INVESTIGATIONS ON Xanthomonas cucurbitae (EX. BRYAN) VAUTERIN et al. CAUSING BACTERIAL LEAF SPOT OF CUCURBITS

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2019-10-23
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COLLEGE OF HORTICULTURE AND FORESTRY, DR Y S P UHF, NERI, HAMIRPUR
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Bacterial leaf spot is emerging as an important bacterial disease of cucurbits especially in bottle gourd, pumpkin and squashes. Bacterial leaf spot was found to occur in moderate to severe form in various locations of three districts viz., Hamirpur, Una and Bilaspur of Himachal Pradesh surveyed during 2018. Magnitude of disease severity ranged from 21.10 to 98.20 per cent in bottle gourd whereas, in case of pumpkin, disease severity ranged from 5.70 to 56.90 per cent. The incidence of the disease on fruits of bottle gourd ranged from 20 to 100 per cent at various locations surveyed, however in pumpkin, no incidence of disease was recorded on fruits. On nutrient sodium chloride agar, colonies of both isolates were yellow in color, mucoid, circular, smooth textured and having a diameter of 2-4 mm. Both the isolates tested positive for Esculin hydrolysis and protein digestion tests and negative for Gram’s staining reaction. Pathogenicity test on leaves of healthy seedlings of bottle gourd and pumpkin resulted in incubation period of 2 and 4 days, respectively. Among various temperature regimes ranging from 15-35°C tested for both isolates, the optimum temperature was observed to be 25 and 30°C for bottle gourd (5.35×107 cfu/ml) and pumpkin (4.15×107 cfu/ml) isolates, respectively. Out of six pH levels ranging from 4.0 – 9.0 tested to see the effect on bacterial growth, the optimum pH for maximum growth of the bacteria was observed to be 6.0 and 7.0 for bottle gourd (44.16×107 cfu/ml) and pumpkin (31.62×107 cfu/ml) isolates, respectively. However, no growth of any of the isolates was recorded at pH 4.0. The bacterium grew best in nutrient glucose broth (9.31×107cfu/ml; bottle gourd isolate), nutrient sodium chloride broth (7.70×107cfu/ml and 7.33×107cfu/ml; bottle gourd and pumpkin isolate) and yeast extract calcium carbonate broth (8.11×107cfu/ml and 10.63×107cfu/ml; bottle gourd and pumpkin). An inoculum density of 108 cfu/ml and syringe inoculation method was found to be best for the artificial inoculation of susceptible plants of bottle gourd and pumpkin. Disease was found to be negatively and significantly correlated with the average temperature and cumulative rainfall whereas positively and significantly correlated with the average relative humidity. Multiple regression equation indicated that 89.27 % and 89.19 % variation in the disease severity of bacterial spot in case of bottle gourd and pumpkin, respectively could be attributed to these three weather parameters under consideration.
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