assessing the effect of resistant-susceptible associations and determining thresholds for...

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Assessing the effect of resistant-susceptible associations and determiningthresholds for associations in suppressing leaf and neck blast of rice

N.P. Castilla a, L. Willocquet a,*, S. Suwarno b, S. Santoso b, A. Nasution b, Y. Sulaeman b,S. Savary a, C.M. Vera Cruz a

a International Rice Research Institute, Plant Breeding, Genetics and Biotechnology Division, DAPO Box 7777, Metro Manila, Philippinesb Indonesian Center of Rice Research, Sukamandi, Indonesia

a r t i c l e i n f o

Article history:Received 29 May 2009Received in revised form24 November 2009Accepted 25 November 2009

Keywords:Magnaporthe oryzaeRiceBlastDisease-managementHost genetic diversity

a b s t r a c t

Blast has become a major problem in the upland rice of Indonesia, as the improved resistant varietieswith high yield performances released in the 90’s became susceptible only a few years after their release.The present study investigated the efficiency of association of susceptible and resistant varieties inreducing leaf and neck blast and increasing yield. Two sets of experiments were conducted in fields withassociations through using different proportions of resistant variety, and two types of susceptible variety(moderately and very susceptible) in the association. The efficiency of the association was measured bycomparing disease intensity and yield in the variety when grown in association and when grown asa pure stand. ANOVAs and logistic regressions indicated that resistant-susceptible variety association wasmore efficient in reducing neck blast than leaf blast. Associations were more efficient (1) when theproportion of resistant variety was increased, (2) when a moderately susceptible variety was usedinstead of a very susceptible variety, and (3) when disease level in the pure stand was higher. Associa-tions did not penalize the performances of the resistant variety, neither in terms of disease intensity, noryield. Logistic regressions allowed to estimate the probability of a given association to reduce disease (orto increase yield) above a preset threshold, according to the proportion of resistant variety and to thetype of susceptible variety.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

Rice blast, caused by Magnaporthe oryzae, is an importantdisease in upland and rainfed tropical and subtropical areas (Ou,1987; Zeigler et al., 1994). The pathogen can infect several organs ofthe rice plant: leaves, collars, necks and panicles (Ou, 1987;Pinnschmidt et al., 1995). Neck blast is particularly damaging, asinfections will result in significant yield reduction of the damagedpanicle (Ou, 1987). Blast is one of the major diseases in the uplandrice (Teng et al., 1990), including in Indonesia (Suwarno et al.,2001a). In conventional upland rice production, traditional varie-ties are in general resistant to blast, and have a good grain quality.These traditional varieties however have generally a low attainableyield, a long duration cycle, and a large plant height (Suwarno et al.,2001a). Breeding efforts have been focusing on improved varietieswith resistance to blast and higher yield. These varieties, however,when extensively grown over large areas, become susceptible toblast fungus several years after their release (Suwarno et al., 2001a).

New management strategies are required to enable the efficientand sustainable use of these improved varieties.

The association of resistant and susceptible varieties has beensuggested decades ago as a disease management tool (Leonard,1969). Such a strategy has proven to be efficient for severalpathosystems (Garrett and Mundt, 1999), for perennial and annualcrops. The epidemiological characteristics of M. oryzae allow pre-dicting that variety association could be an efficient control of blast(Garrett and Mundt, 1999). Indeed, such a strategy has been appliedsuccessfully for blast management at a large scale in the Yunnanprovince of China (Zhu et al., 2000). Generalizing the efficiency ofthis disease control strategy in upland rice conditions, with varie-ties having different levels of susceptibility to blast, however,remains to be demonstrated and analysed so as to provide guidancefor the type of association (type of component, proportion ofresistant variety) under upland rice conditions in Indonesia.

The objectives of the present study were: (1) to assess the effectof the proportion of resistant variety in the association on leaf blast,neck blast, and yield, as compared to pure susceptible and resistantstands; (2) to assess the effect of the level of susceptibility of thenon-resistant component on the efficiency of association in

* Corresponding author. Tel.: þ63 2 580 5600; fax: þ63 2 580 5699.E-mail address: [email protected] (L. Willocquet).

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Crop Protection

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0261-2194/$ – see front matter � 2009 Elsevier Ltd. All rights reserved.doi:10.1016/j.cropro.2009.11.016

Crop Protection 29 (2010) 390–400


suppressing disease; and (3) to predict the impact of these twofactors (proportion of resistant and level of susceptibility of thesusceptible variety) on disease suppression and on yield gain.

2. Materials and methods

2.1. General description of experiments

Field experiments were conducted to quantify the effects ofproportion of resistant variety and the level of susceptibility of thenon-resistant component in the association, on the efficiency of theresistant-susceptible association to reduce blast and increase yield.Data were also collected on the resistant component of the associ-ation in order to quantify the effect of association on disease intensityand yield on the resistant variety. A first group of two experiments(2004–2005) was conducted with proportions of 4 resistant and 1susceptible varieties (4Rþ 1S) and 3 resistant to 3 susceptible vari-eties (3Rþ 3S), and with two susceptible varieties: one verysusceptible, and the other moderately susceptible to blast. A secondgroup of two experiments (2006–2007) was conducted with lowerproportions of resistant component (1Rþ 2S until 1Rþ 6S), and withthe two susceptible varieties used in the first group of experiments.In each experiment, plots were established with the associations tobe tested, and with the corresponding susceptible and resistantvarieties grown as sole crop. The effects of association on disease andyield were assessed by comparing disease and yield levels betweenthe varieties when grown in an association, and when grown in purestand. Daily temperature and rainfall data corresponding to the siteand timeframe of these experiments were retrieved from the NASA-POWER web site (http://power.larc.nasa.gov/).

2.2. Varieties used in the experiments

The varieties used for the experiments were chosen on the basis oftheir characteristics in terms of level of resistance to blast, and of typeof variety - traditional vs. improved (Suwarno et al., 2001a,b). In bothgroups of experiments, two improved varieties susceptible to blastwere used: Cirata (S1), a short variety released in 1997 that is highlysusceptible to blast; and Way Rarem (S2), a variety moderatelysusceptible to blast released in 1994. Both varieties were actuallyresistant to blast when released in Indonesia, and that resistance wasovercome by the pathogen population within a few years after thevarieties were released (Suwarno et al., 2001a). In the first group ofexperiments, two resistant varieties were used: Situ Patenggang (R1),an improved variety, and Lampung Arak (R2), a traditional, tall,variety. Sirendah (R3), a traditional, tall variety, moderately resistantto leaf blast and highly resistant to neck blast was used in the secondgroup of experiments. Improved varieties have a crop cycle durationof 115–120 days, and are 1–1.1 m high, whereas traditional varietieshave a 130–135 day crop cycle, and are 1.50–1.60 m high.

2.3. Experimental design and crop management

All experiments were conducted in farmers’ fields in the uplandarea of Rama Murti III (4�510 S; 105�230 E), a rice-growing village ofthe Lampung province of Sumatra, Indonesia. In all experiments,seeds were sown using the dibbling method, with 4–5 seeds placedper hole. Sowing was performed at the onset of rains. Intra- andinter-row distance between hills was 20 and 25 cm, respectively.

In the first group of experiments (2004 and 2005), sowing wasdone on November 10–13, 2003, and Dec 4–8, 2004, for the first andsecond year of experiment, respectively. Chicken manure wasapplied at a rate of 2500 kg/ha within the week after sowing, and N,P, and K were applied at rates of 150,140, and 100 kg/ha, respectively,in two splits: at tillering and early booting stages. Improved and

traditional varieties were harvested on 5–8 March and on 24 March,respectively, in the first year of experiment, and on 1–2 April and 15April in the second year. In this first group of experiments, seventreatments were established: three associations of resistant andsusceptible variety (4R1þ1S1, 4R1þ1S2, 3R2þ 3S2), and fourcontrols where the varieties were grown as pure stands (S1, S2, R1,and R2). In the association 4Rþ 1S, four consecutive rows of ricewere established with the resistant variety, followed by 1 rowestablished with the susceptible variety. Repetitive patterns of threeconsecutive rows of R2 and three consecutive rows of S2 wereestablished in the 3R2þ 3S2 association. The seven treatments wereestablished with four replications, according to a randomly completeblock design. Individual plots were 2.5� 5.5 m, and a 5-m widebuffer area between plots was established with the resistant(Suwarno et al., 2001a,b) variety Limboto to reduce interplot inter-ference (Kiyosawa and Shiyomi, 1972; Wolfe, 1985).

In the second group of experiments (2006 and 2007), seedswere sown on November 12–16, 2005, and January 10–13, 2007 forthe first and second experimental year, respectively. Chickenmanure was applied at a rate of 2300 kg/ha within the week aftersowing, and N, P, and K were applied at rates of 180, 60, and 60 kg/ha, respectively, in two splits: at tillering and early booting stages.Improved and traditional varieties were harvested on 17–20 Marchand on 15 April, respectively, in the first year of experiment, and on5–8 May and 19 May in the second year. The second group ofexperiments was established according to a split-plot design with 4replications, where the main unit was the type of susceptiblevariety (2 levels: S1, very susceptible, and S2, moderately suscep-tible) and the sub-unit was the proportion of resistant variety (pureresistant, 1Rþ 2S, 1Rþ 4S, 1Rþ 6S, and pure susceptible stand).Each individual plot was 6� 8.5 m, and a 6-m wide buffer areabetween plots was established with the resistant variety Limboto.

2.4. Disease assessment and yield measurement

Assessments of leaf blast were performed between panicleinitiation and early booting stage for the improved varieties, whichcorresponded to maximum tillering to panicle initiation for thetraditional, longer duration, varieties. Assessments of neck blastwere performed at maturity, and hard dough to ripening for theimproved and traditional varieties, respectively.

At each assessment, 20 hills per variety were selected at randomin each individual plot for disease assessment. Thus, 20 hills wereassessed in plots with pure stands, and 40 hills were assessed in plotswhere an association of resistant and susceptible variety had beenestablished. During the first assessment, the number of leaves with atleast one leaf blast lesion was counted on each assessed hill; 5 tillersper hill were selected at random, and the number of leaves per tillerwas recorded, as well as disease severity for all leaves on each of thesetillers on a 0–25% scale (0, 0.5, 1, 2, 5, 10, and 25% grades were used).These data allowed computing the average of leaf blast severity (%diseased leaf area), and disease incidence (% diseased leaves) for eachhill assessed. In the second assessment, the number of panicles withneck blast symptoms, and the total number of panicles were countedfor each assessed hill. Mean leaf blast severity, leaf blast incidence,and neck blast incidence (% panicles with neck blast symptoms) wascomputed for each variety in each individual plot.

Disease suppression by associations was measured by comparingthe disease intensity of the susceptible variety when in associationwith a resistant variety, and when grown as a pure stand. In the sameway, the disease change in the resistant variety was assessed bycomparing disease intensity of the variety when grown in associationwith the susceptible variety, and when grown as a pure stand.

In all experiments, a 5 m2 area was harvested in each individualplot, the corresponding grain yield was measured, and adjusted to

N.P. Castilla et al. / Crop Protection 29 (2010) 390–400 391


0

5

10

15

20

4R1S S 4R1S S

Leaf

blas

tinc

iden

ce(%

)

2004 2005

B

40

60

80

100

4R1S S 4R1S S

Nec

kbl

asti

ncid

ence

(%)

2004 2005

C

0.0

0.2

0.4

0.6

0.8

1.0

4R1S R 4R1S R

Leaf

blas

tinc

iden

ce(%

)

2004 2005

F

0

1

2

3

4

4R1S R 4R1S R

Nec

kbl

asti

ncid

ence

(%)

2004 2005

G

0

1

2

3

4

5

4R1S S 4R1S S

Yie

ld(t/

Ha)

2004 2005

D

0

1

2

3

4

5

4R1S R 4R1S R

Yie

ld(t/

Ha)

2004 2005

H

0.0

0.5

1.0

1.5

2.0

4R1S S 4R1S S

Leaf

blas

tsev

erity

(%)

2004 2005

A

0.00

0.05

0.10

0.15

0.20

4R1S R 4R1S R

Leaf

blas

tsev

erity

(%)

2004 2005

E

Suceptible cultivar Resistant cultivar

Fig. 1. Blast intensity and yield in susceptible and resistant varieties, in association (proportion of 4 resistant for 1 susceptible variety) or as pure stand, in the first experiment, withCirata (S1) as susceptible variety, and Situ patenggang (R1) as resistant variety. In each graph, disease intensity or yield are compared for the variety grown solely, or in association,for the two years of experiment. A to D: disease intensity and yield for the susceptible variety; E to H: disease intensity and yield for the resistant variety. A and E: leaf blast severity;B and F: leaf blast incidence; C and G: Neck blast incidence; D and H: yield. 4R1S: association with a proportion of 4 resistant for 1 susceptible variety; S: susceptible variety grownsolely; R: resistant variety grown solely. In each graph, bars represent means over four replications, and error bars represent the standard error of the mean.

N.P. Castilla et al. / Crop Protection 29 (2010) 390–400392


a moisture content of 14%. In plots with association of resistant andsusceptible varieties, the yield of each variety was measuredseparately, and computed as the corresponding yield, would the hilldensity of the variety correspond to that of a pure crop:

Y ¼ Ym=prop

where Ym is the yield measured in the plot, and prop is theproportion of the corresponding variety in the association.

0

1

2

3

4

5

4R1S S 4R1S S

Leaf

blas

tinc

iden

ce(%

)

2004 2005

B

0

20

40

60

4R1S S 4R1S S

Nec

kbl

asti

ncid

ence

(%)

2004 2005

C

0.0

0.5

1.0

1.5

2.0

4R1S R 4R1S R

Leaf

blas

tinc

iden

ce(%

)

2004 2005

F

0

1

2

3

4

5

4R1S R 4R1S R

Nec

kbl

asti

ncid

ence

(%)

2004 2005

G

0

2

4

6

8

4R1S S 4R1S S

Yie

ld(t/

Ha)

2004 2005

D

0

2

4

6

8

4R1S R 4R1S R

Yie

ld (t

/Ha)

2004 2005

H

0.0

0.1

0.2

0.3

0.4

0.5

4R1S S 4R1S S

Leaf

blas

tsev

erity

(%)

2004 2005

A

0.00

0.02

0.04

0.06

0.08

0.10

4R1S R 4R1S R

Leaf

blas

tsev

erity

(%)

2004 2005

E

Susceptible cultivar Resistant cultivar

Fig. 2. Blast intensity and yield in susceptible and resistant varieties, in association (proportion of 4 resistant for 1 susceptible variety) or as pure stand, in the first experiment, withWay Rarem (S2) as susceptible variety, and Situ patenggang (R1) as resistant variety. Same detailed legend as in Fig. 1.



2.5. Data analyses

Data analysis was conducted in three steps. First theplots with disease intensity and yield means of the varieties

(susceptible and resistant) when in pure stand and in associationwere visually examined. Second, analyses of variance were per-formed to test the effect of the proportion of resistant variety,and of the level of susceptibility in the non-resistant component,

0

2

4

6

3R3S S 3R3S S

Leaf

bla

st in

cide

nce

(%)

2004 2005

B

0

20

40

60

3R3S S 3R3S S

Nec

k bl

ast i

ncid

ence

(%)

2004 2005

C

0

1

2

3

4

5

3R3S R 3R3S R

Leaf

bla

st in

cide

nce

(%)

2004 2005

F

0

1

2

3

3R3S R 3R3S R

Nec

k bl

ast i

ncid

ence

(%)

2004 2005

G

0

2

4

6

3R3S S 3R3S S

Yie

ld (t

/Ha)

2004 2005

D

0

2

4

6

3R3S R 3R3S R

Yie

ld (t

/Ha)

2004 2005

H

0.0

0.1

0.2

0.3

0.4

3R3S S 3R3S S

Leaf

bla

st s

ever

ity (%

)

2004 2005

A

0.0

0.1

0.2

0.3

0.4

3R3S R 3R3S R

Leaf

bla

st s

ever

ity (%

)

2004 2005

E

Susceptible cultivar Resistant cultivar

Fig. 3. Blast intensity and yield in susceptible and resistant varieties, in association (proportion of 3 resistant for 3 susceptible variety) or as pure stand, in the first experiment, withWay Rarem (S2) as susceptible variety, and Situ patenggang (R1) as resistant variety. Same detailed legend as in Fig. 1, except: 3R3S: association with a proportion of 3 resistant for 3susceptible variety.



on disease intensity and yield. Finally, all data sets werecombined and analysed in a series of logistic regressions topredict the effect of association in suppressing blast, and inincreasing yield.

2.5.1. First group of experimentsFor each year of experiment and for each association studied

(4R1þ1S1; 4R1þ1S2; and 3R2þ 3S2), mean values for diseaseintensity measurements (leaf blast severity, leaf blast incidence,and neck blast incidence) and mean yield of the susceptiblecomponent were compared when grown in association with theresistant variety, or when grown alone. This was done first by visualexamination of bar charts of the means, and second by performinga mixed model ANOVA (Garrett et al., 2004; Schabenberger andPierce, 2002) for each variable (disease intensities and yield). Thelatter analysis allowed testing the effect of association on diseaseintensities and on yield of the susceptible variety. In the same way,the effect of association on disease intensities and on yield of theresistant variety was analysed using graphs and mixed modelANOVAs. The procedure MIXED of SAS was used, and data ofdisease intensity were transformed using the arcsine trans-formation prior to the analyses to stabilize variances (Gomez andGomez, 1984).

2.5.2. Second group of experimentsIn each year (2006 and 2007), the effects of (1) the type of

susceptible variety (S1 or S2) involved in the association and (2)the proportion of susceptible variety in the association (2, 4, and6 susceptible for 1 resistant) on disease intensity and yield of thesusceptible variety were analysed in a series of mixed modelANOVAs. The analyses considered a split-plot design, with typeof susceptible variety as main plot, proportion of resistant varietyas sub-unit, and replication as a random effect. ANOVAs wereperformed on leaf blast severity, leaf blast incidence, neck blastincidence, and yield, using the procedure MIXED of SAS. The datawere transformed using the arcsine transformation prior to theanalyses. The same types of analyses were performed to test theeffects of type of susceptible variety and of proportion of resis-tant variety on disease intensity and yield of the resistantvariety.

2.5.3. Combined analysisA series of logistic regressions was performed using data from

the four years of experiments in order to identify the factorsassociated with the effect of association in reducing disease, and inincreasing yield of the susceptible variety. The binary logistic modelconsidered was (Agresti, 2002; Harrell, 2001):

Ln½PðY ¼ 1Þ=ð1� PðY ¼ 1ÞÞ� ¼ aþ biXi (1)

where Y is a binary (0,1) response variable taking the value 1 if thereduction in disease in the association, as compared tothe susceptible variety grown solely, is above a given threshold. Inthe case of yield, Y takes the value 1 if the increase in yield (yieldgain) of the susceptible variety in the association is above a giventhreshold. For leaf blast severity, leaf blast incidence, and neck blastincidence, the disease reduction was computed as:

RedðDÞ ¼ 1��

Ds;mono � Ds;assoc��

Ds;mono�

(2)

where D is leaf blast severity, leaf blast incidence, or neck blastincidence; Ds, mono is the disease intensity of the susceptible varietywhen grown alone in the plot; and Ds, assoc is the disease intensity ofthe susceptible variety when in association with a resistant variety.

For yield, the yield gain is computed as:

GainðYLÞ ¼��

Ys;assoc � Ys;mono��

Ys;mono�

(3)

where YL is yield, Ys, mono is the yield of the susceptible varietywhen grown alone in the plot; and Ys,assoc is the yield of thesusceptible variety when in association with a resistant variety.

Logistic regressions involving disease (leaf blast severity, leafblast incidence, and neck blast incidence) reduction were testedwith the following predictors: (i) ratio of the planting density of theresistant over the susceptible variety in the association (RoverS);(ii) type of susceptible variety (S1 or S2) grown in the association;and (iii) disease level of the susceptible and resistant variety grownalone. Logistic regressions on the yield gain included as predictors(i) RoverS, (ii) the type of susceptible variety, and (iii) disease (leafblast and neck incidence) and yield levels of the correspondingsusceptible and resistant varieties when grown alone. For eachregression tested, the complete model was tested in a first stage,

Table 1Analyses of variance with mixed models for the effect of association on disease intensity and yield in the three associations tested in two years (2004 and 2005).

Variable Variety Year Assoc. 4 R1 and 1 S1 Assoc. 4 R1 and 1 S2 Assoc. 3 R2 and 3 S2

F Value Pr> F F Value Pr> F F Value Pr> F

Leaf blast severitya Susceptible 2004 0.07 0.80 0.58 0.50 0.51 0.530.71 0.46 2.97 0.18 0.11 0.76

Resistant 2004 1 0.39 0.81 0.40 1.17 0.361 0.39 1 0.39 0.03 0.88

Leaf blast incidencea Susceptible 2004 0.18 0.70 0.43 0.54 0.5 0.532005 0.53 0.52 2.83 0.19 0.24 0.66

Resistant 2004 1 0.39 7.02 0.01 0.83 0.432005 1 0.39 1 0.39 0.09 0.77

Neck blasta Susceptible 2004 2.28 0.23 15.83 0.06 23.28 0.022005 17.92 0.02 11.24 0.04 0.18 0.70

Resistant 2004 0.51 0.63 8.14 0.06 1 0.392005 0.94 0.39 1.33 0.33 3.39 0.12

Yield Susceptible 2004 8.41 0.03 0.27 0.63 2.81 0.142005 6.76 0.08 1.02 0.39 0.93 0.41

Resistant 2004 0.72 0.50 0.44 0.56 0.05 0.832005 0.56 0.51 0.37 0.59 0.13 0.75

S1: Cirata; S2: Way Rarem; R1: Situ Patenggang; R2: Lampung Arak.a Data transformed by arsin(sqrt(x/100)) prior to ANOVA.



and in a second, final stage only predictors contributing signifi-cantly (P< 0.05) to the regression were retained.

Logistic regressions were conducted when considering threethresholds: 0, 10%, and 50%. A threshold of zero represents thepresence of any effect of association in reducing disease (orincreasing yield), even at low level. Thresholds of 10% and 50%represent the effect of association in reducing disease (or increasingyield), by 10% and 50%, respectively.

3. Results

3.1. First group of experiments: effect of three associations ondisease intensity and yield (2004 and 2005)

3.1.1. Association of 4 R1 and 1 S1In both years, disease intensity (leaf blast severity, leaf blast

incidence, neck blast incidence) were much larger, and grain yield

0

10

20

30

40

50

1R2S 1R4S 1R6S S 1R2S 1R4S 1R6S S

2006 2007

Leaf

bla

st in

cide

nce

(%)

B

95

96

97

98

99

100


Nec

k bl

ast i

ncid

ence

(%)

2006 2007

C

0

5

10

15

1R2S 1R4S 1R6S R 1R2S 1R4S 1R6S R

Leaf

bla

st in

cide

nce

(%)

2006 2007

F

0

1

2

3


Nec

k bl

ast i

ncid

ence

(%)

2006 2007

G

0

1

2

3


Yiel

d (t/

Ha)

2006 2007

D

0

1

2

3

4

5


Yie

ld (t

/Ha)

2006 2007

H

0

0.5

1

1.5

2

2.5

3


Leaf

bla

st s

ever

ity (%

)

2006 2007

A

0

0.1

0.2

0.3

0.4


Leaf

bla

st s

ever

ity (%

)

2006 2007

E

Susceptible variety Resistant variety

Fig. 4. Blast intensity and yield in susceptible and resistant varieties, in association (varying proportion of resistant to susceptible variety) or as pure stand, in the secondexperiment, with Cirata (S1) as susceptible variety, and Sirendah (R3) as resistant variety. In each graph, disease intensity or yield are compared for the variety grown solely, or inassociation, for the two years of experiment. A to D: disease intensity and yield for the susceptible variety; E to H: disease intensity and yield for the resistant variety; A and E: leafblast severity; B and F: leaf blast incidence; C and G: Neck blast incidence; D and H: yield. 1R2S: association with a proportion of 1 resistant for 2 susceptible variety; 1R4S:association with a proportion of 1 resistant for 4 susceptible variety; 1R6S: association with a proportion of 1 resistant for 6 susceptible variety; S: susceptible variety grown solely;R: resistant variety grown solely. In each graph, bars represent means over four replications, and error bars represent the standard error of the mean.



was lower for the susceptible (Fig. 1A–D) than for the resistant(Fig. 1E–H) variety. Maximum leaf blast severity, leaf blast inci-dence, and neck blast incidence were 0.6%, 9%, and 99%, respec-tively, for the susceptible variety, while the corresponding diseaseintensities were 0.02, 0.4, and 1.7% for the resistant variety.Maximum yield was 3.3 and 4.8 t/ha for the susceptible and resis-tant variety, respectively.

A lower disease intensity (leaf blast severity, leaf blast incidence,and neck blast incidence) and higher yield were observed for thenon-resistant component when grown in association than whenalone in both years. Neck blast and yield had similar ranges in 2004and 2005, while much higher leaf blast was observed in 2004 thanin 2005 (Fig. 1 A–D).

In the case of the resistant variety, disease intensity was similaror higher when the variety was grown in association than whengrown alone in both years (Fig. 1E–H). Yields were similar whencomparing association and pure stand, and when comparing bothyears (2004 and 2005; Fig. 1H).

3.1.2. Association 4 R1 and 1 S2For this association, a lower level of disease intensity on the

non-resistant component was also observed in the association(Fig. 2A–C). Disease intensity was much lower for S2 (Fig. 2A–C)than for S1 (Fig. 1A–C), and yield was higher for S2 (Fig. 2D) than forS1 (Fig. 1D). Disease intensities (leaf and neck blast) were muchhigher in 2004 than in 2005. In the case of the resistant variety,

0

2

4

6


Leaf

blas

tinc

iden

ce(%

)

2006 2007

v

B

0

10

20

30

40

50


Nec

kbl

asti

ncid

ence

(%)

2006 2007

C

0

2

4

6

8


Leaf

blas

tinc

iden

ce(%

)

2006 2007

F

0

1

2

3


Nec

kbl

asti

ncid

ence

(%)

2006 2007

G

0

1

2

3

4


Yie

ld(t/

Ha)

2006 2007

D

0

1

2

3

4

5


Yie

ld(t/

Ha)

2006 2007

H

0

0.1

0.2

0.3


Leaf

blas

tsev

erity

(%)

2006 2007

A

0

0.1

0.2

0.3


Leaf

blas

tinc

iden

ce(%

)

2006 2007

E

Susceptible variety Resistant variety

Fig. 5. Blast intensity and yield in susceptible and resistant varieties, in association (varying proportion of resistant to susceptible variety) or as pure stand, in the secondexperiment, with Way Rarem (S2) as susceptible variety, and Sirendah (R3) as resistant variety.Same detailed legend as in Fig. 4.



similar patterns were observed in this association (4R1þ1S2;Fig. 2E–H) than in the 4R1þ1S1 association (Fig. 1E–H).

3.1.3. Association 3 R1 and 3 S2In this association, disease intensities were similar when the

susceptible variety was grown in association, except for leaf blastseverity and neck blast incidence in 2004, which were lower whenthe susceptible variety was grown in association with the resistantvariety (Fig. 3A, C). A yield gain of the susceptible variety whengrown in association, was also observed in both years (Fig. 3D).

3.1.4. Analyses of varianceResults of ANOVAs to test the effect of association on disease

intensities and yield, for the three associations (4R1þ1S1,4R1þ1S2, 3R2þ 3-S2) are shown in Table 1. A significant (P< 0.05)effect of association was found mainly for neck blast incidence onthe susceptible variety (3 cases out of 6 year by associationcombinations). A significant effect was also found for the effect onleaf blast incidence in the resistant variety in 2004 in the 4R1þ1S2association, and for yield of the susceptible variety for the4R1þ1S1 association in 2004.

3.2. Second group of experiments: effect of proportion of resistantvariety and of susceptible variety type on disease intensity and yield(2006 and 2007)

When considering the associations of S1 and R3 with differentproportions of S1 (Fig. 4), a trend of decreasing proportion ofresistant variety corresponding to a decrease in disease intensitywas found only for neck blast incidence in 2006 (Fig. 4C). Whencomparing years, leaf blast was higher in 2007 than in 2006, andyield was larger in 2006 than in 2007 for both varieties (Fig. 4).

The association of R3 and S2 indicated an increase of diseasewhen the proportion of susceptible variety increased for leaf blastand neck blast incidence in 2006 (Fig. 5B, C). Leaf blast was higherin 2007 than in 2006 (Fig. 5A, B), while neck blast was lower in2007 than in 2006 (Fig. 5C). As for the other association tested (R3and S1), yield was lower for both varieties in 2007 than in 2006, butwas not affected by the proportion of susceptible variety in theassociation (Fig. 5D, H). In this association, disease levels in S2 weremuch lower than disease levels observed for S1 (Fig. 4).

The results of ANOVAs (Table 2) indicated that the effect of theproportion of resistant variety in the association was significant(P< 0.05) only for neck blast on the resistant variety in 2006, andwas nearly significant (0.07) on the corresponding susceptiblevariety. The type of susceptible variety used in the association (S1or S2) had a significant effect (P< 0.05) on disease intensity(severity and incidence on leaf blast, neck blast) and yield of thesusceptible variety in both years (Table 2).

3.3. Combined analysis of the four-year data set: guidelines for riceassociations to control blast

The logistic regression for reduction of leaf blast severity indi-cated that disease severity on the corresponding susceptible purestand was the only significant predictor, with a positive estimate,for thresholds of 0 and 10% (Table 3). No significant regression wasderived for a 50% threshold in disease severity reduction. Aregression for leaf blast incidence could be fitted only fora threshold of 50%. In that case, leaf blast incidence on the corre-sponding resistant pure stand was the only significant parameter,with a negative estimate.

When analyzing the reduction of neck blast incidence and theincrease in yield, the proportion of resistant variety over suscep-tible variety in the association (RoverS) and the type of susceptiblevariety (S1 or S2) were consistently significant, and with a positiveestimate (Table 3). Therefore, the odds for reduction in neck blastincidence and for yield gain were increased (1) if the proportion ofresistant variety in the association increased, and (2) if themoderately susceptible variety instead of the very susceptiblevariety was used in the associations. Neck blast incidence on thepure susceptible stand contributed to several regressions on neckblast reduction and yield gain, with a positive estimate, indicatingthat the odds of the association having an effect are increased whendisease level is higher on the corresponding susceptible variety.

4. Discussion

The overall results obtained in this series of experimentsconform to former studies (Mundt, 1994; Zhu et al., 2000), inshowing that the association of resistant and susceptible ricevarieties can efficiently control blast. Analysis of these

Table 2Analyses of variance with mixed models for the effect of proportion of resistant variety and type of susceptible variety on disease intensity and yield in a resistant-susceptiblevariety association in two years (2006 and 2007).

Variable Variety Year Effect of susceptible varietyb Effect of proportion of R

F Value Pr> F F Value Pr> F

Leaf blast severitya Susceptible 2006 21.91 0.0184 1.71 0.202007 166.73 <0.0001 0.15 0.93

Resistant 2006 0.2 0.6723 0.25 0.862007 0.4 0.5324 2.4 0.10

Leaf blast incidencea Susceptible 2006 75.10 <0.0001 1.04 0.402007 119.46 0.0016 0.37 0.77

Resistant 2006 2.46 0.2089 2.38 0.112007 1.32 0.2628 2.01 0.14

Neck blasta Susceptible 2006 102.85 <0.0001 2.84 0.072007 2707.75 <0.0001 0.52 0.67

Resistant 2006 0.10 0.7573 4.01 0.022007 0.22 0.646 1.94 0.15

Yield Susceptible 2006 16.27 0.0312 0.64 0.602007 10.80 0.0462 1.29 0.31

Resistant 2006 0.06 0.8184 2.39 0.122007 1.11 0.3017 0.99 0.41

a Data transformed by arsin(sqrt(x/100)) prior to ANOVA.b Cirata (S1) or Way Rarem (S2).



experimental data however provides further insights on (1) prob-abilities associated with the expected reduction of disease (andyield increase), depending on the proportion of resistant varietyand on the level of susceptibility of the non-resistant component –providing thus a formal basis for decision on the use of this controltool; (2) the simultaneous analysis of leaf blast and neck blastreduction from varietal association; and (3) the effect of associationon disease and yield of the resistant variety.

Graphic comparison of patterns of means (Figs. 1–5), theiranalyses (ANOVAs, Tables 1 and 2), and logistic regressions (Table3) indicate that depending on year-to-year experimental condi-tions, and depending on the set of varieties used, the associationdid not consistently reduce leaf blast in the susceptible variety,but did consistently reduce neck blast (Table 3). This differencemay be attributed to three causes. First, larger variances about themeans for leaf blast than for neck blast hampered to statisticallydetect differences (Figs. 1–5). Second, the numerical effect ofassociation was larger for neck blast than for leaf blast (Figs. 1–5).Third, the difference between resistant and susceptible varietiesmay be higher in the case of neck blast than in the case of leafblast.

This study also indicates that the effect of the association wasgreater when a moderately susceptible variety rather than a verysusceptible variety was used: associations with a moderatelysusceptible variety should be preferred to associations involvinga very susceptible component.

A low level of (leaf and neck) blast was observed on the resistantcomponent in all experiments. When these resistant varieties wereassociated with a susceptible variety, disease intensity was often,but not always, increased, but nevertheless always remained to lowlevels (e.g., below 4% for neck blast incidence). Furthermore, yieldswere never significantly affected when the resistant varieties wereintercropped with the susceptible ones. This indicates that resis-tant-susceptible association does not penalize the performance ofthe resistant variety.

Two differences are observed when comparing leaf and neckblast levels in the very susceptible variety Cirata (S1) and themoderately susceptible variety Way Rarem (S2). First, the rela-tionship between leaf blast and neck blast appears complex: lowlevel of leaf blast can be associated with high levels of neck blast(Cirata, 2005), and also high levels of leaf blast can be associatedwith low level of neck blast (Way Rarem, 2007). Such relationships,

Table 3Logistic regressions on disease intensity reduction and rice yield increase in associations.

Binaryvariable analysed

Explanatoryvariables testeda

Threshold forbinary variable

Likelihood Somer’s Explanatory variablesretainedb

Estimate statistics

Ratio P D Estimate SE P

Leaf RoverS 0% 6.84 0.0089 0.363 Intercept �0.73 0.30 0.016Blast typevarS sevmonoS 0.90 0.37 0.014

Severity LBsevmonoS 10% 6.84 0.0089 0.363 Intercept �0.73 0.30 0.016Reduction LBsevmonoR sevmonoS 0.90 0.37 0.014

50% no significant parameter

Leaf RoverS 0% no significant parameter

Blast typevarS

Incidence LBmonoS 10% no significant parameter

Reduction LBmonoR

50% 17.52 <0.0001 0.514 Intercept �0.38 0.33 0.240LBmonoR �2.24 1.12 0.046

Neck RoverS 0% 41.69 <0.0001 0.752 Intercept �22.68 9.35 0.015Blast typevarS RoverS 2.55 1.05 0.015Reduction NBmonoS typevarS 18.91 8.11 0.020

NBmonoR NBmonoS 0.41 0.17 0.017

10% 42.89 <0.0001 0.864 Intercept �5.61 1.73 0.001RoverS 1.06 0.31 0.001typevarS 4.29 1.42 0.003NBmonoS 0.06 0.03 0.033

50% 16.28 0.0003 0.58 Intercept �2.66 0.61 <0.0001RoverS 0.57 0.21 0.007typevarS 1.18 0.45 0.008

Yield RoverS 0% no significant parameter

Gain typevarSNBmonoS 10% 16.53 0.0009 0.523 Intercept �3.70 1.06 0.001NBmonoR RoverS 0.48 0.18 0.009LBmonoS typevarS 1.45 0.68 0.033LBmonoR NBmonoS 0.04 0.02 0.006

YmonoS 50% 22.84 <0.0001 0.672 Intercept �0.50 0.73 0.495YmonoR RoverS 1.20 0.33 0.000

typevarS 0.93 0.54 0.082YmonoS �1.40 0.51 0.006

a RoverS: Proportion of resistant over susceptible variety in the association. typevarS: very susceptible (S1; value given in the regression is�1) or moderately susceptible(S2; value given in the regression is þ1). LBsevmonoS: leaf blast severity when the susceptible variety is grown alone. LBsevmonoR: leaf blast severity when the resistantvariety is grown alone. LBmonoS: leaf blast incidence when the susceptible variety is grown alone. LBmonoR: leaf blast incidence when the resistant variety is grown alone.NBmonoS: neck blast incidence when the susceptible variety is grown alone. NBmonoR: neck blast incidence when the resistant variety is grown alone. YmonoS: rice yieldwhen the susceptible variety is grown alone. YmonoR: rice yield when the resistant variety is grown alone.

b Only significant variables were retained in the model.



and how they are influenced by host plant resistance, requirefurther analysis. Second, the efficiency of resistance to neck blast inWay Rarem was not consistent over years: whereas neck blastincidence was very low (3–4%) in 2005 and 2007, it was muchlarger (30–40%) in 2004 and 2006, which was however lower thanneck blast incidence observed for Cirata in these years (80–100%).This difference in neck blast incidence could not be explained bythe level of leaf blast incidence (for example, high leaf blast inci-dence in 2007 was associated with low neck blast). One possibleexplanation may be that the component of resistance to neck blastin Way Rarem is (1) specific to the neck blast pathogen, whosepopulations may vary from one year to another, and (2) efficienttowards only a fraction of the pathogen population; this fractionwas large in 2005 and 2007, and small in 2004 and 2006. Thishypothesis is supported by the fact that Way Rarem showsa differential reaction of resistance to leaf blast depending on theisolate (Suwarno et al., 2001a). Again, further studies would benecessary to examine the level of specificity of resistance to neckblast, in order to guide the deployment of resistant varieties overspace and time.

The patterns of weather (temperature, rainfall) conditionsduring the experiments could not be directly associated with thevariation in leaf blast levels observed between years. The very lowlevels of yield observed in 2007 could be related to a 14 day-periodof drought which occurred early in the crop cycle, between 19 and33 days after sowing.

The logistic regressions indicated that disease reductionoccurred more often when the level of disease in the susceptiblevariety grown as sole crop increased. This may be explained by thefact that the variety association effect accumulates over diseasecycles (Garrett and Mundt, 1999; Leonard, 1969): if the number ofdisease cycles increases, disease intensity will increase in thesusceptible variety grown alone, and thus the efficiency of theassociation will be increased.

Results of the logistic regressions can be translated into termsthat can be useful for decision making in the case of the use ofresistant-susceptible association for blast management. Such anapproach has been used in other pathosystems, for other diseasemanagement tools, for example for Sclerotinia stem rot of oilseed rape (Yuen et al., 1996), Sclerotinia stem rot of soybean (Milaet al., 2004), and Stewart’s disease of corn (Esker et al., 2006). Forexample, the regression obtained for a neck blast reduction of50% (Table 3) indicates that such a reduction can be expectedfrom the establishment of an association of 2, 4, and 6 resistantvarieties with one moderately susceptible variety with a proba-bility of 41%, 69%, and 87%, respectively. Corresponding proba-bilities when the association is established with a resistant anda very susceptible variety are much lower, i.e., 7%, 17%, and 40%,respectively.

The results obtained from these 4-year experiments confirm theefficiency of associating resistant and susceptible varieties inreducing neck blast and increasing yield of the susceptible variety,and indicate that this effect is observed in upland rice croppingsystems. The results further indicate that the efficiency is increasedwhen a moderately susceptible variety is used, as compared to

a highly susceptible variety. The combined analyses based onlogistic regressions can provide a basis to estimate expectedreduction in disease, and gain in yield, depending on the suscep-tible variety type (very or moderately susceptible) and on theproportion of resistant and susceptible varieties established in theassociation, and thus provide a basis for deciding the type of vari-eties to use, and at which proportion.

Acknowledgment

This study was partly funded by the Asian Development Bank(ADB).

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