Understanding the status and epidemiological studies of downy mildew in Knol-khol across sub-tropical region of India

Abstract

Knol-khol (Brassica oleracea var. gongylodes L.) is a cool-season crop widely cultivated in Jammu and Kashmir, West Bengal, and Karnataka. This study aimed to explore various aspects of downy mildew in Knol-khol, assessing disease status, and epidemiological studies. Symptoms on Knol-khol manifested as small angular spots with a pale yellow color, later turning black. On the lower side of the leaves, affected areas were covered with a grey-white downy growth containing numerous conidiophores bearing conidia. Morphological characteristics of Peronospora parasitica showed aseptate, hyaline conidia, and globose to spherical in shape, with an average length and breadth of 22.99 µm and 20.67 µm, respectively. Survey results in the Jammu division indicated considerable variation in disease incidence and intensity across locations in districts, Jammu and Samba, on different days after transplanting (DAT) - 30, 40, 50, and 60. Overall mean incidence and intensity recorded during these DAT periods were 12.66%, 9.73%; 21.77%, 17.48%; 41.07%, 36.02%; and 53.15%, 43.67%, respectively. The disease was significantly influenced by weather conditions during the cropping period, with a significant variation of 99%. Correlation data between weather parameters and disease intensity revealed highly significant positive correlations of 0.94 and 0.84 with the disease index for maximum and minimum temperatures. A highly significant negative correlation of -0.83 and -0.72 was observed with maximum and minimum relative humidity, respectively. However, a non-significant and negative correlation of -0.49 was noted with rainfall.