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

Pyrethrum (Chrysanthemum cineraefolium) is cultivated on a large scale in the states of Uttar Pradesh and Uttranchal in north India, where root rot and wilt disease caused by Rhizoctonia solani AG-4 was recorded for the first time in 1999-2000. It has become a limiting factor for commercialisation of this valuable crop in the region.

Infection initially appears in the form of dark brown, necrotic lesions on the roots and basal parts of the plant, which later turn into root rot and wilt symptoms, leading to premature death and desiccation of infected plants. Isolations from infected tissue invariably produced Rhizoctonia species. Based on cultural and morphological characters, the common isolates of Rhizoctonia were identified as Rhizoctonia solani Kühn (AG-4). The identification was confirmed with tests against isolates of known anastomosis group (AG). In pathogenicity tests, inoculum from one of the isolates recovered from pyrethrum was multiplied on a soil:sand:cornmeal mixture (6:3:1 v/v) for 10 days at 25°C and 20 g was added to soil around the basal portion of plants in 10 cm pots. In controls, no inoculum was added. Plants were kept under glasshouse conditions at 25°C and 80-90% relative humidity. The isolate from pyrethrum was highly pathogenic on pyrethrum, compared to isolates of Rhizoctonia taken from geranium and periwinkle, tested in the same way. Initial symptoms were the formation of minute, tan brown necrotic lesions on the roots of inoculated plants, 3-4 days after inoculation, which in later increased in size and often coalesced to form dark brown patches on the infected roots; the pathogen then invaded the basal stem and leaves which ultimately resulted in wilt of infected plants 5-7 days after inoculation. Control plants were free from infection. Re-isolation from artificially infected tissue consistently yielded R. solani of the same morphological type, thus fulfilling Koch's postulate.

Pyrethrum cultivation has been affected by root rot caused by Fusarium solani in Kashmir valley (Thakur & Husain, 1971) and wilt caused by Phytophthora cambivora and Rhizoctonia solani in Palni Hills of southern India (Ramakrishnan & Soumini, 1948). However, in the latter case the anastomosis group of R. solani was not identified.