Evaluation of non-injury inoculation technique for assessing Sclerotinia stem rot (Sclerotinia sclerotiorum) in oilseed Brassica.

The field assessment technique to evaluate the plants with a fungal phytopathogen for their tolerance to the disease is one of the crucial steps in dissecting their genetic control and in developing the resistant crop varieties. The objective behind this study was to develop and evaluate a field-based non-injury method of inoculation technique for Sclerotinia stem rot (SSR) in oilseed Brassica, caused by Sclerotinia sclerotiorum (Lib.) de Bary. The non-injury method of screening technique involves stem inoculation using a five days old mycelial mat on potato dextrose agar (PDA) plug placed on the top of sterile water-soaked cotton pad firmly wrapped over the internodal region with parafilm at the basal portion of the stem (15-20 cm above the ground) in the field. Inoculation without injury substantiates the natural means of infection in the field and the use of moist cotton pad keeps humidity for longer to initiate infection even in case of adverse climatic conditions. Disease development on the inoculated stem was measured by the length and width of the lesion. The symptom appears with water-soaked lesion formation and spreading deeper and wider on the stem in >90% of inoculated plants. During the experiment, about 800 Brassica germplasms including their wild relatives were screened and evaluated for three consecutive years using near-natural (non-injury) method of disease inoculation in the field. The Inoculation severity index (ISI) obtained during these years at Pusa, New Delhi were significantly similar and correlated with the natural infection measured in terms of disease severity index (DSI) on selected germplasm in the sick plot at ICAR-DRMR, Bharatpur. The significant correlations obtained among the used Brassica lines that were earlier not subjected for natural screening suggest the potential of this technique in evaluating the breeding material for SSR before confirmation with natural infection in the field.