Screening of Rice Genotypes for Leaf and Neck Blast Disease Resistance

Author: Jyoti P. Jirankali*, C.A. Deepak, M.P. Rajanna, B.S. Chethana and S. Ramesh

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Abstract

Rice blast, caused by Pyricularia oryzae with high pathogen plasticity and mutation rate considered as the most damaging disease in rice. It is responsible for yield losses of about 10% to 30% annually. In favorable conditions, this disease can devastate entire rice plants within 15 to 20 days and cause yield losses of up to 100%. There is always a continuous need of screening germplasm lines to identify resistant genotypes, which is a sustainable approach to disease management. A total of 143 rice genotypes, with resistant (Tetep) and susceptible check (HR12) were screened against leaf blast and neck blast (Pyricularia oryzae) disease under high disease pressure. It is found that for leaf blast, 3 rice genotypes were resistant, while for neck blast, genotype (G226) was highly resistant, and 32 genotypes were resistant. Leaf and neck infection were positively correlated and non-significant (r = 0.228). While genotypes G177 and G701 were resistant to both leaf and neck blast, and can be used in blast resistance breeding programs as prospective resistant sources.

Keywords

Breeding lines, Leaf blast, Pyricularia oryzae, Neck blast, Screening and Resistance

Conclusion

Identification or screening of the available germplasm against a disease is a fundamental work, before the start of gene introgression or heterosis breeding. In the present study, we identified that genotypes viz. G177 and G701 were resistant to both leaf and neck blast, while none of the genotypes were highly resistant to leaf blast. While the genotype G226 was highly resistant to neck blast. From these findings we conclude that these entries could be used as parents in blast resistance breeding programs, at the area of study.

References

INTRODUCTION Pyricularia oryzae, a heterothallic ascomycete, causes rice blast, which is one of the most important biotic limitations on rice productivity (Deng et al., 2017). Rice blast may infect the plant at any stage of development. Over the previous few decades, India and Japan have seen repeated outbreaks and recurrent breakdowns of rice blast resistance, resulting in output losses of 20–100 percent (Khush and Jena 2009; Sharma et al., 2012). Disease kills seedlings in nurseries and crops at the tillering stage when environmental circumstances are ideal. The use of host resistance genes is critical to its control (Zhai et al., 2014). Leaf blast lowers the quantity of bearing panicles and the weight of individual grains by stunting plant height (Thruston 1998). Barren panicles develop from stem node infections, while late neck infections result in 'broken necks,' chalky kernels, and sterile grains (Candole et al., 1999). Leaf blast also boosts plant respiration while lowering maximal photosynthetic rate at light saturation and early light efficiency (Pinnschmidt et al., 1994). Under intensive agriculture and high nitrogen levels in highland areas, disease becomes prominent and pandemic (Bonman 1992). Host cultivars, resistant to leaf and panicle blast, are the most widely used method of disease control (Bonman 1992). Partial resistant cultivars are more effective to manage blast in irrigated rice of the tropics (Bonman and Mackill 1988; Yeh and Bonman 1986). Some partial resistant cultivars showed durable resistance (Johnson, 1981). Variability and population biology of the blast fungus (Correa- Victoria and Zeigler 1993; Zeigler et al., 1994); behavior of resistance genes (McCouch et al., 1994), and host–parasite interaction in the rice-blast pathosystem (Notteghem and Silue 1992) are essential in the breeding programme. Effective and efficient screening techniques are keys in successful breeding programs for blast resistance. The continuously evolving genome of Pyricularia oryzae as well as existence of geographically diverse strains are challenges for the rice breeders. Genome studies of the rice blast fungus have revealed high probabilities of transposons mediated inactivation of genes involved in host specificity. Moreover, the high genetic variability in Pyricularia oryzae allows the fungus to broaden the host range and infect formerly resistant genotypes (Dean et al., 2005). It is therefore important to build a repository of resistant accessions. Thus, the present experiment was conducted to examine and screen out the different rice lines resistant to both leaf and neck blast as well as to show the relationship between them. MATERIAL AND METHODS A set of 143 rice genotypes of rice germplasm material were obtained from the International Rice Gene bank (IRG) of the International Rice Research Institute (IRRI), Philippines as listed in Annexure 2. The International Rice Germplasm Collection (IRGC) of IRG holds more than 120,000 accessions from different geographical regions of the world (129 countries) and were screened phenotypically for blast resistance at ZARS, V. C. Farm, Mandya adopting Uniform Blast Nursery design. Each test entries were sown in a single row of 50 cm long with row to row spacing of 10 cm with two rows of local susceptible check (HR 12) after every ten test entries and resistant check of two rows (Tetep) was planted in every bed (Fig. 1), respectively. Isolate and inoculum production. The pure culture of Pyricularia oryzae was grown on rice flour agar medium (2% rice flour, 0.2% yeast extract and 2% agar) and incubated at 25oC for 12 hours per day of fluorescent light conditions for 8-10 days. Fungal colonies were scraped out of the surface for further sporulation and incubated under the same culture conditions for 1 to 2 days. After conidia formation, the conidia were harvested using sterile distilled water. The inoculums were adjusted to a concentration of 5 × 104 conidia per ml using sterilized distilled water to which 0.1% Twenty 20 was added before spraying. The spore suspension was inoculated using atomizer at fourth leaf stage after the sunset at around 6 pm. After inoculation, the plants were covered with polythene for 14 h i.e., from 6pm to 8 am for 3-4 days till the symptom appearance. Rice lines evaluation to leaf and neck blast: Artificial blast nursery for leaf blast. Seeds of rice variety HR12 were planted as a border row in 20 cm diameter containers containing wetland soil for leaf blast scoring. Using a pneumatic hand sprayer, the decanted spore solution containing 5×104 spores per ml was sprayed at fourth leaf stage after the sunset at around 6 pm To increase disease incidence, spraying was repeated every three days, when the susceptible check was extensively infected with blast, with a leaf blast score of 9, the observations were made. Individual plants in each submission were graded on a 0-9 scale for leaf blast intensity using the Standard Evaluation System (SES, IRRI, 1996) given in Table 1. Open field conditions for neck blast natural infection. Screening for natural infection against neck blast was carried at I block, ZARS, V. C. Farm, Mandya during winter season which is the most favorable condition for neck blast disease development (Fig. 1). The sowing was carried out as per the guidelines given by IRRI. The observation of disease reaction was recorded when the susceptible check was severely infected by neck blast. Disease severity was assessed on 10 plants of each entry for neck blast and infested neck area. The observations were recorded when the susceptible check was severely infected with blast with a neck blast score of 9. Individual plants in each entry were scored based on the neck blast severity following Standard Evaluation System on 0-9 scale (SES, IRRI, 2013) given in Table 2. Disease assessment and statistical analysis. Disease scoring was done at weekly intervals after inoculation at different growth stages. Area Under Disease Progress Curve (AUDPC) was calculated for quantitative disease resistance assessment using the following formula (Das et al., 1992). where xi = disease severity on the ith date, ti = date on which the disease was scored (ith day), n = number of dates on which disease was scored. AUDPC measures the amount of disease as well as rate of progress, and unit less. Similarly, for the neck blast, total numbers of infected necks were scored, counted and disease incidence (DI) % was calculated using formula DI% = (number of infected plants/total number of plants counted in a plot). Based on the neck incidence percentage, lines were classified as resistant (R) with 0–15% neck infection, moderately resistant (MR) with 15.1–30% infection, moderately susceptible (MS) 30.1–50% with infection, and 50.1–100% infection as susceptible. Simple correlation coefficient and regression was determined to test the mean and interaction effect between leaf and neck infection using Microsoft Excel (2000). Individual plants in each entry were assessed on a 0-9 scale for leaf blast intensity using the Standard Evaluation System (SES, IRRI, 1996). RESULTS AND DISCUSSION The blast pathogen affects different parts of a rice plant during pathogenesis. One of the serious forms of rice blast is neck blast. However, due to the very complex nature of Pyricularia oryzae, the epidemiology of pathogen is not completely understood and the screening technique for neck blast is not standardized. In contrast to neck blast, the leaf blast is well studied and the screening method for the same is precisely standardized. A set of 143 lines with checks (Tetep-resistant check, HR 12-susceptible check) were evaluated for the blast resistance using uniform blast nursery method, with artificial inoculation of Pyricularia oryzae, following 0-9 standard evaluation scale for rice blast (SES IRRI, 1996 and 2013) and all test entries were categorized into different categories based on their response to Pyricularia oryzae. It was identified that the lines G177, G701, G814, G830 and Tetep (Resistance check), shown resistance reaction, with a phenotypic score of 1 but none of test entries shown to be highly resistant with a score of 0 (Fig. 2). However, the 24 lines corresponded to moderate resistance. Further, 31 lines were found to be moderately susceptible against leaf blast disease with phenotypic scores of 4 and 5. Twenty seven lines were found to show susceptible reaction to leaf blast disease with phenotypic scores of 6 and 7. The highest susceptibility with phenotypic score of 8 and 9 was recorded by 57 lines and susceptible check HR 12 (Table 3). Similar field screening experiments were conducted for identification of location specific blast resistant lines by Srijan et al. (2015), Hosagoudar and Jairam Amadabade (2017); Vinayak et al. (2018) also. Under natural hotspot screening of different landraces for neck blast resistance, it was observed that genotype G226 is highly resistant against neck blast. Thirty-two genotypes showed resistant reaction with a score of 1. However, 37 were moderately resistant against neck blast disease with phenotypic scores of 3.32 entries were moderately susceptible against neck blast with a score of 5. While 27 genotypes were found to be susceptible with a score of 7 and remaining 14 entries were highly susceptible with a score of 9, against neck blast (Table 4). Genotype G226 was highly resistant to neck blast and moderately resistant to leaf blast. While G177, G701 were resistant to both neck and leaf blast. G238, G270, G332, G352, G353 (a), G362, G365, G368, G379, G493, G699, G704, G823 were resistant to neck blast and moderately resistant to leaf blast. Puri et al. (2009) reported differential behavior of lines, (Barkhe 1006, Barkhe 1032, Barkhe 3004 were resistant to neck blast and had intermediate reaction to leaf) to leaf and neck blast, as our findings. Leaf and neck blast infection was positively correlated and non-significant (r = 0.228) (Fig. 4). Leaf blast susceptible varieties have shown resistance to neck blast and vice versa (Ono and Suzuki 1960). Ou (1985); Ou and Nuque (1963) reported lines resistant to leaf blast to seedling stage, are completely resistant to neck blast and susceptible at the seedling stage are susceptible to neck blast. Bhardwaj and Singh (1983); Balal et al. (1977) also showed the positive correlation between leaf and neck infection. However, Koh et al. (1987); Bonman (1992) found some cultivars resistant in seedling stage appeared susceptible to neck infection. Area under disease progressive curve. AUDPC were calculated based on the disease severity percentage, using the formula as presented in the materials and methods chapter. Lowest total AUDPC was observed on G177 with a value of 22.5, whereas highest value was observed on G13, G86, G89, G127, G131, G142, G153, G178, G209, G259, G271, G275, G333, G376, G382, G384, G385, G565, G736 followed by G82, G129 and G322. Based on the Total AUDPC value, rice genotypes were listed on the five categories from resistant to highly susceptible which are shown in the Tables 3 and 4. The AUPDC Values along with their disease score after every week has been depicted in Annexure 1. Disease progress in rice lines. Rice lines showed increasing disease progress and AUDPC value up to 25 days after inoculation (DAI) and trend remained constant (Fig. 3). In G1, AUDPC was increasing at a higher rate compared to G42, G167, G365 and G505. In G1, G42, G167, G365, G505, G161, G205, G236, G270, G701, G814, G830 it was constant throughout all days. In G21, G37, G52, G122, G133, G137, G138, G147, G184, G187, G191, G204, G208, G215, G232, G246, G250, G263, G268, G332, G360, G375, G404, G657, G740, G773, G782, G812. G823, G832 AUDPC was increasing after 28 DAI. G82, G129 and G322 had higher and G177 had minimum AUDPC value. None of the genotypes had decreasing AUDPC values. G86, G89, G127, G131, G142, G153, G178, G209, G259, G271, G275, G333, G376, G382, G384, G385, G565, G736 a higher level AUDPC value was recorded, showing constant susceptible disease reaction with a score of 9. As shown in Table 5, among the selected top 12 genotypes resistant to blast disease, G177 was resistant with AUDPC value 22.5. AUDPC value of genotypes G701, G814, G830 were constant with score of 27, whereas for G704 and G365 disease progress was increasing after 34 DAS with AUPDC value of 31.5 and 36 respectively, for G740, G823 AUPDC value was 40.5, disease progress was increasing after 25 DAS. G1 showed a drastic increase after 34 DAS. G279 and G505 showed increased disease rate after 25 and 34 DAS, both with AUPDC value of 49.5 (Fig. 5). Thus, the rice genotypes used in this study having different genetic background showed different interaction to leaf blast. Such result was also supported by the work of Chaudhary et al., (2001) and Puri et al. (2006). Several researchers have reported having higher degree of blast resistance (Chaudhary et al., 2005 and Joshi et al., 2017). The most important challenge in front of the rice scientists is to do accumulation of resistance genes which could be used against continuously evolving and geographically diverse races of P. oryzae (Sharma et al., 2012). Thus, such studies need to be continued to monitor virulence of the blast pathogen and to identify new sources of resistance which will help in national breeding program for the development of blast resistant rice varieties in future.

How to cite this article

Jyoti P. Jirankali, C.A. Deepak, M.P. Rajanna, B.S. Chethana and S. Ramesh (2022). Screening of Rice Genotypes for Leaf and Neck Blast Disease Resistance. Biological Forum – An International Journal, 14(1): 1741-1750.