4. REVIEW OF LITERATURE:
The literature pertinent to the proposed study has been reviewed as under:
4.1 Economic losses:
Kernel Smut is a major problem in rice hybrids (Sharma et al. 1996; SHU. Z.Y et al. 1993) in India and China. Incidence of kernel smut has been recorded up to 15-20 percent on some of the CMS lines in Punjab which has proved a major bottleneck in seed production and release of rice hybrids.(Ayado et al 1993; PAN. X. X. et al 1995; Liang et al1996; Fan et al. 1994; Agarwal et al.1981).
There are few reports on the study of pathogenic variability of the pathogen however some workers have studied the Cultural and physiological variability among the isolates of T. barclayana. ( Singh 1998; kumar et al. 2001). Pannu et al. 2002 categorized the Punjab isolates in the four distinct groups, based on morphological, cultural and pathogenic variations. Polulation variations have also been observed in punjab isolates using random polymorphic DNA analysis (Chahal et al. 2007). Genetic variations, based, both on RADP and RELP analysis, have also been observed by (Pimental et al. 1998), which indicated that T. barclayana populations were comprised of two distinct taxa with one cluster corresponding to infecting isolate and other to isolates infecting species of panicum and paspalum.
The pathogen infests only a few grains in a panicle and often only a part of the grain. Padwick et al (1944) assign the fungus to the genus Neovossia and named it Neovossia horrida (Tak.) Tullis et al (1952) conducted further studies on spore germination and inoculated the pathogen to several grass hosts (Pennisetum spp.), on which they found that it produced spores essentially identical to those of Neovossia barclayana Brefeld. Thus, they agreed with Padwick and Khan that the fungus belonged to Neovossia and named it Neovossia barclayana which has priority. Fischer et al (1953), however, retained the name Tilletia horrida Tak. Fischer and Halton (1957) did not recognize the pathogen as a species of Neovossia. However, Duran and Fischer (1961) agreed that the fungus on rice was the same as that found on grasses by Tullis and Johnson (1952) but regarded the mature spores to be more of the genus Tilletia than Neovossia. They, therefore, disposed it as Tililetia barclayana (Bref.) Sacc and Syd., which is now generally used. Later on, Singh and Pavgi (1972 ) and Singh et al (1979) studied the cytology of teliospore germination and development of fungus and were of the opinion that it belonged to Neovossia. (Lori et al. 2006) ver, to date, both the names are being used by different workers. The fungus was reported to be homothallic based on presence of two nuclei in detached sporidia (Singh et al. 1970) nevertheless, later studies suggest the heterothallic nature of the fungus based on inoculation with single and paired monosporidia.
4.3 Etiology and spore germination:
Teliospores of the fungus are thick-walled that germinate under specific conditions. The smut spores remain dormant for 4-5 months after the time of harvest (Shetty et al. 1988; WEI et al. 1934; Chahal et al. 1993). The teliospores have also been reported to become dormant under cold conditions (Chahal et al. 1992; Whitney 1972). The optimum temperature for teliospore germination was observed 25-30°C (Liang et al. 1996, Chahal et al. 1993).
The fungus produce filliform and allentiodes sporodia on PDA and host extract media (Chahal et al. 1999). The mature spore contains a single diploid nucleus, that enlarges and undergoes successive divisions in the spore itself to produce 32 to 76 sporidia in a whorl at the tip of the promycelium. The single nucleus undergoes a mitotic division making the sporidium binucleate. On these sporidia, sterigmata is developed which bear hyaline,curved, secondary sporidia measuring 7.5-13.0 x 1.2-1.8 µm in size. The two nuclei from the primary sporidium pass into the secondary sporidium. These nuclei undergo a conjugate division to form nuclei for subsequent secondary sporidia. On germination, the secondary sporida produce infection hyphae, which is dikaryotic (Singh et al. 1987).
4.4 DISEASE CYCLE AND SPREAD
Chlamydospores (Teliospores) of the fungus are the primary source of infection, which survive in the soil and are carried through infected seed (OU et al. 1985, Singh et al. 1987). Sharma et al. (1999) reported that at soil surface, the chlamydospores remained viable for two years and there germination was recorded even after 3 years. However, at 5-15 cm soil depth the spores remained viable for more than one year and in traces up to second year. Nevertheless, under laboratory conditions, when the spores were stored in glass bottles, they remained viable for four years. Liang et al. (1996) reported that chlamydospores lost viability after three years of indoor storage..
4.6 Host resistance:
There are diverse reports on the reaction of varieties to bunt infection in india. Early maturing varieties are reported to suffer more than late maturing varieties (Choudary et al. 1951). Chauhan and verma (1964) identified 22 resistant genotypes belonging to early, medium and late maturing varieties. They reported that in Punjab, 76 percent early maturing and 62 percent medium maturing varieties were highly susceptible, while the late maturing varieties escape the infection. However, Kameshwar Row (1962), in Orissa, found that severity of the disease was higher on medium duration varieties. Singh and Pavgi (1970) did not find any resistance in the 412 varieties tested in asssam. Muthusamy and ahmed (1977) reported IR36 as the most resistant variety among the 19 cultivars tested. Trimurthy and singh (1982) found pankaj, jagriti, garima, patel85 and safri 17 as resistant cultivars, while Srinivasan 1982 identified Ad5426, Adl6674 and Ad14758 as resistant varieties against the disease. Usually, short grained cultivars had more resistance as compared to long grained cultivars. Biswas et al (2003). Disease incidence and severity was found to have positive correlation with anthesis period and negative correlation was observed with pollen concentration (Chahal et al. 2003)indicating that rice genotypes with shorter anthesis period and higher concentration of pollen are less prone to ks as compared to genotype having other combinations.
The cytoplasmic male sterile lines and are usually more susceptible to kernel smut with high disease severity and incidence as compared to inbred cultivars (MEW et al. 1998). Sharma et al. (1999) and pan et al. (1995) attributed the higher susceptibility of cms lines to exceptionally longer duration of floret opening. Kumar et al. (1997) found low incidence of disease in rice hybrid, Perh 207 as compared to other 23 hybrids and 4 commercial varieties tested.
Seed treatment with fungicides is ineffective because of the non systemic nature of the disease and occurrence of infection only at flowering stage. (Templeton et al. 1968;Sharma et al. 1999). Fungicidal sprays have been tried at flowering stage of the crop. Templeton et al. (1977) tried two systemic fungicides (plantvax and vitavax) and found that Plantvax applied at late boot stage reduced the bunt infection. Benomyl, Maneb, Chlorothalonil or Triphhenyltin hydroxide as ineffective to control the disease (Whitney et al. 1977). Among the large number of fungicides screened as foliar sprays, triazole compounds namely, Tilt (propiconazole), Folicur( tebuconazole) and Cantaf ( hexaconazole) have been found quite effective (Sharma R.C. and S.S GILL (1997). These fungicides are effective at the rate of 0.1 percent when sprayed twice, at 5 percent flower initiation stage and after 10 days . Folicur, Tilt and Score 10 (difenoconazole) have been reported to inhibit sporidial production under in- vitro conditions (Kaur et al. 2002).