Cultural, Morphological and Pathogenic Characterization of Isolates of Alternaria helianthi Causing Sunflower Blight**

Twenty-five isolates of Alternaria helianthi were collected in pure form from Directorate of Oilseeds Research, Rajendranagar, Hyderabad for cultural, morphological and pathogenic characterization. All the isolates showed considerable variation in respect of colony margin, colony growth, pigmentation, colour of aerial mycelium and sporulation. Six isolates (Ah-2, Ah-5, Ah-7, Ah-18, Ah-23 and Ah-25) showed circular growth pattern and the remaining isolates recorded irregular growth with wavy margin. Based on variation in pigmentation and colour of aerial mycelium, 25 isolates were categorized into five types. Variation in colony diameter varied from 21.0 mm (Ah-19) to 42.0 mm (Ah-24). Isolates Ah-2, Ah-12, Ah-21, Ah-24 and Ah-25 recorded abundant sporulation (>12 × 10⁴ spores ml⁻¹), isolates Ah-3, Ah-14 and Ah-18 showed scanty sporulation (< 4× 10⁴ spores ml⁻¹) while the remaining displayed moderate sporulation. The range of average conidial length and width varied from 124.2 (Ah-23) to 158.4 μm (Ah-12) and from 30.7 (Ah-9) to 40.1 μm (Ah-25), respectively. The average septation was highest in isolate Ah-7 (8.9), while that was least in Ah-3 (6.0). Based on similarity in morphological and colony growth characters, the 25 isolates were categorized into seven groups and from each group one representative member was evaluated for variability in pathogenicity by using five differential cultivars (DRSH-1, DRSF108, KBSH-44, KBSH-1, and NDSH-1) along with one susceptible genotype (Morden). Though all seven isolates induced symptoms on all the differentials, their level of virulence varied greatly across the host cultivars. The isolate Ah-21 was found highly virulent with highest mean virulence index (15.2) and Ah-18 was found least virulent with lowest mean virulence index (4.6).