Biology and Management of Alternaria leaf spot of Soybean [Glycine max (L.) Merr.] Incited by Alternaria alternata (Fr.) Keissler

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Date
2020-06-26
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MPUT, UDAIPUR
Abstract
The present study was undertaken to study Biology and Management of Alternaria leaf spot of Soybean, as the disease is quite destructive in all the soybean growing areas. The studies were aimed to work out distribution of Alternaria leaf spot around Udaipur, Chittorgarh, Pratapgarh, Kota, Baran and Jhalawar, assessment of losses, cultural morphological and pathogenic variability of the pathogen, effect of different environment factors and to develop sustainable management strategy using fungicides, botanicals and host plant resistance. The survey of Alternaria leaf spot in soybean conducted for two years in Udaipur, Chittorgarh, Pratapgarh, Kota, Baran and Jhalawar districts of Rajasthan reveal that mean PDI from different districts, that disease pressure was slightly higher in the year 2019 as compared to year 2018. Maximum mean per cent disease index (PDI) of Alternaria leaf spot was recorded in Udaipur district whereas minimum disease severity of Alternaria leaf spot was recorded in Kota district. It was found that the Per cent disease index increased with generating disease through inoculum density (1 x 103 conidia ml-1) of A. alternata and significant reduction in grain yield was observed as compared to the inoculated protected and inoculated unprotected control. The foliar infection was also related to seed infection and seed collected from the inoculated unprotected plots of disease yielded colonies of Alternaria spp. There were distinct variations in symptoms observed in the disease samples collected during Kharif 2018 from the six major soybean growing districts of Rajasthan. Isolates of A. alternata recovered from various samples collected from Udaipur (UDP), Chittorgarh (CHI), Pratapgarh (PRA), Kota (KOT), Baran (BAR) and * Research Scholar, Department of Plant Pathology, Rajasthan College of Agriculture, MPUAT, Udaipur * * Professor & Head, Department of Plant Pathology, Rajasthan College of Agriculture, MPUAT, Udaipur Jhalawar (JHI) showed variations in symptoms on inoculation on pot-grown plants and also in growth characteristics, colony diameter, amount of sporulation and conidial morphology. The isolates showed considerable variations in latent period, symptomatology and disease severity on soybean. The latent period ranged between 35 to 63 hrs. in soybean. Interestingly, the isolate UDP Aa-1 of A. alternata was most virulent and expressed typical symptoms of leaf spotting. Of the twelve isolates, the maximum disease severity (65.2%) in soybean was observed with the isolate UDP Aa-1, while minimum disease severity (26.6%) in soybean was recorded due to the isolate KOT Aa-2. These results suggest that considerable variability existed in the population of A. alternata. Incubation period for growth A. alternata was evaluated for 3-25 days period. The maximum mycelial weight was 846.7 mg (UDP Aa-1) and minimum mycelial weight was 414.8 mg (KOT Aa-2) recorded at 20 days after incubation. The most virulent isolate of UDP Aa-1 of A. alternata could grow on wide range of pH and temperature. However, Maximum mycelial growth and sporulation of was recorded at pH 6.5 and 25 0C. Disease progress was influenced by different weather factors viz., temperature, relative humidity and rainfall. The maximum AUDPC value (area under disease progressive curve) was 389.45 and 394.42 on plants inoculated on 8th July and the lowest 266.18 and 259.18 was on that inoculated on 29th July 2018 and 2019. Of the four fungicides tested in vitro, combi product Azoxystrobin 8.3% + Mancozeb 66.7% WG completely inhibited the mycelial growth of all the six isolates of A. alternata followed by Azoxystrobin 23% SC which was also found quite effective at 500 and 1000 ppm concentrations. Neem formulations “neem oil at 0.5%” was found most effective in vitro. Under field conditions, combination of Azoxystrobin 8.3% + Mancozeb 66.7% WG at 0.36% and Neem oil at 0.5% was found most effective when applied as mixed foliar spray. Screening of cultivars showed JS-9305, JS-9752 and RVS 2002-04 were found moderately resistant and none of the cultivars found completely free from Alternaria leaf spot of soybean
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Biology and Management of Alternaria leaf spot of Soybean
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Fagodiya R.K. And Trivedi A.
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