Effect of plant age and spore load of Alternaria tagetica on development of blight of marigold

Marigold (Tagetes erecta) is a commercially cultivated flower crop that has been associated with auspicious occasions since ancient times. Blight caused by Alternaria tagetica has been found to inflict serious losses to this crop. There are several gaps in our knowledge on epidemiology of the disease. The present study was carried out to study the effect of plant age and spore load concentration on susceptibility of marigold to A. tagetica. Plants of marigold cv. Pusa Narangi Gainda, of varying age, were artificially inoculated with the fungus and weekly observations on disease incidence and severity recorded. Maximum disease incidence and severity were observed on 12 week-old plants, followed by 10, 8, 6 and 4 week-old plants, respectively, during 2015–16 and 2016 17 crop seasons. The effect of spore load of the fungus on development of the disease was, studied on two marigold varieties, viz., Punjab Gainda No. 1 and Pusa Narangi Gainda. The disease incidence increased with increase in spore load concentration in both the cultivars. Per cent disease severity also showed a similar trend. The per cent disease severity was found to be significantly more in cv. Pusa Narangi Gainda than Punjab Gainda No. 1, during both the crop seasons. It may be inferred that there is an increased need for management of the disease during later stages of crop growth, since the vulnerability of the marigold plants to Alternaria increased with age. High disease pressure will also have to be avoided as it further complicates the situation.