In this article we will discuss about:- 1. Occurrence of Sheath Rot Disease of Rice 2. Pathogen of Sheath Rot Disease of Rice 3. Other Possible Organism Producing Sheath Rot Symptoms 4. Seed Health Evaluation 5. Inoculation 6. Mode of Infection 7. Symptoms 8. Role of Insect Injury in Sheath Rot Development 9. Disease Management 10. Varietals Resistance.
Contents:
- Occurrence of Sheath Rot Disease of Rice
- Pathogen of Sheath Rot Disease of Rice
- Other Possible Organism Producing Sheath Rot Symptoms
- Seed Health Evaluation of Sheath Rot Disease of Rice
- Inoculation of Sheath Rot Disease of Rice
- Mode of Infection of Sheath Rot Disease of Rice
- Symptoms of Sheath Rot Disease of Rice
- Role of Insect Injury in Sheath Rot Development
- Disease Management of Sheath Rot Disease of Rice
- Varietals Resistance of Sheath Rot Disease of Rice
1. Occurrence of Sheath Rot Disease of Rice:
The occurrence of the sheath rot has also been recorded from Japan, Vietnam, Thailand, United Sates, Peru, Brazil and Bangladesh, Nigeria and Punjab in Pakistan. In India the occurrence of sheath rot was also reported for the first time by Agnihothrudu in 1973. Further Amin (1974) and Reddy (1974) reported the sheath rot incidence from Andhra Pradesh.
The occurrence was recorded from several rice growing states of India, viz. Andhra Pradesh, West Bengal and Bihar, Tamil Nadu, Orissa, Kerala, Punjab, Uttar Pradesh, Rajasthan and Madhya Pradesh.
Sarocladium oryzae (Acrocylindrium oryzae) the incitant of sheath rot of rice was considered the main cause by several workers. Sheath rot caused by Sarocladium oryzae was also reported by several workers from different parts of the world from Pakistan, Argentina, Cameroon, Indonesia, Senegal and China, respectively.
Severe incidence of this disease increases chaffy grain, reduction in yield as well as poor quality of the grain. Thrimurty (1980) also reported that Sarocladium oryzae was found to be associated with sheath rot infected panicles of rice in Chhattisgarh region and also reported that sheath rot incidence on some popular rice varieties increased the number of chaffy grains in infected panicle than healthy.
2. Pathogen of Sheath Rot Disease of Rice:
The typical characters of Sarocladium oryzae (Acrocylindrium oryzae) (Sawada) Gams and Hawksworth (1975) the causal organism was described by several workers as: white, septate mycelium with sparsely branched conidiophore, slightly thicker than vegetative hyphae with terminal branches, phialides arising from conidiophores or directly from undifferentiated vegetative hyphae, 30-40 mm long and 1.5-2.0 mm wide at the base when formed at the appex of slender conidiophores, 6-20 mm long and 1.0-1.5 mm wide when arising in dense broom like fascicles, tips tapering to 0.6-1.0 mm in width, lacking any distinct collarette and conidia borne on tip, conidia formed in slimy masses, cylindrical with rounded ends, sometimes becoming slightly curved, hyaline, thin walled, smooth, one celled with a 3.5-7.0 x 0.8-1.5 mm. chlaymydospore absent, branches of conidiophores arises in whorls.
3. Other Possible Organism Producing Sheath Rot Symptoms:
The incitant of sheath rot was reported to be different in different geographical locations where rice is grown. Bacterium, Pseudomonas fuscovagine was found to be associated with sheath rot from Latin America, Asia and Africa.
Brown sheath rot of rice caused by Gaemannomyces graminis var. graminis, reported by Nayak and Chakrabarti 1987, from West Bengal, India. Akhtar, (1988) from Pakistan Reported that Pseudomonas syringae pv. syringae also causes bacterial sheath rot.
The disease was characterized by brown necrotic lesions and florets that did emerge but did not fill normally. Different species of Fusarium (F. moniliforme, F. avenacea, F. graminearum, F. proliferatum) were also reported to be associated as causal agents for sheat rot development in rice from different rice growing regions.
Cartwright (1996) reported that F. proliferatum was found to be associated with sheath rot infected panicle of cv. Bengal, it was severe on certain Bengal and Cypress field in Southwestern and Northeastern Arkansas.
Devi and singh (1995) studied that Fusarium avenacea (Gibberella avenacea), F. graminearum (G. zeae ) and F. moniliforme (G. fujikuroi. ) survived in rice seeds for more than 13 months. G. zeae and G. fujikuroi were isolated from both husk and kernel and G. avenacea only from husk.
All 3 pathogens remained viable for 12 months in infected leaf sheaths buried up to a depth of 7 cm., but viability was lost after 10 months at 10 and 15 cm.
The fungi survived for 14 months on the soil surface for 20 months in diseased leaf sheaths at room temp (14-29°C). Mazzanti-de-Castanon and Gutierrez-de- Arriola (1995) reported Helicoceras oryzae, the causal agent of reddish brown sheath rot in rice crops from Northeast Argentina.
Padasht-Dehkaee and Hedjaroude (1996) reported Gibberella zeae, the telomorph of Fusarium graminearum, the causal agent of sheath rot in Gulian province, Iran. This was the first report of G. zeae on rice from Iran.
Saifulla (1996) found Curvularia lunata (Cochlibolous lunatus) as a new causal agent of sheath rot disease of rice. It was identified on the commercial rice variety IET 7191 and its pathogenicity was confirmed. Cartwright (1999) also reported the black sheath rot of rice caused by Gaemanomyces graminis var. graminis from Arkansas, after evaluating the 70 rice varieties.
4. Seed Health Evaluation of Sheath Rot Disease of Rice:
A solution of sodium hypochlorite as a mild disinfectant, containing approximately 1% w/w available chlorine is generally used for surface sterilization of cereals seed before estimating the seed borne microflora by agar plate method. Among the 55 fungi that reported to be pathogenic, 43 are identified as seed borne or seed transmissible.
Seed health refers primarily to the presence or absence of disease causing organism of various kinds. The role of seed borne pathogens in seed health, disease management and epidemic occurrences are also needs to be critically assessed.
Mew (1987) emphasized the regular survey works on disease occurrence at regional, country and continental basis which not only ensures in formulating good disease management programme but also checks any new introduction through seed.
For the detection of the seed borne fungal infection, generally incubation methods were followed. Important incubation methods includes standard blotter method, agar plate method, deep freezing method, 2,4-D method and modified blotter method.
Among these blotter and agar plate methods were reported to be more important and considered as standard methods for routine seed health evaluation. Agar plate method found to be most efficient in detecting fungal species.
Sarocladium oryzae, the incitant of sheath rot was found to be both externally and internally. Viswanathan and Mariappan (1980) reported that most of the infected seed died (almost 45%) even before to emergence and at seedling stage.
Singh (1996) stated that the laboratory tests showed that Sarocladium oryzae was found only on discolored rice seed and mainly carried externally (77.5%) and hot water treatment of seeds at 52°C for 30 min. controlled the seed borne fungus.
Rajan (1981) also reported seed borne nature of the Sarocladium oryzae and suggested the mechanical separation of the infected seeds with brine solution. Nsemwa and Wolffhechel (1999) tested 47 rice seed samples of different varieties from two agro-ecological zones of Tanzania.
They found that the samples were associated with Alternaria padwickii and Sarocladium oryzae and not also previously reported. The infection level of these fungal species were higher in seeds from the Lake Nyasa Basin, might be due to higher humidity in the vicinity of the Lake Nyasa Basin.
Vachaspati (2000) investigated seed borne fungi and seed discolouration of freshly harvested seeds of hybrid rice. The fungal species Alternaria padwickii, Cochlibolus miyabeanus, Drechslera australiensis, Fusarium pallidoroseum and Magnaporthe grisea could be detected from the seed coat and the endosperm and Cochlibolus lunatus, Curvularia ovoidea, C. oryzae, Gibbrella fujikuroi, Magnaporthe salvinii, Nigrospora oryzae and Sarocladium oryzae were detected from all parts of the seed i.e., seed coat, endosperm and embryo.
In an elaborate study at International Rice Research Institute, Manila, Merca and Mew (1987) reported the presence of Sarocladium oryzae with seed lots, out-going seeds before treatment to a tune of 4.02 mean per cent among the 1684 seed lots tested during 1987.
The seed lots incoming to IRRI, Manila were also found to be infected by Sarocladium oryzae to an extent of 5.39 per cent even after treatment among the 627 seed lots verified. Importance of seed health evaluation was also revealed by several workers.
5. Inoculation of Sheath Rot Disease of Rice:
Different inoculation methods were used for inciting the sheath rot disease development, the flag leaf sheaths were inoculated with spore suspension with the help of sterilized pippet. Gill (1993) used injection method with the 10 days old pathogen culture.
Infected rice grain inoculation method was followed by Shahjahan (1986) from USA for screening of the resistant varieties. Vidhyasekaran and Lewin (1987) also followed the infected grain inserting in to the boot of the rice plant.
Hazarika and Phookan (1998) compared 3 inoculation methods, single grain insertion, injection of spore suspension and spraying of spore suspension for sheath rot disease development. They found that single grain insertion method was best for disease incidence creation.
6. Mode of Infection of Sheath Rot Disease of Rice:
Sheath rot pathogen mainly attacks the uppermost (flag) leaf sheath and infection was more facilitated in the presence of an injury. Shahjahan (1977) reported that the pathogen could enter the host through stomata or injuries and colonizes intercellular in the vascular and mesophyl region of the susceptible cultivars.
Reddy (1984) reported that pathogen produces cellulolytic and pectolytic enzyme in-vitro. Such enzymes might have also a positive role in infection and colonization of the pathogen. Sarocladium oryzae could infect the rice plant at all stages of growth, but was most destructive at boot stage.
Zhuge (1985) reported that the pathogen could infect the leaf sheath, leaf blade midrib and seeds. Reddy (1989) reported that an inoculum concentration of 8 x 104 spores/ ml as the potential dose for effective infection and disease development.
Further increase in spore concentration to the level of 10 x 104 spores/ml reduced the infection, which might be due to autolysis of the spores at high concentration. Manibhushanrao (1996) opined that histopathological studies need to be conducted in order to understand the exact mode of penetration and invasion of the host tissue, as little information is available.
7. Symptoms of Sheath Rot Disease of Rice:
According to Tasugi and Ikeda (1956) typical symptoms of sheath rot are characterized by the production of grayish brown blotches on the flag leaf sheath.
Agnihothrudu (1973) reported that grayish brown lesions with powdery masses of conidia were present in the flag leaf sheath. Dark brown to chocolate brown spots on flag leaf resulting in partial or incomplete emergence of panicles reported to be the most common symptom induced by Sarocladium oryzae.
Under severe infection the panicle shows large number of chaffy and discolored grains. Ou (1985) described as an oblong or sometimes-irregular spots of 0.5-1.5 cm. in length with brown margins and gray centres or grayish brown throughout.
The enlarged spots coaleses and coveres most of the leaf sheath. The young panicles remain within the sheath or partially emerge. Also an abundant whitish powdery growth inside affected sheaths and the rotting of the young panicles occurs.
Infected Debris:
Singh and Raju (1981) found that Sarocladium oryzae survived for 10 months in the infected sheaths incubated under field conditions. Deka and Phookan (1998) reported that survival of Sarocladium oryzae varied with the type of soil investigated, survival was best (210 days) in sterilized soils at 30 +1°C. Survival was also high (180 days) in naturally infected field soil and sterilized soil maintained outdoors.
Host Range of Sarocladium oryzae:
The host range of Sarocladium oryzae, the rice sheath rot causal organism has not been widely studied. Few reports are available on the host range of this pathogen. Balakrishanan and Nair (1981) reported that field weeds Cyperus diformis, Echinochloa crusgalli, Monochoria vaginalis and Cyperus teneriffe were naturally infected hosts of A. (Sarocladium) oryzae.
Subsequently among the 20 species of weed and wild rice tested, Digitaria cilians, E. colona, E. crusgalli var. hispidula, Chloris barbata, Ischemum rugosum and Leptochloa chinensis were reported to be infected by sheath rot pathogen. Echinochloa colona, was also found to play a role as an alternate host for sheath rot pathogen.
Boa and Brady (1987) reported that three grasses viz. E. colona, E. crusgalli and C. diformis with typical symptoms were found in and around sheath rot disease affected rice fields. The pathogen also causes blight in Bambusa sp.
Deka and Phookan (1992) reported some alternate hosts from Assam, which included Echinochloa colona, Monochoria vaginalis, Hyrnenachne assamica, Leersia hexandra, Panicum. walense, Oryzae rufipogon and Eleusine indica.
Variability in Sarocladium Oryzae:
Morphological:
Mew and Mishra (1994) described that Sarocladium oryzae colonies on potato dextrose agar grow very slow and compact. Aerial mycelia are sparse, orange, reverse darker orange.
Hyphae are branched, septate, measuring roughly to 2.5 mm in diameter. Characteristic gnarled and wider hyphae are also usually seen. Condiophores are branched irregularly and in whorls. On vegetative hyphae, phialides develop singly or on slender conidiaphores in fascicular manner.
Agnihothrudu (1973) first described the pathogen A. oryzae from India. The description he has given is as follows: fungus obtuse, white, sparsely branched up to 2 mm in diameter.
Conidiophores arising from the mycelium, up to 3 mm in diameter, hyaline, smooth, macronematous , mononematous, once or twice branched, with apical conidiogenous cells in groups of 2-5, monophialidic, discrete, elongate, cylindrical, single, individual, intercalary philades also observed on the conidiophores.
Phialides flask shaped, elongate, narrow towards the apex. Phialoconidia acrogenous, simple, hyaline produced successively, cylindrical to sub avicular 4-10 mm, in nature.
Brady (1980) reported that the Sarocladium oryzae produces less regularly verticulate conidiophores and shorter conidia with rounded ends as compared to S. attenuation which had more regular and longer unicellular conidia with truncated ends.
Gams and Hawksworth (1975) also reported that both species co-exist at times and had only minor variations like pigmentation and round or truncated ends of the spores.
Further reports on morphological characters were also studied and described by Ou, 1985; Ahamed 1975; Joe and Manibhushanrao, 1996 and Purkayastha and Ghosal, 1982. Bridge (1989) described and illustrated the conidial variation in Sarocladium.
Relative Virulence:
Shahjahan (1986) studied the growth and virulence of two isolates of Sarocladium oryzae. Phookan and Hazarika (1994) obtained 8 isolates of Sarocladium oryzae from various rice growing areas of Northeast India. Morphometric differences among 8 isolates were not significant.
The isolates responded differently to various media, temperature and pH levels. Pathogenic variability was significant in tests on 8 rice genotype. Joe and Manibhushanrao (1995) arranged different isolates of Sarocladium oryzae in order of virulence based on a pathogenicity test on rice.
They estimated the mycelial constituents, cell wall degrading enzymes and phenolics of these isolates. The relative capacity of the isolates to secrete the enzymes such as CMCase, PTE, invertease and protease in-vitro showed a positive correlation with their degree of pathogenicity to rice, Sarocladium oryzae was identified as the more aggressive species than S. attenuatum.
8. Role of Insect Injury in Sheath Rot Development:
Chen and Chein (1964) and Chin (1974) reported that stem borer infested plants showed more sheath rot infection, indicating that injury plays an important role in infection.
Hsieh (1977) studied the association of mite (Steneotarsonemus madecarsus) and sheath rot pathogen. Further from their studies Hsieh (1980) reported that mites and thrips act as a vector for transmitting the sheath rot pathogen from diseased to healthy plant.
These insects were found to carry the conidia of the fungus all year round. Naidu (1983) also reported that brown plant hopper (Nilaparvata lugens) infestation favoured for more sheath rot incidence. It might be due to injury caused by the hopper during feeding which might have facilitated easy infection. Rice tungro infection and mealy bug infection also favoured the sheath rot infection development.
Ramabadran (1990) reported that spread of the disease was highly dependent on environmental conditions and was severe in densely planted fields and those infected by stem borers. Earhead bug (Leptocorisa acuta) also favoured the sheath rot fungus infection.
Lakshmanan (1992) also reported that mealy bugs (Brovennia rehi) acted as a vector for sheath rot pathogen and these bugs were found to be infested with conidia of Sarocladium, both internally and externally.
Response to Different Temperatures:
Kavamura (1940) found that, the Acrocylindrium oryzae (Sarocladium oryzae) grew at temperature range 13-37°C with optimum growth at 30-31″C. Chen (1957) stated that, the isolates of the fungus differed in their response to temperature and the pathogen was readily killed when incubated at 50°C for 5 min.
Optimum temperature for the growth of Sarocladium oryzae was 24-32°C. The colony diameter and sporulation of the fungus were maximum at temperature 30°C.
Singh and Raju (1981) also reported that the development of disease was maximum when the minimum temperature was 17-20°C and with minimum RH 40-50% at flowering. Sinha and Sinha (1996) reported that high temperature (27-30°C), high RH (92-100%) and light precipitation from October to the first week of December were conducive for disease development and spread.
Yield Loss:
The pathogen mainly causes damage to the flag leaf sheath, which encloses the panicle and hence effecting the emergence of panicle, increasing the chaffy and half-filled grains depending upon the severity. Thus, the yield losses are caused by the infection. The damage caused by the fungus nearly 3-20% losses were in general and sometimes as much as 85 per cent under severe infection.
Severe yield losses were also reported from abroad viz., Taiwan, Philippines, Thailand, United States. Mohan and Subramanian (1977) also reported nearly 6.2% yield loss in Jagannath, and 57.4% yield reduction in Co39 due to sheath rot. Losses to an extent of 9.6% to 26% by way of reduction in grain yield were reported by Chakrabarty and Biswas (1978).
Estrada (1984) found 52.8% yield loss due to sheath rot under severe infected conditions. Srinivasan (1980) reported that severe infection of sheath rot had reduced the yield up to 90% under Indian conditions. Enormous reduction in yields were also reported from different states in India.
Thrimurty (1980) reported that sheath rot infection increased the chaffy grain percentage in some popular rice grown in Chhattisgarh region. Similar observation was also reported by Srinivasan (1980).
In an elaborate study conducted at International Rice Research Institute, Philippines (1981), a positive relation was observed between the quantitative data on flag leaf sheath infection, panicle exscertion, grain filling and discoloration and with that of disease severity.
Tikko (1985) and Surin (1988) reported that sheath rot infection reduced the number of tillers/panicle, number of grains/panicle and also the 1000-grain weight and yield reduction were directly proportional to the severity. Singh (1985) and Dhal (1998) reported that yield losses varied from 1.7 to 54.7% in 6 different cultivars under conditions of artificial inoculation in field.
Similar reports of reduction in 1000 grain weight, poor grain filling was observed due to sheath rot. Upadhyay and Diwakar (1984), Sachan and Agarwal (1995) and Jaykumar (1995), also reported grain discoloration due to sheath rot.
9. Disease Management of Sheath Rot Disease of Rice:
Different fungicides were tried by several researchers in order to know their effect on Sarocladium oryzae in in-vitro. Fungicides, Bavistin (Carbendazim), Benlate (Benomyl), Hinosan (Ediphenphos) and Dithane M-45 (Dithiocarbamate) inhibited the growth of Sarocladium oryzae.
Purkayastha and Roychaudhari (1977) tested the efficacy of 6 fungicides i.e. Benomyl, Vitavax, MBC, Kitazin, DDUP and Aureofungin against Sarocladium oryzae and found that among these fungicides MBC was most effective, in reducing the growth and sporulation.
Chein and Huang (1979) reported that Bavistin (Carbendazim), Busan (TCMTB) and Benlate (Benomyl) were found effective in-vitro. Raju and Singh (1981) tested the efficacy of 10 fungicides and found that 3 sprays of Bavistin, Benlate or Brestanol significantly reduced the disease severity. Corroborating with the above reports, Benomyl and Carbendazim were also found to be highly effective in reducing the disease severity.
Thrimurty (1986) studied on chemical control of sheath rot in Chhattisgarh region and found that Hinosan, Bavistin and Dithane M-45 significantly reduced Sarocladium oryzae infection as compared to other chemicals tested.
Alagarsamy, (1986) reported that Captafol at a concentration of 0.125 per cent reduce the disease severity and found to be best among the seven fungicides tested. In contradiction to the above reports, Lewin and Vidhyasekaran (1987) reported that fungicides could inhibit the test fungus in cultural conditions, but failed to reduce the disease severity in field conditions.
Vidhyasekaran and Lewin (1987) concluded that sprays of Carbendazim every 3 or 5 days, Captafol at 3, 5 or 10 days intervals, controlled fungus completely. But fungicides sprayed at longer intervals were ineffective. On the contrary Ramabadran (1990) stated that disease is difficult to control by chemical application.
They also suggested that seed treatment with Benomyl and Panoctine (guazating) is useful to manage this disease. Karmakar (1992) tested Carbendazim at 0.1%, protected the plants against Sarocladium oryzae and reduced the disease severity.
Viswanathan and Narayanasamy (1993) evaluated Mancozeb, Edifenphos and Tricyclazole for the control of sheath rot of rice, using 3 rice varieties. They found that Mancozeb + Tricyclazole gave the best control.
Fungicide treatment also increased the number of productive tillers and grain yield. Das (1997) evaluated 6 fungicides (Edifenphos, Thiophonate methyl, Carbendazim, Kitazin, Validamycin and Mancozeb) for the control of sheath rot of rice. Two sprays of Thiophonate methyl or Carbendazim at 0.1% were highly effective in controlling the disease.
10. Varietals Resistance of Sheath Rot Disease of Rice:
Use of resistant varieties in disease management practice is the least expensive, easiest, safest and one of the most effective means. In sheath rot management many cultivars were evaluated by several workers.
Chen and Chein (1964) observed some degree of differences in susceptibility to the disease in field conditions in Taiwan. Ahamed (1974) observed that Karikalan, Karuna, TKM6 and Sigadis showed high degree of resistance. Amin (1974) and Raichoudhari and Purkayastha (1980) found that semi dwarf and dwarf varieties showed more susceptibility than tall cultivars.
Generally, Japonica varieties were reported to be less susceptible to sheath rot. In contradiction to the above findings, Fang (1980) and Liang (1980) found Japonica cvs. to be more susceptible than Indica cvs. Lakshmanan (1991) also screened 87 breeding lines derived from O. officinalis and 15 lines showed high level of resistance under artificial inoculated conditions. Reddy and Ghosh (1993) also reported resistant rice to sheath rot.
Naik (1976) reported IR-24, IR-26, CR-44-120-1, RP-825-71-4-l,RP-884-81-l and RP-874-112-1-6 as resistant to sheath rot from Orissa. Amin (1976) screened 243 varieties and 1050 progeny lines.
The cvs. Tetep, ARC 7117, Ramtulsi, Manoharsali, D-25-4, Sigadis, SR 26B, Intan, Zenith, Dissi Hatif, Raminad str 3 and Tadukan all tall varieties were highly resistant. Shahjahan (1977) reported that Bluebella, Labella, Labonnet, Nova 66 and Starbonnet were found as highly susceptible to sheath rot.
Kannaiyan (1978) screened 130 rice varieties against sheath rot pathogen and found that AS5821, AS6975, AS2992, 1287, 3226 and 6403 were free from sheath rot infection, while 18 were resistant, showing disease incidence ranging from 2 to 20%. Mohan and Subramanian (1972) tested reaction of 16 rices against Sarocladium oryzae, and reported that infection per cent ranged between 3.6% in Jagannath to 43.6% in Co39.
Lin and Ts Ai (1977) reported that Taichung Sen Yu 229 was resistant. Chien and Thseng (1982) screened several cultivars against sheath rot from Taiwan. Estrada (1979) tested 3 screening methods for sheath rot resistance evaluation.
Raju and Singh (1978) reported that out of 46 varieties inoculated under field conditions Sigadis and Homthong were resistant and URR-171-12 and Ratna were moderately resistant. Mukerjee (1980) tested 170 varieties against sheath rot, 5 varieties namely Masuri, Vishnubhog, Gajgor, Kala Namak and Usha, were found resistant to this disease.
Among 17 varieties evaluated, Tainung 61 and Japonica varieties showed less per cent of infected panicles, brown grains and grain sterility than Indica varieties. Purkayastha (1983) reported that momilactone was present in higher concentration in tall cultivars resistant to sheath rot and in smaller amounts in susceptible semidwarf cultivars. The antifungal nature of this compound was plso confirmed by TLC and bioassay.
Paromita (1986) screened 83 germplasm lines against S. attenuatum, 7 cultivars were found resistant, 4 from early maturing group and 3 of the late maturing group.
Saswati Biswas (1988) reported that peroxidase and polyphenol oxidase activities increased in rice plants inoculated with Acrocylindrium (Sarocladium} oryzae. Phenylalanine ammonia lyase (PAL) activity decreased gradually upto 7 clays in healthy and inoculated plants, but levels were higher in inoculated plants.
All enzymes levels increased after gibberellic acid (GA3) treatment, but decreased in plants treated with Aureofungin or Carbendazim. Shukla (1995) evaluated 183 accessions against the major diseases of rice including sheath rot. Each accession was tested for a minimum of 2 seasons.
Most accessions showed fairly high tolerance to sheath rot. Kumari (1998) tested field tolerance levels of Indica rice varieties for major rice diseases. They reported that rich diversity in cultivated and the traditional rice varieties offered a pool of resistance genes against disease like blast, sheath blight, sheath rot, brown spot, false smut etc. that causes considerable damage to rice crop.
Number of researchers also screened different cultivars for sheath rot resistance.
Upadhyay and Diwakar (1984) from Chhattisgarh, reported that among several commercially grown rice varieties, Asha, Usha, Safri-17, Dubraj and Madhuri showed some resistance during 1981 and 1982 under field conditions.
Sthapit (1991) observed that indigenous germplasm showed good tolerance for sheath rot in Nepal. Velazhahan and Ramabadran (1992) tested the effect of 5 levels of potassium (0,50,100,150 and 200 kg/ha) on the phenolic content of rice, at different stages of pathogenesis.
Increasing the potassium level (0-200 kg/ha.) reduced disease intensity and increased grain yield. The application of potash also increased the phenolic content. Photoperiod insensitive variety Rajavadlu was reported to be resistant by Kashikar (1994).
Hemlatha (1999) tested a crude toxin preparation from Sarocladium oryzae as a molecular sieve, using in-vitro techniques for selection of resistant somaclones of rice. Sakthivel and Gnanamanickam (1986) identified cerulenin as a toxic metabolite produced by Sarocladium oryzae in rice tissues showing sheath rot symptoms.
Efficacy of Plant Leaf Extracts on Sarocladium Oryzae Growth:
Bio-pesticides are becoming popular because of their eco-friendly nature. Botanical product extracts were tried by several researchers in order to know their bio-pesticidal efficacy against wide range of pathogens. On sheath rot causing pathogen also efficacy of botanicals formulations were tested and reported.
Narsimhan (1993) reported that neem seed kernal extract at a concentration of 5 per cent, when applied as foliar spray at booting stage followed with a repeated application after 10 days reduced the infection and increased yield. Its effect was comparable with that of Carbendazim treatment.
In general, Azadirachtin formulations were found to be promising at most of the test locations in reducing sheath rot incidence. Neem derivatives (neem oil, neem seed kernal extract) and leaf extract of Vitex negundo, Accacia leucocephala and Polyalthia longifolia reduced sheath rot significantly. Jeeva and Ramabadran (1993) screened 23 plants for inhibition of conidial germination of Sarocladium oryzae.
They found that cold water extract of 4 plants (Arachis hypogea, Caesalpinia pulcherrima, Euphorbia hirta and Ipomea crassicaulis) were inhibitory to sheath rot pathogen and ethanolic extract from I. crassicaulis gave the maximum inhibition.
Extracts of Caesalpinia pulcherrima and Impomea crassicaulis also reduced the abnormality incited by sheath rot pathogen in rice. Rajappan (1997) stated that leaf extracts of Ipomea spp. effectively controlled the growth of sheath rot pathogen.
Narasimhan (1998) stated that Neem oil and Pungam oil based EC formualtion [Viz. Neem oil 60 EC (acetic acid), Neem oil 60 EC (citric acid) and Neem oil + Pungam oil 60 EC (citric acid)] were evaluated for their efficacy against sheath rot pathogen. All the 3 formulations efficiently inhibited the mycelial growth of the pathogen in-vitro.
Rajappan (1999) found that fresh and stored sample of neem oil based EC formulation (0.225, 0.4 and 0.9%) showed a significant reduction in the mycelial growth of Sarocladium oryzae at all the 3 concentrations and storage duration (0, 3, 6 and 9 months) of the neem oil in comparison with control.
Pramanick and Phookan (1998) reported that among 10 plant extracts tested against Sarocladium oryzae in-vitro an aqueous extract of Ocirnum sanctum was most effective followed by Eucalyptus citriodora and Azadirachta indica.
Lesions on rice plant (cd. IR-36) were also significantly restricted by pre inoculation spraying with these 3 plant extracts. Rajappan (2000) also reported that neem oil inhibited the growth of Sarocladium oryzae.
Avdhesh and Satapathy (1977) reported that extract from different plant parts (viz. leaf, flower, stem and root) of Vinca rosea were found antifungal to Helminthosporium nodorum, Sclerotium rolfsii, Pestalotia sp., Fusarium oxysporum, Colletotrichum sp. and Aspergillus niger.
It inhibited the spore germination, sporulation and mycelial growth of fungi. Mishra and Tiwari (1992) tested the ethanol extract and essential oil of the leaf of Polyalthia longifolia against 5 rice pathogen viz. Pyricularia oryzae, Rhizoctonia solani, Fusarium moniliforme, Aspergillus niger and Curvularia lunata. They found that ethanolic extract was more effective, with broad-spectrum fungitoxicity than essential oil.
Several reports were published by different workers on the efficacy of plant products in inhibiting the growth of different plant pathogens of important crops.