A root rot and wilt disease of pyrethrum (Chrysanthemum cineraefolium) caused by Rhizoctonia solani AG‐4 in the north Indian plains

Abstract

‘Safed musli’ (‘moosli’) ( Chlorophytum borivilianum ) is a commercially important medicinal plant in India. Cultivation of this crop in Lucknow, Pantnagar and adjoining areas in northern India was severely affected by a leaf blight disease in 2003 and 2004. Initial symptoms were the appearance of minute, pin-head, circular, reddish-brown lesions on the leaves, which form longitudinal streaks along the midrib, veins and margin of the infected leaves. Severely affected leaves dry prematurely and plants fail to produce healthy fingers (tubers) used for medicinal purposes. The disease occurred in August and September, after the onset of the rainy season, and caused losses of up to 30% in severely affected fields. Isolations from the young and mature necrotic lesions on potato dextrose agar (PDA) invariably yielded a Colletotrichum species . The fungus initially produced white to grey mycelial growth which became reddish brown due to copious sporulation after 5–7 days. Acervuli were numerous, globose to saucer shape with large number of dark brown setae 96– 124 μ m long. Conidiophores were short, simple and hyaline. Conidia were aseptate, fusiform, sickle-shape and single celled. Those from host material were 10–15 (13) × 3–4 (3) μ m and 10–21 (14) × 3–5 (4) μ m after 7– 10 days’ growth on PDA. Based on these morphological features, the fungus was identified as Colletotrichum capsici . Isolates were morphologically identical to C. capsici isolates infecting Ocimum basilicum (Alam et al ., 1981) . To prove pathogenicity, 45-day-old healthy plants, bearing five to six leaves, were sprayed with C. capsici isolates from Lucknow and Pantnagar regions, in aqueous suspensions of 1·5 × 10 6 conidia mL − 1 . Symptoms identical to those found occurring naturally were observed 3–5 days after inoculation. After 7–12 days the original lesions had spread lengthwise and often coalesced to form long, necrotic streaks, producing typical leaf blight symptoms. The inoculated plants died prematurely. Control plants sprayed only with water remained healthy. Colletotrichum capsici was reisolated from the inoculated plants. This is the first record of C. capsici on ‘safed musli’ from India. Colletotrichum chlorophytumi has been recorded on Chlorophytum spp. growing in pots in various gardens in Allahabad (Sudhir Chandra Tandon, 1965). This species has slightly longer and broader conidia, and infections resulted in leaf spots that, at maturity, had three distinct zones of colour, a feature not seen with C. capsici on C. borivilianum .