Indian Phytopath. 66 (3) : 284-286 (2013) Fungitoxicant acting as ergosterol biosynthesis inhibitors (ESBIs) are widely used to control apple scab, caused by Venturia inaequalis in Kashmir valley (Anonymous, 2006). This group of fungitoxicants prevent the fungus from synthesizing ergosterol, an important components of cell and mitochondrial membrane, by inhibiting the reaction catalysed by enzyme 14-α-demethylase (Sholberg and Haag, 1993). The commercial use of fungitoxicant particularly systemic one introduces concern about the development of resistance in the pathogen (Lelancette et al., 1987; Koller et al., 2004). In order to monitor resistance to fungitoxicant it is important to know the baseline sensitivity of a number of isolates from a population before they have been exposed to a new fungitoxicants. Therefore, present study was first time conducted in Kashmir valley to determine the sensitivity of isolates of V. inaequalis in order to determine the comparable baseline sensitivity and discriminatory concentration of ESBI fungitoxicants used in the control of apple scab in Kashmir valley. MATERIAL AND METHODS Isolates of V. inaequalis were collected from six isolated orchards in Kashmir valley in 2006 and 2007. These isolates originated from a single conidium (monoconidial) isolated from one lesion per sample leaf, with diseased leaves randomly collected from atleast three apple trees cv. Red Delicious per orchard. Thirty monoconidial isolates from each of three orchards of Shalimar I, Zakura (district Srinagar) and Kunnil (district Kupwara) localities of Kashmir valley were collected. These isolates were never been exposed to sterol biosynthesis inhibitors. Thus represented as baseline or wild types isolates (Smith et al., 1991). Fifty isolates were collected from apple orchards of Shalimar II and Harwan (district Srinagar) and Pattan (district Baramulla) that had previously exposed to fungitoxicant 1 to 3 times atleast for the last 7-10 years. The diseased leaves showing typical symptoms were first examined for associated fungus by tearing the diseased portions with the help of tearing needle and observed under microscope (100 X). Thereafter, the diseased leaf along with some healthy leaf portion were cut into small bits using sterilized blade for isolation. The bits were surface sterilized in HgCl 2 solution (1%) for 20-30 second followed by three washing in sterilized distilled water. After blotting dry with pre-sterilized blatting paper, surface sterilized diseased bits were aseptically transfer to potato dextrose agar containing petriplates and incubated for 7-10 days at 20±1 o C. Single spore isolation technique (Johnston and Booth, 1983) was used for getting purified culture of the fungus. The isolate were allowed to grow for 2-3 weeks at 20±1 o C temperature. The pure culture were preserved at 4 o C temperature in refrigerator for use to assessment of the fungitoxicant sensitivity. Poisioned food technique (Nene and Thapliyal, 2002) was used to determined the sensitivity test based on both ED 50 and relative growth (RG) values. Colony fragment of 3 mm in diameter were take from the growing edge of 3 weeks old culture and transfer aseptically into PDA plates with seven fungitoxicant concentration of 0.0, 0.0001, 0.001, 0.01, 0.1, 1.00 and 10 μg ml -1 . Two replicates per isolate were maintained in a completely randomized design (Koller et al., 1997). These petriplates were incubated at 20 ± 1 o C. After four weeks of incubation sensitivity test was assessed by recording the radial mycelial growth of test isolate (minus the diameter of the incubation plug) with measuring scale. The ED 50 values were determined by recording the relative Venturia inaequalis sensitivity to ergosterol biosythesis inhibitors in Kashmir valley NASSREEN F. KACHO*, SABA BANDAY and SABIHA ASHRAF Division of Plant Pathology, Sher-e-Kashmir University of Agricultural Sciences and Technology of Kashmir, Shalimar Campus 191 121, Jammu & Kashmir, India ABSTRACT: Monoconidial isolates of Venturia inaequalis were collected in 2006 and 2007 from apple orchards in Kashmir valley that had never been exposed or atleast treated with only 1-3 spray of fungitoxicants acting as ESBI. Sensitiveness of baseline and exposed isolates to two ESBIs hexaconazole and fenarimol were determined by a sensitivity test based on relative growth of mycelial colonies. For both fungitoxicants the population was constructed each with 240 monoconidial isolates. Baseline population were constructed with 90 monocondial isolates (30 from each orchard) for both fungitoxicants. The ED 50 was determined for the baseline population which ranged from 0.0061 to 0.0233 μg ml -1 with a combined value of 0.0135 μg ml -1 for hexaconazole, while it ranged from 0.0284 to 0.0472 μg ml -1 with a combined value of 0.0352 μg ml -1 for fenarimol. For the combined population hexaconazole was 2.6 times more active than fenarimol. A discriminatory dose was selected close to ED 50 values to test sensitivity of exposed population of 50 isolates from each orchards. The single discriminatory dose for hexaconazole and fenarimol were observed to be 0.02 and 0.05 μg ml -1 , respectively. Key words: ESBI, sensitivity, Venturia inaequalis RESEARCH ARTICLE *Corresponding author: ftmkacho898@gmail.com