in IR36 than in IR50.Moodmi; and low seed rate were equally effective In reducing terminal diseaseseverity in 1R36. Seed rate gave the biggest effect with IR50, whereas floodingwas not important.

Siiin it leant differences in r-values and terminal disease severities wereobserved due to floodini;, seed rate and rice cultivar (Table 3). Effects of seed rateand flooding; were additive. Flooding reduced the rate of disease progress in allculti\ars tested (Fig. 1). Disease progress was slower in seedbeds sown at the lowseed rate than those sown at a high seed rate (Fig. 1). Slow disease progress alsoresulted in lower terminal disease severities (Table 3).

Cultivar IR30 had significantly higher r-values and terminal disease severitieswhen compared to IR36, IR58 and 1R64 (Table 3). Seedlings grown in uplandbeds at the high seed rate had higher r-values than seedhngs raised at the low seedrate. Flooding significantly reduced the r-values regardless of seed rate used.

Rainfall was low (mean 1.8 mm) and of short duration (17.8 hr total) duringthe final 17 days of experiment 2. In spite of the long duration of leaf wetness

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Days after seedingFig. 1. Blast disease progress on seedling plants of three partially resistant cultivars (IR36,1R58, IR64)and a susceptible cultivar IR50, grown in flooded or upland seedbeds at 200 or 100 g seed/m^ in 1989

(experiment 3)

Effects of Seedbed Management on Blast Development 79

caused by dew and the polyethylene cover, initial disease symptoms appearedthree weeks after seeding. In experiments 1 and 3, the first lesions appeared twoweeks after seeding.

Discussion

In general, disease progressed rapidly and resulted in higher terminal sever-ity values on the susceptible cultivar IR50, as compared to the partially resistantcultivars IR36, IR58, and IR64. However, seedlings of partially resistant cultivarsraised in upland seedbeds with high seed rate were also heavily infected. Thisresult is logical, since blast is greatly influenced by agronomic management andenvironmental factors and cultivars exhibiting resistance in one agro-ecosystemmay be susceptible in another (BONMAN and MACKILL 1988).

The lower seed rate greatly reduced the rate of disease progress and terminaldisease severity, regardless of seedbed water management. Effects of seed rate onblast development were similar to those reported for plant spacing experiments(EL REFAEI 1977, SUZUKI 1975). Longer dew periods and higher blast incidencehave been recorded in closely spaced plantings. Low seed rate alone greatlyreduced the rate of disease progress on IR50 in the second experiment (Table 2),but the reduction was not as pronounced in the third experiment (Table 3).Ambient air-temperature, relative humidity, and hours of leaf wetness werefavorable for blast development during the three experimental periods (KiNG-SOLVER et al. 1984), although less rain fall in a shorter duration during the secondexperiment.

The degree of disease reduction due to flooding varied with cukivar and seedrate. Flooding was only effective in managing blast of IR36 and not of IR50.

Lower blast incidence was observed in partially resistant cultivars sown inwet beds at a low seed rate than at a high seed rate. In case of the susceptiblecultivar IR50, low seed rate was effective in reducing the rate of disease progressand terminal disease severity (Table 2). However, it was not effective in control-ling seedling blast (Table 3). Thus, effectiveness of low seed rate in controllingseedling blast in susceptible rice varied with environmental conditions.

The slightly higher leaf wetness duration (18.3 h) caused by dew or poly-ethylene cover during the low rainfall period (experiment 2) was not as effectivein causing as severe an epidemic in raised seedbeds of IR36 as was leaf wetnessduration of 16.3 or 16.6 h during a higher and more prolonged rainfall (experi-ment 1 and 3). The effectiveness of leaf wetness caused by rain on plant diseasedevelopment is not the same as that caused by fog, dew, or other means (JONES

1986). Perhaps, a device needs to be developed to distinguish the leaf wetnessduration caused by rain and those caused by dew, fog, or guttation.

Microclimatic factors, leaf wetness duration, air-temperature and relativehumidity measured with two thermohygrographs varied little between floodedand upland beds. This agrees with the results reported earlier (KIM et al. 1988).Increased nitrogen uptake by the seedlings grown in unflooded nurseries has beenassociated with the high susceptibility exhibited by those plants (OTANI 1952,KOZAKA 1965).

80 SAM and BONMAN

Detailed investigations are needed to understand the mechanism of flooding-mcdiated resistance to P. grisea. Mean soil moisture tensions of raised or upland-beds at 5 cm depth were 0, 8.07, and 7.46 kPa in the first, second, and thirdexperiments, respectively. These soil moisture tension values indicated that soilwas saturated in the first experiment, while soil moisture were at field capacity orunder only slight moisture stress without leaf rolhng, in the other experiments.Rapid disease progress in the absence of leaf rolling indicated that seedling blastcould become severe in the raised beds even in the absence of drought.

Recently, KiM et al. (1988) have shown that leaf blast of a susceptible cultivarcan be effectively controlled by flooding direct-seeded rice under warm temperatehumid conditions in Louisiana. They found that cultivars susceptible underupland conditions behaved as partially resistant when flooded. In the presentstudy, however, flooding alone failed to control blast in seedlings of a susceptiblecultivar raised in seedbeds under tropical conditions. This difference in findingswith susceptible cultivar grown in two different agro-ecosystems support thehypothesis that environmental conditions, plant age, genotype and culturalpractices might influence the effectiveness of flooding-mediated resistance effectson disease.

Our results show that flooding alone can decrease the rate of disease progressin partially resistant cultivars; yet a combination of low seed rate and floodingwas more effective in managing the disease. Therefore, integration of partialresistance, low seed rate, and flooding may lead to successful management of leafblast at the seedling stage. Low seed rate alone or in combination with floodingmight be only sufficient for managing leaf blast in susceptible cultivars raised inenvironments less conducive to blast development.

Literature

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EL REFAEI, M. I., 1977; Epidemiology of rice blast disease in the tropics with special reference to theleaf wetness in relation to disease development. Ph. D. Thesis, Indian Agricultural researchInstitute. 195 pp.

GILL, M. A., and J. M. BONMAN, 1988: Effects of water deficit on rice blast. I. Influence of waterdeficit on components of resistance. J. PL Prot. Tropics 5, 61—66.

HASHIOKA, Y., 1950: Studies on the mechanism of prevalence of the rice blast disease in the tropics.Tech. Bull. Taiwan Agric. Res. Inst. 8, 1—225.

HEMMI, T., 1933: Experimental studies on the relation of environmental factors to the occurrence andseverity of blast disease in rice plants. Phytopath. Z. 6, 305—324.

, and T. ABE, 1932: Studies on the rice blast disease. II. Relation of the environment to thedevelopment of blast disease. Bull. Dep. Agric. For. Japan 47, 204 pp.

JONES, A. L., 1986: Role of wet periods in predicting foliar disease. In: LEONARD, K. J., and W. E.FRY (eds). Plant Disease Epidemiology, Vol. 1, pp. 87—100. Macmillan Publishing Com-pany, New York-

KAHN, R. P., and J. L. LIBBY, 1958: The effect of environmental factors and plant age on the infectionof rice by the blast fungus, Pyricularia oryzae. Phytopathology 48, 25—30.

KIM, C . H . , 1986: Effect of water management on the etiology and epidemiology of rice blast causedby Pyricularia oryzae Cav. Ph. D. Thesis, Louisiana State University and Agricultural andMechanical College, 170 pp.

Effects of Seedbed Matiagement on Blast Development 81

, D. R. MACKENZIE, and M. C. RUSH, 1988: Field testing a computerized forecasting system forrice blast disease. Phytopathology 78, 931—934.

KiNGSOLVER, C. H., T. H. BARKDALE, and M. A. MARCHETI, 1984: Rice blast epidemiology. PaAgric. Exp. Stn. Bull. 853, 33 pp.

KOZAKA, T., 1965: Control of rice blast by cultivation practices in Japan, in: The Rice Blast Disease.Proc. Symposium at IRRI, 1963, pp. 421—438. Johns Hopkins Press, Baltimore, Maryland.

OTANI, Y., 1952: Studies on the relation between the principal components of rice phnt and itssusceptibility to blast disease. Ann. Phytopathol. Soc. Japan. 16, 97—102.

Ou, S. H., 1985: Rice Diseases. Commonwealth Mycological Institute, Kew, Surrey, England,380 pp.

SUZUKI, H . , 1934: Studies on the influence of some environmental factors on the susceptibility of therice plant to blast and Helminthosporium disease and on the anatomical characters of the plant.I. Influence of the difference of soil moisture. J. Coll. Agric. Tokyo Imp. Univ. 13, 45—108.

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, 1935 b: Studies on the influence of some environmental factors on the susceptibility of the riceplant to rice blast and Helminthosporium diseases and on the anatomical characters of theplant. IIL Influence of differences in soil moisture and m the amount of fertihzer and silicagiven. J. Coll. Agric. Tokyo Imp. Univ. 13, 277—332.

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ToKUNAGA, Y., and Y. OTA, 1959: Studies on the blast disease of rice in humus rich paddy field, withspecial reference to soil conditions. 4. Occurrence of blast disease on rice plant transplantedsuccessively in the nursery bed. Bull. Tohoku Natl, Agr. Expt. Sta. 16, 6—12.

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ViLLAREAL, R. L., D. R. MACKENZIE, R. R. NELSON, and W. R. COFFMAN, 1980: Apparent infectionrates of Pyricularia oryzae on different nee cultivars. Phytopathology 70, 122^1—1226.

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J. Phytopathology, Bd. 136, Heft 1