VALIDATION OP CLIMATIC DISEASE SEVERITY VALUES OP POWDERY MILDEW (Uncinula necator (Schw.) Burr.) OP GRAPE (Vitis vinifera L.) UNDER FIELD CONDITIONS
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Date
1993-09-18
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MAHATMA PHULE KRISHI VIDYAPEETH Rahuri-413 722 (Maharashtra.)
Abstract
The experiment entitled "Validation of climatic disease severity values of powdery mildew (Uncinula necator (Schw.) Burr.) of grape (Vitie vinifera L.) under field conditions" was carried out during the post monsoon season of the year 1992-93 at Modi Baug Garden of Horticulture Section, College of Agriculture, Pune and on cultivatars' fields at four different locations in the major grape growing areas of Western Maharashtra. At Pune location five microclimatic compartments, each consisting of nine vines were erected with transparent polythene sheets and wooden frame. Varying microclimatic conditions, viz. temperature and relative humidity were created in each of these compartments by giving irrigation at different intervals, i.e., 3, 6, 9, 12 and 15d, respectively. Micrometeorological observations on temperature and relative humidity, within the canopy of vines, in these compartments were recorded for 15 minutes at hourly interval with Delta T psychrometer. Biological observations of powdery mildew on berries (%IN), spore load on berries (SL) and per cent disease intensity on leaves (PDI) were recorded at an interval of 3d. Observations on ambient meteorological conditions and disease incidence and progress were recorded on a vine selected just outside these compartments. Normal irrigations practice of lOd interval was followed for this vine. In the multilocation experiment, daily meteorological observations on temperature and relative humidity were recorded, starting from the date of pruning till the first visible symptoms of powdery mildew were noticed, by installing a single Stevenson screen in close proximity to five vines selected on windward side. At Pune location the incidence and multiplication of disease was rapid in the months of December and January when the climate was cool and humid compared to the climate in the months of November and February. The vines suffered early and more from the attack of disease where irrigations were applied at shorter intervals than the vines irrigated at longer intervals. The most critical minimum and maximum temperature for rapid multiplication of powdery mildew ranged from 10.6 to 14.5'C and 26.1 to 30.4*C with an average of 12.2 and 28.2*C, respectively. Similarly, the most critical relative humidity at 0730 h and 1430 h ranged between 91 to 97% and 53 to 61% with an average of 93 and 58%, respectively. The most critical minimum and maximum temperature for zero rate of multiplication for powdery mildew were 7.7 and 33.3 *C, respectively, while relative humidity at 0730 h and 1430 h were 100 and 47%, respectively. Thus, temperature in the range of 10.6 and 30.4 'C and relative humidity greater than 53% favoured the development of powdery mildew on grape. Below 7.7 *C temperature and 47% relative humidity and above 33.3'C and 100% relative humidity powdery mildew showed negative rate of multiplication. Disease severity values in the range of 0 to 2 were generated from these findings and were tested under the multilocation experiment. Rating of DSV to daily weather parameters at each location showed that none of the values either for jtemperature or relative humidity, 6-8 day prior to the incidence of the disease was zero. Favourable climatic conditions prevailing for last 6 to 8 consecutive days were responsible for the visible symptoms of the disease, which can be considered as the incubation period of the pathogen. The total sum of DSV of all parameters during whole incubation period at different locations ranged between 42 and 54, with an average of 48. From the findings of the present investigations, the cultivators can be advised to take up the need based spraying programme instead of fixed scheduled sprayings, when cumulative DSV approaches 34 and congenial climate prevails for consecutive five days. This operational meteorological information may result in not only fewer application of fungicides and hence permissible residues on the fruits, but also better disease management through better timing of spray and reduced environmental pollution.
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