Rice yellow mottle virus (RYMV) is a major biotic constraint for rice production in Africa. The resistance-breaking ability of Tanzanian RYMV strains and phylotypes (S4lm (Tz526), S4lv (Tz516), S4ug (Tz508), S5 (Tz429, Tz445), S6c (Tz486) and S6w (Tz539)) were tested by inoculating rice cultivars with RYMV 1 resistant alleles (Gigante ( rymv 1-2), Tog12387 ( rymv 1-3), Tog5681 ( rymv 1-3), Tog5438 ( rymv 1-4), Tog5672 ( rymv 1-4+ rymv 2) and Tog5674 ( rymv 1-5)) in a screen house. The results revealed multiple resistance-breaking strains and phylotypes on resistant cultivars Gigante, Tog12387, Tog5438 and Tog5681. However, the resistance breakdown was highly variable depending on the strain used, and disease severity ranged from 11% - 75.3%. The virulence potential of RYMV phylotype S4lm (Tz526) was similar to phylotype S6w (Tz539). The impact of strains and phylotypes on yield and its components in rice cultivars revealed highly significant differences (P ≤ 0.001). The lowest percent plant height reduction (2.8%), number of tillers per plant (2.5%), 1000 grain weight (2.7%), spikelet sterility (3.5%) and yield (5%) was recorded in rice cultivar Gigante inoculated with RYM V phylotype S6c (Tz486). Phylotype S6c (Tz486) despite being less virulent compared to other strains, its virus titer in rice cultivar Gigante (1.833) was higher than S5 (Tz429, Tz445) inoculated on Tog5674 (0.171, 0.207) and S6w (Tz539) inoculated on Tog5681 (0.283). The resistant-breaking strain S5 (Tz445) multiplied in resistant rice cultivar Tog5674 without inducing visible symptoms but showed positive reaction to ELISA with low virus titer. The strain S5 overcame wide range of resistant alleles including rymv1-2, rymv 1-3, rymv 1-4 and rymv 1-5 resistance, with exception of rymv 1-4 + rymv 2. The current results gave a new perspective for future identification of resistance-breaking mutations through sequencing of the RYMV genome in infected rice cultivars and mutagenesis of an infectious viral clone useful for future RYMV resistant breeding programs.
High genetic diversity of RYMV and evolution of resistant-breaking strains and phylotypes are challenges in developing durable disease resistant varieties. Management of RYMV has relied on the use of resistant varieties. Insect vector control and prophylactic measures such as high surveillance of seedbeds, fields and weed reservoirs are also used for management of RYMV despite time-consum- ing and variable efficiency [
The RYMV disease is characterized by mottling and yellowing symptoms, stunted growth, reduction of tiller formation and grain sterility [
Diverse groups of Rice yellow mottle virus strains are widely distributed in Tanzania [
High level of RYMV genetic diversity have accelerated emergence of new virulent strains which are capable for overcoming resistance of cultivated rice plants. The existence of high genetic diversity of RYMV in Tanzania [
The highly resistant rice cultivar Gigante has been reported to be effective against range of different RYMV strains from Central and West Africa [
Selection and breeding for disease resistant varieties is considered as the best means for control of RYMV disease and have successfully been done in several countries in Africa [
Identification of resistant-breaking RYMV strains in rice cultivars with RYMV1 resistant genes in Tanzania will enable identification of suitable resistant genotypes to improve local rice cultivars. Lack of information on the distribution of virulent strains and their reaction on differential rice genotypes, slows the process of breeding for RYMV resistance in Tanzania. The purpose of this study, therefore, was (i) to determine the pathogenic variation of Tanzanian RYMV strains and phylotypes against rice cultivars with known resistant genes and (ii) to assess the virulence and multiple resistant-breaking ability of RYMV strains and phylotypes on highly resistant cultivars as well as partially resistant and susceptible rice varieties and their effect on yield.
Infected leaf samples with typical symptoms of different RYMV strains and phylotypes used in this study are shown in
The sources of rice genotypes with known resistant genes used to screen Tanzanian RYMV strains and phylotypes are shown in
The resistance-breaking ability of different RYMV strains and phylotypes from
Isolate | Region | District | Date | Strain | Reference |
---|---|---|---|---|---|
Tz516 | Arusha | Monduli | 2014 | S4lv | Hubert et al. [ |
Tz508 | Kilimanjaro | Moshi | 2014 | S4ug | Hubert et al. [ |
Tz526 | Morogoro | Kilombero | 2013 | S4lm | Hubert et al. [ |
Tz429 | Morogoro | Kilombero | 2013 | S5 | Hubert et al. [ |
Tz445 | Morogoro | Ulanga | 2014 | S5 | Hubert et al. [ |
Tz486 | Morogoro | Ulanga | 2014 | S6c | Hubert et al. [ |
Tz539 | Morogoro | Kilombero | 2013 | S6w | Hubert et al. [ |
S4lv = Strain 4-Lake Victoria, S4ug = Strain 4-Uganda, S4lm = Strain 4-Lake Malawi, S6c = Strain 6-coast area, S6w = Strain 6-wide, Tz = Tanzania.
Rice genotype | Resistant genes | Sources of seeds | Reference |
---|---|---|---|
Azucena (Partial R.) | QTLs | IRD, France | Albar et al. [ |
IR64 (S. control) | rymv1-1 | IRD, France | Ndjiondjop et al. [ |
Gigante | rymv1-2 | IRD, France | Ndjiondjop et al. [ |
Tog12387 | rymv1-3 | AfricaRice | Jaw [ |
Tog5681 | rymv1-3 | IRD, France | Albar et al. [ |
Tog5438 | rymv1-4 | IRD, France | Thiemélé et al. [ |
Tog5672 | rymv1-4+ rymv2 | IRD, France | Thiemélé et al. [ |
Tog5674 | rymv1-5 | IRD, France | Thiemélé et al. [ |
SARO-5 (S. control) | Unknown | SUA | Msomba et al. [ |
SUA = Sokoine University of Agriculture, IRD = French National Research Institute for Sustainable Development, QTLs = Quantitative trait locus, S = Susceptible, R = Resistant.
Tanzania was evaluated on rice cultivars with RYMV1 gene. The reaction of the known rice resistant cultivars Gigante (rymv1-2), Tog12387 (rymv1-3), Tog 5681 (rymv1-3), Tog5438 (rymv1-4), Tog 5672 (rymv1-4+rymv2) and Tog 5674 (rymv1-5) against RYMV Tanzanian strains and phylotypes was tested in a screen house. Split-plot design with three replications was adopted. The strains were considered as the main-plot and the cultivar as the sub-plot. Partially resistant (Azucena), and susceptible local variety (SARO-5) and (IR64) were included as controls.
Sixty seeds of each variety were planted in each replication (2 seeds/hole) in plastic trays, measuring 48 cm length, 34 cm width and 9.5 cm depth filled with 10 kg of sterilized forest soil. The soil was mixed with N:P:K (15:15:15) at a rate of 8 g/tray before sowing of seeds followed by split application of Urea (8 g) 7 days after inoculation (DAI) and at early stage of flowering. Thirty plants of each cultivar in each replication were used. Trays were constantly irrigated with fresh tap water on the daily basis until maturity.
Inoculum was prepared by grinding infected rice leaves using a mortar and pestle in phosphate buffer saline with 0.5% Tween-20 (PBST 1X) at a ratio of 1:10 w/v [
Disease severity was assessed on individual plant basis using a rating scale of 1 - 9 [
The resistance-breakdown of each resistant rice cultivar were also compared by evaluating the area under disease progress curve (AUDPC) for each strain with the formula: AUDPC = Σ [(Si+1 + Si)/2] [ti+1 ? ti] i=1, where: Si = disease severity at the ith observation and ti = time (days) at the ith observation [
The last fully expanded leaf of each rice plant was collected 42 DAI for ELISA test. The rice leaves of each tested rice cultivar that did not show symptoms were also collected separately at 60 DAI for ELISA test.
Direct antibody sandwich enzyme-linked immunosorbent assay (DAS-ELISA) was used to test for the presence of RYMV in leaves harvested from inoculated rice cultivars following procedures described by Pinel et al. [
The effect of RYMV strains and phylotypes on the yield components of rice cultivars was evaluated according to procedures developed by Zouzou et al. [
where: Y/p = yield loss per panicle (%)
Data obtained were used to assess both inoculated and non-inoculated seedlings of each cultivar and thus, the impact of the RYMV disease on growth of the rice plants. Mean values were calculated and the impact of the disease was assessed using the following formula:
where: Ni = mean values on the seedlings not inoculated
I = mean values on the seedlings inoculated
Rice yellow mottle virus disease severity data were analyzed using GenStat Software Package (14th edition). Prior analysis data were arcsine transformed [
The following statistical model was used for analysis:
where Yij = Response of variables investigated, µ = General mean, Ci = ith effect of cultivars, Rj = jth effect of RYMV strains, CRij = Interaction due to cultivars and RYMV strains, Eij = Experimental error.
The resistance-breaking ability of Tanzanian RYMV strains and phylotypes were evaluated using rice differential cultivars with RYMV1 resistance gene. Disease progress and disease reaction classes are shown in
Rice cultivar | Incidence (%) | Severity (%) | ||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
S4lm (Tz526) | S4lv (Tz516) | S4ug (Tz508) | S5 (Tz445) | S5 (Tz429) | S6c (Tz486) | S6w (Tz539) | S4lm (Tz526) | S4lv (Tz516) | S4ug (Tz508) | S5 (Tz445) | S5 (Tz429) | S6c (Tz486) | S6w (Tz539) | |
Azucena | 19.6bc | 8.5ab | 3.4a | 54.6d | 45d | 1.1a | 35.5c | 31.3c | 19.5b | 17b | 50c | 51.3d | 15.9c | 37.5c |
Gigante | 10.6ab | 17.8bc | 2.2a | 47.1c | 38.8b | 1.3a | 17b | 19.4a | 24.7bc | 13.8a | 52d | 50.8c | 13.3b | 25.8b |
Tog12387 | 45.5d | 47.2d | 33.4c | 100f | 83.3f | 22.8b | 54.7d | 50e | 46.1d | 41.4d | 75.3f | 74f | 38.3d | 51d |
Tog5438 | 48.9d | 18.6c | 8.1b | 85.8e | 80.7e | 1a | 38.3c | 40.5d | 27.9c | 22.1c | 73e | 71.1e | 13.9b | 39.5c |
Tog5672 | 0.5a | 0.5a | 0.5a | 0.5a | 0.5a | 0.5a | 0.5a | 11.1a | 11.1a | 11.1a | 11.1a | 11.1a | 11.1a | 11.1a |
Tog5674 | 0.5a | 0.5a | 0.5a | 0.5a | 0.5a | 0.5a | 0.5a | 11.1ab | 11.1a | 11.1a | 11.1a | 11.1a | 11.1a | 11.1a |
Tog5681 | 26.2c | 0.5a | 0.5a | 40.9b | 40.6c | 0.5a | 13b | 32.2c | 11.1a | 11.1a | 38.2b | 38b | 11.1a | 29b |
S. control | ||||||||||||||
IR64 | 80.6e | 77.6e | 65.1d | 100f | 100g | 68.7c | 72.4e | 63.2f | 62e | 59.2e | 80.4g | 82g | 57.1e | 64.8e |
SARO-5 | 100f | 100f | 95e | 100f | 100g | 83.7d | 100.5f | 84.2g | 80.8f | 79.5f | 93.7h | 95.4h | 75.4f | 85f |
GM | 37 | 30.2 | 23.2 | 59 | 54.5 | 20 | 36.9 | 38.1 | 32.7 | 29.6 | 53.9 | 53.9 | 27.5 | 39.4 |
F test | *** | *** | *** | *** | *** | *** | *** | *** | *** | *** | *** | *** | *** | *** |
LSD 5% | 10.23 | 9.29 | 2.84 | 1.56 | 0.9 | 1.31 | 9.5 | 8.27 | 7.56 | 3.06 | 0.38 | 0.21 | 1.1 | 7.32 |
CV% | 16 | 17.8 | 7.1 | 1.5 | 1 | 3.8 | 14.9 | 12.5 | 13.3 | 6 | 0.4 | 0.2 | 2.3 | 10.7 |
*Values are means of three replicates. Numbers followed by the same letter in a column are not significantly different at P = 0.05, using Duncan’s Multiple Range Test. *** = Highly significantly different (P < 0.001), S = Susceptible, GM = Grand mean.
results showed multiple resistance-breaking in resistant cultivars Gigante (rymv1-2), Tog12387 (rymv1-3), Tog5681 (rymv1-3) and Tog5438 (rymv1-4) inoculated with Tanzanian RYMV strains S4lm (Tz526), S5 (Tz429, Tz445) and S6w (Tz539) respectively collected in Kilombero and Ulanga districts, Morogoro region (
The incidence of RYMV also varied significantly (P ≤ 0.05) between rice cultivars, strains and phylotypes (
Genotype | Resistant gene | S4lm (Tz526) | S4lv (Tz516) | S4ug (Tz508) | S5 (Tz445) | S5 (Tz429) | S6c (Tz486) | S6w (Tz539) | |||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
DSc | DRn | DSc | DRn | DSc | DRn | DSc | DRn | DSc | DRn | DSc | DRn | DSc | DRn | ||
Azucena | QTLs | 5.7c | MR | 4b | R | 3b | R | 8c | S | 8c | S | 3b | R | 6.7c | S |
Gigante | rymv1-2 | 4b | R | 4.6bc | MR | 1.6a | R | 8.1c | S | 8c | S | 1.6a | R | 5b | MR |
Tog12387 | rymv1-3 | 8e | S | 7.7d | S | 6.8d | S | 9c | HS | 8.9c | HS | 6.7c | S | 8d | S |
Tog5438 | rymv1-4 | 6.7d | S | 5.4c | MR | 4.6c | MR | 8.9c | HS | 8.8c | HS | 3b | R | 6.7c | S |
Tog5672 | rymv1-4+rymv2 | 1.2a | HR | 1.2a | HR | 1.2a | HR | 1.2a | HR | 1.2a | HR | 1.2a | HR | 1.2a | HR |
Tog5674 | rymv1-5 | 1.2a | HR | 1.2a | HR | 1.2a | HR | 1.2a | HR | 1.2a | HR | 1.2a | HR | 1.2a | HR |
Tog5681 | rymv1-3 | 5.7c | MR | 1.2a | HR | 1.2a | HR | 6.7b | S | 6.7b | S | 1.2a | HR | 5.7bc | MR |
S. control | |||||||||||||||
IR64 | rymv1-1 | 8.6ef | HS | 8.6de | HS | 8.4e | S | 9c | HS | 9c | HS | 8.3d | S | 8.6d | HS |
SARO-5 | Unknown | 9f | HS | 9e | HS | 7.5de | HS | 9c | HS | 9c | HS | 9d | HS | 9d | HS |
GM | 5.567 | 4.77 | 3.94 | 6.79 | 6.76 | 3.91 | 5.79 | ||||||||
F test | 0.8078 | 1.01 | 1.01 | 1.01 | 1.04 | 1.177 | 1.228 | ||||||||
LSD 5% | *** | *** | *** | *** | *** | *** | *** | ||||||||
CV% | 8.4 | 12.2 | 14.8 | 8.6 | 8.9 | 17.4 | 12.3 |
*Values are means of three replicates. Numbers followed by the same letter in a column are not significantly different at P = 0.05, using Duncan’s Multiple Range Test. *** = Highly significantly different (P < 0.001), S = Susceptible, GM = Grand mean, DSc = Disease score, DRn = Disease reaction, S = Susceptible.
Based on 1-9 rating scale, rice cultivars were categorized into five groups (HR, R, MR, S and HS) in accordance with their level of resistance against Tanzanian RYMV strains and phylotypes (
The area under disease progress curve (AUDPC) was statistically significant difference (P ≤ 0.05) between resistant rice cultivars and RYMV strains and phylotypes (
Days to first appearance symptoms on resistant rice cultivars inoculated with RYMV strains and phylotypes are summarized in
Rice genotype | Resistant gene | Days to appearance of symptoms | ||||||
---|---|---|---|---|---|---|---|---|
S4lm Tz526 | S4lv Tz516 | S4ug Tz508 | S5 Tz445 | S5 Tz429 | S6c Tz486 | S6w Tz539 | ||
Azucena | QTLs | 25 | 30 | 34 | 13 | 13 | 38 | 26 |
Gigante | rymv1-2 | 23 | 28 | 40 | 13 | 13 | 42 | 24 |
Tog12387 | rymv1-3 | 13 | 14 | 15 | 10 | 9 | 16 | 13 |
Tog5438 | rymv1-4 | 23 | 30 | 30 | 13 | 13 | 38 | 34 |
Tog5672 | rymv1-4+rymv2 | ns | ns | ns | ns | ns | ns | ns |
Tog5674 | rymv1-5 | ns | ns | ns | ns | 42 | ns | ns |
Tog5681 | rymv1-3 | 35 | ns | ns | 28 | 28 | ns | 30 |
S. control | ||||||||
IR64 | rymv1-1 | 7 | 9 | 15 | 7 | 6 | 15 | 7 |
SARO-5 | Unknown | 6 | 7 | 8 | 5 | 5 | 8 | 7 |
ns = no symptoms appearance on the rice leaves, S = Susceptible.
Tog5681 inoculated with phylotype S4lv (Tz516) and strain S5 (Tz445, Tz429), respectively, 28 DAI. However, rice cultivar Tog5681 inoculated with S6w (Tz539) and S4lm (Tz526), first symptoms were observed 30 and 35 DAI, respectively. Symptoms of RYMV were not observed in non-inoculated controls and rice cultivar Tog5672 until maturity. The rice cultivar Tog5674 inoculated with S5 (Tz445) produced first symptoms 42 DAI (
The titer of RYMV in resistant rice cultivars inoculated with different strains and phylotypes was measured by ELISA in order to confirm the resistance breakdown. Rice yellow mottle virus titer varied significantly (P ≤ 0.05) between differential resistant rice cultivars (
There were highly significant differences (P ≤ 0.001) on performance of yield components between resistant rice cultivars inoculated with different strains and phylotypes (
Rice cultivar | S4lm (Tz526) | S4lv (Tz516) | S4ug (Tz508) | S5 (Tz445) | S5 (Tz429) | S6c (Tz486) | S6w (Tz539) | |
---|---|---|---|---|---|---|---|---|
Plant height reduction (%) | Azucena | 22.61c | 18.25d | 18.95d | 49.99d | 47.01d | 2.69a | 20.12d |
Gigante | 38.13e | 18.98d | 20.4e | 57.09e | 56.07e | 2.76a | 24.04e | |
Tog12387 | 39.1e | 31.03e | 31.89f | 84.87i | 62.85f | 10.06b | 37.18f | |
Tog5438 | 30.32d | 15.9c | 14.84c | 65.3f | 65.93g | 17.66c | 15.9c | |
Tog5672 | 1.15a | 1.27a | 1.62ab | 1.85a | 1.27a | 1.27a | 1.27a | |
Tog5674 | 1.2a | 1.65a | 0.75a | 4.79b | 9.74b | 1.09a | 1.54a | |
Tog5681 | 11.42b | 3.13b | 2.85b | 29.82c | 17.27c | 1.49a | 7.65b | |
S. control | ||||||||
IR64 | 53.62f | 52.47f | 45.58g | 77.08g | 66.27g | 53.62d | 62.59g | |
SARO-5 | 68.93g | 63.24g | 62.11h | 80.98h | 76.88h | 57.11e | 72.11h | |
GRAND MEAN | 29.6 | 22.9 | 22.1 | 50.2 | 44.8 | 16.4 | 26.9 | |
LSD 5% | 1.23 | 1.38 | 1.32 | 0.88 | 0.97 | 2.06 | 1.25 | |
F test | *** | *** | *** | *** | *** | *** | *** | |
CV% | 2.4 | 3.5 | 3.5 | 1 | 1.2 | 7.2 | 2.7 | |
Tiller number reduction (%) | Azucena | 27.27c | 20c | 22.42c | 51.52d | 46.67d | 5.94a | 27.27c |
Gigante | 32.5cd | 16.25bc | 20c | 57.5e | 50d | 2.5a | 25c | |
Tog12387 | 42.22ef | 40.28d | 40.28d | 85.07i | 70.14f | 19.37b | 43.26d | |
Tog5438 | 36.4de | 11.01b | 10.96b | 69.47f | 59.3e | 5.88a | 13.51b | |
Tog5672 | 1.67a | 1.33a | 0.5a | 1.67a | 1.67a | 1.67a | 1.67a | |
Tog5674 | 1.58a | 1.58a | 0.58a | 5.08b | 7.58b | 2.08a | 1.33a | |
Tog5681 | 9.23b | 2.02a | 1.67a | 24.36c | 19.32c | 0.66a | 5.36a | |
S. control | ||||||||
IR64 | 47.65f | 39.59d | 43.62d | 79.86h | 79.86h | 47.65c | 55.7e | |
SARO-5 | 69.84g | 63.81e | 65.32e | 77.38g | 75.87g | 56.27d | 69.84f | |
GRAND MEAN | 29.8 | 21.8 | 22.8 | 50.2 | 45.6 | 15.8 | 27 | |
LSD 5% | 5.92 | 6.1 | 6.08 | 2.48 | 3.4 | 7.91 | 5.57 | |
F test | *** | *** | *** | *** | *** | *** | *** | |
CV% | 11.5 | 16.2 | 15.4 | 2.9 | 4.3 | 29 | 12.1 | |
1000 grains weight reduction (%) | Azucena | 27.53d | 21d | 20d | 50.47d | 53.64d | 8.92c | 25.35d |
Gigante | 39.63f | 27.38e | 20.5d | 57.76e | 60.86f | 2.69b | 30.84e | |
Tog12387 | 37.23e | 30.18f | 31.31e | 93.13h | 67.25g | 13.14c | 35.36f | |
Tog5438 | 42.61g | 13.49c | 12.65c | 58.56f | 57.37e | 8.68c | 9.28c | |
Tog5672 | 0.58a | 0.77a | 0.12a | 0.93a | 1.27a | 1.09a | 0.99a | |
Tog5674 | 1.56b | 0.97ab | 0.66a | 5.98b | 7.2b | 1.73a | 0.88a | |
Tog5681 | 11.26c | 2.18b | 2.37b | 23.23c | 19.43c | 1.3a | 5.9b | |
S. control | ||||||||
IR64 | 53.08h | 48.26g | 49.56f | 73.39g | 76.18h | 49.52e | 63.35g | |
SARO-5 | 76.57i | 71.55h | 70.06g | 96.2i | 86.33i | 60.3f | 64.64h | |
GRAND MEAN | 32.2 | 24 | 23 | 51.1 | 47.7 | 16.4 | 26.3 | |
LSD 5% | 0.58 | 1.24 | 0.59 | 0.51 | 0.52 | 0.65 | 0.6 | |
F test | *** | *** | *** | *** | *** | *** | *** | |
CV% | 1 | 3 | 2 | 0.6 | 0.6 | 2.3 | 1.3 |
*Values are means of three replicates. Numbers followed by the same letter in a column are not significantly different at P = 0.05, using Duncan’s Multiple Range Test. *** = Highly significantly different (P < 0.001).
Tiller production differed significantly (P ≤ 0.05) between inoculated differential rice cultivars (
Reduction in 1000 grain weight of rice cultivars was highly significantly different (P ≤ 0.05) as influenced by RYMV strains and phylotypes except in Tog5672 (
The percentage rice grain yield losses differed significantly (P ≤ 0.001) between the rice cultivars, strains and phylotypes (
Spikelet sterility differed significantly (P ≤ 0.05) between inoculated differential rice cultivars (
Rice cultivar | Yield loss per panicle (%) | ||||||
---|---|---|---|---|---|---|---|
S4lm (Tz526) | S4lv (Tz516) | S4ug (Tz508) | S5 (Tz445) | S5 (Tz429) | S6c (Tz486) | S6w (Tz539) | |
Azucena | 37c | 25.8c | 20.4d | 52.7d | 49.7d | 5.8b | 38.1d |
Gigante | 38.2c | 28.8d | 15.6c | 56.5e | 61.8e | 5b | 37.1d |
Tog12387 | 57.5d | 40.6e | 39.1e | 99.1h | 77.9g | 20.9c | 52.8e |
Tog5438 | 36.4c | 9.8b | 8.7b | 81.2f | 73f | 4.3ab | 22.4c |
Tog5672 | 1.9a | 1.4a | 1a | 2a | 1.2a | 1.1a | 1.1a |
Tog5674 | 1.9a | 1.5a | 0.1a | 6.7b | 11.6b | 1.7a | 1.2a |
Tog5681 | 20.7b | 2.2a | 2.6a | 28.9c | 29.7c | 1.8a | 6.4b |
S. control | |||||||
IR64 | 63.3e | 58.2f | 55.8f | 93.8g | 95.7h | 52.2d | 66.8f |
SARO-5 | 76.4f | 70.6g | 69.3g | 98.9h | 95.7h | 62.1e | 71g |
GRAND MEAN | 37 | 26.6 | 23.6 | 59.8 | 55.1 | 17.2 | 33 |
LSD 5% | 3.09 | 2.82 | 3.85 | 2.54 | 2.31 | 3.04 | 2.65 |
F test | *** | *** | *** | *** | *** | *** | *** |
CV% | 4.8 | 6.1 | 9.4 | 2.5 | 2.4 | 10.2 | 4.6 |
*Values are means of three replicates. Numbers followed by the same letter in a column are not significantly different at P = 0.05, using Duncan’s Multiple Range Test. *** = Highly significantly different (P < 0.001).
Rice cultivar | Spikelet sterility (%) | |||||||
---|---|---|---|---|---|---|---|---|
S4lm (Tz526) | S4lv (Tz516) | S4ug (Tz508) | S5 (Tz445) | S5 (Tz429) | S6c (Tz486) | S6w (Tz539) | Non-i | |
Azucena | 25.01e | 17.11d | 21.03e | 49.84d | 53.98d | 4.26c | 26.28d | 0.27a |
Gigante | 29.61f | 27.36e | 18.92d | 57.95e | 60.42e | 3.52c | 33.53e | 1.12abc |
Tog12387 | 44.31g | 33.81f | 25.44f | 99.63h | 79.75g | 13.4e | 40.4f | 0.94abc |
Tog5438 | 20d | 12.58c | 13.63c | 63.39f | 62.11f | 7.57d | 16.21c | 1.01abc |
Tog5672 | 0.69a | 0.7a | 0.21a | 0.89a | 0.85a | 1.3a | 1.01a | 1.03abc |
Tog5674 | 2.54b | 2.05b | 0.67a | 5.5b | 8.25b | 2.49b | 1.59a | 1.57bc |
Tog5681 | 10.57c | 1.85b | 2.01b | 23.75c | 14.61c | 0.95a | 5.18b | 1.81c |
S. control | ||||||||
IR64 | 55.47h | 39.18g | 43.29g | 75.63g | 85.72i | 49.74f | 61.56g | 0.98abc |
SARO-5 | 76.17i | 67.18h | 68.34h | 99.68h | 83.05h | 56.5g | 75.29h | 0.59ab |
GRAND MEAN | 29.37 | 22.42 | 21.5 | 52.92 | 49.86 | 15.53 | 29.01 | 1.04 |
LSD 5% | 0.94 | 0.746 | 0.818 | 0.447 | 0.456 | 0.814 | 0.854 | 1.012 |
F test | *** | *** | *** | *** | *** | *** | *** | * |
CV% | 1.8 | 1.9 | 2.2 | 0.5 | 0.5 | 3 | 1.7 | 56.5 |
*Values are means of three replicates. Numbers followed by the same letter in a column are not significantly different at the 5% probability level by the Duncan’s Multiple Range Test. *** = Highly significantly different (P < 0.001), * = Significantly different (P < 0.01), Non-i = Non-inoculated, S = Susceptible.
with RYMV phylotypes S4lv (Tz516), S4ug (Tz508) and S6c (Tz486) had similar spikelet sterility to non-inoculated plants (healthy controls).
Great pathogenic variation of Tanzanian RYMV strains was detected in resistance breakdown of the rice cultivars with one resistant gene (RYMV1). Emergence of new pathogen strains through genetic mutation and recombination may result to virulent strains which are capable for overcoming the resistance of commercial rice varieties used in crop production world-wide. The resistance breaking of cultivars carrying rymv1 resistant allele has been studied and genetic determinants established [
The rice cultivar Tog12387 was reported by Jaw [
The stable resistance in rice cultivar Tog5672 to Tanzanian RYMV strains and phylotypes observed in this study could be due to the presence of its second resistance gene on RYMV2 locus [
Days to RYMV symptoms appearance varied significantly (P ≤ 0.05) between inoculated cultivars. However, assessment of resistant rice cultivars by symptoms is not enough to determine the virulence of the virus as some cultivars had genotypic features of hiding symptoms [
Breeding for resistance is widely focused by researchers as the key strategy for RYMV disease control [
The capability of RYMV to overcome RYMV resistance gene at molecular level has been studied [
The ability of Tanzanian RYMV strains and phylotypes to overcome resistance conferred by RYMV1 gene was determined. However, the analysis on molecular basis of resistance breakdown is recommended based on identification of RB mutations by sequencing of the RYMV genome in infected rice cultivars and mutagenesis of an infectious viral clone. The results indicate that RYMV1 gene resistance breakdown was highly variable depending on the strain used, and disease severity ranged from 11% - 75.3%. Rice yellow mottle virus pathogen caused significant yield losses ranging from 5% - 99% in resistant rice cultivars depending on the RYMV strain used. This study also showed that, multiple resistant-breaking occurred in resistant rice cultivars Gigante (rymv1-2), Tog12387 (rymv1-3), Tog5681 (rymv1-3), Tog5438 (rymv1-4) inoculated with RYMV strains S4lm, S4lv, S4ug, S5, S6c and S6w. However, resistant-breaking did not occur in rice cultivar Tog5681 inoculated with RYMV phylotypes S4lv, S4ug and S6c. Therefore, rice cultivar Tog5681 could be a good source of resistance in areas where S4lv, S4ug and S6c are widely distributed. The results indicate high capacity of RYMV strain S5 to overcome rymv1-2, rymv1-3, rymv1-4 and rymv1-5 resistance that may be due specific features of strain S5 biology, thus, calls for the study on interaction between rice cultivars and survival of S5. Further research is also needed to determine alleles that may be resistant against RYMV strain S5.
Rice cultivar Tog5672 with allele rymv1-4 + rymv2 remained effective against Tanzanian RYMV strains and phylotypes used in this study. Based on these findings, the rice cultivar Tog5672 might have resistance durability factors that should be further studied and directed towards breeding to introgression of such resistance in susceptible rice cultivars to RYMV prevalent in Tanzania. The development of resistant rice cultivars must take into account variability of RYMV strains in targeted areas. Better understanding of the factors that favor the emergence of pathogen virulence is essential for planning strategies for breeding and the use of resistance that will result in durable protection. Furthermore, the continued screening for resistance of rice cultivars to RYMV is recommended in order to identify the resistance controlled by multiple genes but also with highly durable resistance.
We thank the Innovative Agricultural Research Initiative (iAGRI) for providing funds for this work. Thanks are due to Institut de Recherche pour le Developement (IRD), France, AfricaRice and Sokoine University of Agriculture (SUA), Tanzania for supplying rice plant genotypes and providing research facilities.
Hubert, J., Lyimo, H.J.F. and Luzi-Kihupi, A. (2017) Pathogenic Variation and Occurrence of Multiple Resistance-Breaking Rice yellow mottle virus Strains in Tanzania. American Journal of Plant Sciences, 8, 1820-1841. https://doi.org/10.4236/ajps.2017.88124