“Epidemiology, virulence and molecular diversity in blast [Magnaporthe grisea (Hebert) Barr.] of pearl millet [Pennisetum glaucum (L.) R. Br.] and resistance in the host to diverse pathotypes”
Blast has emerged as an important disease in the major pearl millet growing areas in India. The present investigation was undertaken to study cultural, pathogenic and molecular diversity in the M. grisea isolates infecting pearl millet, effect of temperature and leaf wetness duration on blast development and to screen pearl millet lines for resistance to blast disease. Culture characters of four monoconidial isolates of M. grisea MgPM 45, MgPM 53, MgPM 56 and MgPM 118 were tested on OMA and PDA medium at different pH ranging from pH 5.5 to pH 8.0 (pH 5.5, pH 6.0, pH 6.5, pH 7.0, pH 7.5 and pH 8.0) and different incubation temperatures 22oC, 24oC, 26oC, 28oC and 30oC to select optimum conditions for growth and sporulation of the fungus. Results of this study indicated that pH of 6.5 and temperature of 28oC is ideal for the growth and sporulation of M. grisea adapted to pearl millet. Based on these results, variability in the cultural and morphological characteristics of 65 isolates of M. grisea was studied on OMA medium at pH 6.5 by incubating at 28oC. Culture morphology varied significantly among isolates. A range of colour variation in the medium was also observed from buff colour to black among field isolates with smooth or rough margin. The radial growth of the M. grisea isolates varied significantly; maximum radial growth of 4.25 cm was recorded for isolates MgPM 125 and MgPM 162 whereas minimum radial growth of 2.30 cm was recorded for the pearl millet isolate MgPM 148. Large variation was also observed for sporulation among field isolates. It was observed that isolates with grayish black and brownish black growth with sector formation produced more spores. In majority of the isolates, maximum sporulation was confined to sectored region. These 65 isolates were also tested for
pathogenic variation on a set of 10 host differentials (ICMB 93333, ICMB 95444, ICMB 97222-P1, ICMB 01333, ICMB 02444, ICMR 06444, 863B-P2, ICMR 06222 ICMR 11003 and IP 21187). The mean blast severity across the differentials was maximum for isolate MgPM 138 and minimum severity was observed for MgPM 132. Based on reaction type (avirulent/virulent), the 65 isolates were grouped into 28 different pathotypes. Pathotype G22 comprising isolates MgPM 121, MgPM 137, MgPM 138, MgPM 145, MgPM 148 from Rajasthan and MgPM 173 and MgPM 174 from Uttar Pradesh appeared as most virulent as it could infect all the 10 host differentials whereas pathotype G2 comprising MgPM 127, MgPM 129, MgPM 132, MgPM 149, MgPM 158, MgPM 159 and MgPM 39 was least virulent. These isolates could be grouped in five main clusters based on the results of molecular diversity study using URP markers. Among them cluster I (32) and III (31) included more than 95 per cent isolates whereas cluster II, IV and V contained 2-3 isolates. Studies on the effect of leaf wetness duration on disease development showed an overall increase in leaf blast severity, lesion length (mm), number of lesions per plant, lesion sporulation and leaf sporulation with the increase in leaf wetness duration (LWD). Based on the results it can be concluded, that both leaf wetness duration and temperature were essential for blast on pearl millet which becomes more severe at longer wetness durations beyond 48 hours during optimum day/night with a temperature ranging from 25±1/20±1oC to 30±1/22±1oC. For the identification of blast resistance, 160 designated B-lines of pearl millet were screened under greenhouse conditions against five pathotype-isolates viz., MgPM 45, MgPM 53, MgPM 56, MgPM 118 and MgPM 119. Multiple-pathotype (3-5) resistance was found in 23 lines. Eight lines (81B, ICMB 88004, ICMB 92444, ICMB 97222-P1, ICMB 02111, ICMB 07111, ICMB 09333 and ICMB 09999) were found resistant to all the five pathotypes. Similarly for the identification of stable sources of adult plant resistance, 28 lines were evaluated in the disease nursery (PMBVN) at six locations, Aurangabad, Dhule, Durgapura, Gwalior, Jamnagar and Patancheru during 2013 and 2014. None of the entries in the blast nursery was resistant at all the test locations. However, ICMR 06444 was found resistant at three (Gwalior, Jamnagar and Patancheru) locations and showed moderate resistance at other three locations. ICMB 01333, ICMR 11009 and HHB 146 improved (a hybrid) were resistant at Gwalior and Jamnagar. Pearl millet lines identified in this study that are resistant at 2-3 locations can be selected for use in pearl millet breeding programs aiming to develop blast resistant hybrids
