First report of Sclerotinia sclerotiorum on Mimosa pudica in India

Mimosa pudica (sensitive plant), a native of tropical America is grown in many parts of the world including India. The plant contains a range of phytochemicals that have been used in traditional medicine (Ahmad et al., 2012).  Mimosa pudica is also used as green manure, cover crop and forage for cattle, but it can also be a weed in tropical agriculture.

Between November 2016 and February 2017, defoliation and stem rot was observed on M. pudica in Medziphema, Nagaland, India (25.7573°N, 93.8330°E). Initial symptoms included paleness of the leaves which gradually drooped, wilted and fell. The fungus attacked the plants from leaves to twigs close to the soil (Fig. 1). Occasionally, white mycelia and sclerotia were present on infected tissue of twigs and stems (Fig. 2). Approximately 20 to 50 % plants in three locations were infected. Diseased leaves and stems were collected, washed and cut into small pieces (5 × 5 mm). The tissue pieces were surface sterilised with 1% sodium hypochlorite for one minute, washed with sterile distilled water, plated on potato dextrose agar (PDA) and incubated in the dark at 22 ±2°C. The fungus completely covered the Petri plate (9 cm diameter) by the fourth day (growth rate 1 mm/h) and after 8 days it produced large black sclerotia (up to 6.2 mm in length). Morphological and microscopic observations were consistent with Sclerotinia sclerotiorum.

Koch's postulates were fulfilled by inoculating a mycelial plug from the isolate (2 mm diameter) on the leaves and twigs of three 75-day-old M. pudica plants grown in pots (25 cm diameter) in a glasshouse (Fig. 3). Inoculated plants were maintained at 25°C, 90% relative humidity and 12 hr alternating light and dark periods. Three uninoculated controls were also maintained under the same conditions. On the fifth day inoculated plants showed symptom initiation consistent with field observations including wilting of leaves and defoliation, and after 10 days, infection spread from the side shoots to the main stem. Two-three days later, cottony white mycelia and sclerotia were seen on the infected tissue around the point of inoculation (Fig. 4). Control plants remained symptomless and healthy. The fungus, re-isolated from the infected plant parts of the inoculated plants, exhibited identical morphological and microscopic features with the fungus originally obtained from infected field plants (Fig. 5). The culture was deposited at ICAR-NBAIM, UP, India with Accession No. NAIMCC-F-03354. Fungal genomic DNA was extracted using the CTAB method and amplification of the ITS 1 - ITS 4 region with primer pairs, ITS 1 (5'-TCCGTAGGTGAACCTGCGG-3') and ITS 4 (5'- TCCTCCGCTTATTGATATGC-3') (White et al., 1990) yielded a c. 500 bp amplicon which was sequenced. The nucleotide sequence (GenBank Accession No. MF563996) showed 100% identity to S. sclerotiorum (KM221197). The pathogen is known to be virulent on a number of hosts worldwide (Boland & Hall, 1994) as well as in India (Mondal et al., 2015).

Review of the literature revealed that this is the first report of leaf and stem rot of Mimosa pudica caused by S. sclerotiorum. This new host may aid survival and spread of the ubiquitous pathogen to other new hosts and threaten the production system if effective management strategies are not implemented.