ASSESSMENT OF THE ROLE OF SOIL MOISTURE AND TEMPERATURE IN CAUSING DRY ROOT ROT BY Rhizoctonia bataticola (Taub.) Butler IN PIGEONPEA (Cajanus cajan (L) Millsp.)
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Date
2018
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Acharya N.G. Ranga Agricultural University
Abstract
Pigeonpea cultivation is subjected to high yield losses every year. Dry root rot or Macrophomina stem canker caused by Rhizoctonia bataticola (Taub.) Butler [synonym- Macrophomina phaseolina (Tassi) Goidanich] of pigeonpea is an emerging and serious problem in late sown or summer crops and in perennial or ratooned pigeonpea. A very little work has been done on dry root rot pathogen in pigeonpea crop. Keeping in mind the importance of this emerging disease with respect to changing climate, the present study involving cultural, morphological and pathological variability among the isolates, effect of temperature, moisture and pH on the growth and development of the pathogen and various biochemical alterations occurring with respect to disease development in pigeonpea was studied. All the investigations were conducted under laboratory and glasshouse during 2016-17 at Legumes Pathology Division, International Crops Research Institute for the Semi-Arid Tropics (ICRISAT), Hyderabad.
Cultures of the R. bataticola of pigeonpea was procured from the Legumes Pathology division, ICRISAT. The isolates were sub-cultured, molecular characterized and serially designated as Rb1 to Rb10. Pathogenicity test against pigeonpea variety Bahar using paper towel technique as employed to find out the virulent isolates. Maximum disease severity rating of 8 was observed in Rb 6 inoculated plants followed by 7.3 in Rb 1 and Rb 4 inoculated plants.
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Influence of different media, temperature and pH were tested on the growth of R. bataticola isolates. Carrot agar and PDA was found to support maximum colony growth of all the isolates. The optimum temperature for growth was found to be 35°C at par with 30°C. At 45°C no growth was observed even after 72 HAI. The optimum pH range for growth was found to be 5-7, where the maximum colony growth of 90.00 mm was attained at 48 HAI. At pH 3 and 9 the growth was drastically reduced.
Cultural and morphological variability existed among the isolates tested. The radial growth at 48 HAI ranged from 71.17 mm (Rb 6) to 50.67 mm (Rb 2). The colony colour varied from black to grey except for Rb 1 producing off white colonies. In soybean and mungbean isolates, the colony colour was brown. The colony texture was fluffy to partially fluffy and aerial mycelium was observed in all the isolates. The shape of the sclerotia varied from round to oblong. The colour of sclerotia was recorded to be black in all cases except for soybean and mungbean isolates which were brown. The sclerotial texture varied from smooth to rough. The average length and width of sclerotia varied from 9.97 and 9.71 (Rb 1) to 5.50 and 3.60 (Rb 5). The average sclerotial intensity varied from 15.60 (Rb 5) to 76.15 (Rb 9) The average sclerotial initiation days varied from 2 to 3 days with the exception of Rb 1 showing the maximum of 4.67 days.
Temperature and soil moisture plays an important role in the infection and disease development by the pathogen in susceptible genotypes. Disease severity was observed to increase in plants with the increase in temperatures up to 35°C, above which physiological wilting was observed. In case of different soil moisture regimes, R. bataticola inoculated plants grown at 60% SMC showed maximum reduction in dry weight and thereby disease severity. Progressively the disease severity was observed to decreases with increase in SMC.
A wide range of alterations in the biochemical factors within the plants were activated in response to pathogenic infection. The total sugars was observed to reduce with progressive time points of inoculation, whereas phenols and plant defence enzymes like PO, PPO and PAL was observed to increase due to infection. In case of dry root rot, the disease expression and biochemical activity was higher in plants maintained at 60% SMC as compared to those from 100% SMC.
It is concluded that the climatic factors such as temperature and soil moisture has a direct influence over the growth and disease development by the pathogen R. bataticola. Thus in the advent of climate change, dry root rot of pigeonpea could gain the status of a severe disease of pigeonpea.
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D5729
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