Environmental variables on development of leaf spot and flower blight, Alternaria zinniae in African marigold

Leaf spot and flower blight disease has become an important constraint for marigold cultivation in Himachal Pradesh during recent years. Laboratory and field experiments were conducted during rainy season (Kharif) of 2006 and 2007 to assess the influence of different meteorological parameters on development and spread of leaf spot and flower blight of African marigold cv. Pusa Narangi Gainda caused by Alternaria zinniae. Under laboratory conditions, leaf spot severity was highest at 25°C and less at 35°C, while relative humidity more than 95.1% influenced the spread of the disease to maximum by causing 53.1% disease severity. In epiphytotic conditions, the disease usually appears in May and reached to maximum in August with increase in relative humidity and rainfall in both years 2006 and 2007. The simple, partial and multiple correlations indicated a positive correlation of disease with rainfall and relative humidity while temperature had a negative effect on disease development. The co-efficient of multiple determinations signified 99.2% & 98.6% (leaf spot) and 99.1% and 98.1% (flower blight) variation with the positive weather factors on disease development.