Identification and pathogenicity of Alternaria alternata causing leaf blight of Bacopa monnieri (L.) Wettst. and its biocontrol by Trichoderma species in agrifields--an ecofriendly approach

A destructive leaf blight disease of Bacopa monnieri (L.) Wettst. is recently observed in West Bengal, India. The causal organism was identified as Alternaria alternata based on morphological characteristics and sequence analysis of the internal transcribed spacer (ITS) region. Extensive disease surveys were conducted in order to record the disease intensity by monitoring Bacopa cultivation at two selected areas of West Bengal from January to December, in four consecutive years 2016–2019. Two Trichoderma species, namely T. harzianum and T. asperellum and two fungicides (Bavistin and Blitox 50) were tested against the pathogen in both, in vitro and in the field. The study on the mechanisms of action of biocontrol agents for crop protection and growth promotion were done. The results of identification showed that the pathogen was Alternaria alternata (Fr.) Keissler. Maximum percentage of disease incidence (PDI) for Bacopa monnieri leaf blight disease of pooled data of four years in two places (Nilgung and Titagarh) reached its peak upto 59.95 f% and 64.82 f% respectively during August. Similarly, the pooled data indicated that in both places (Nilgunj and Titagarh) the effect of the disease was minimum such as, 10.28a and 13.30a% respectively in the month of December. The results of in vitro control of A. alternata showed that PIRG (percentage of inhibition of radial growth) exerted by T. harzianum, T. asperellum, Bavistin and Blitox-50 against A. alternata were 75.55ab, 77.22a, 73.68b and 80.50a% respectively. The statistical analysis (p = 0.05) also clearly depicted that the biocontrol agents and Blitox-50 had the same effect but it differed from the effects of Bavistin. In field trials, four years (2016–2019) pooled data analysis showed that PDI in control (untreated) Bacopa plots was much higher (63.02b %) in comparison to their treated counterparts. Plots treated with T. asperellum resulted only 17.40a% PDI which is very low percentage of leaf blight disease incidence. While application of 2% Blitox-50 resulted 21.82a % PDI which was statistically almost similar to T. harzianum (22.44a%) treatment. Thus, T. asperellum or T. harzianum has reduced the disease in vivo significantly. The mechanistic study of interaction of Trichoderma sp. and A. alternata probably by hyperparasitism or volatile metabolites (HCN and CH₃) or siderophore. Moreover, they induced plant defence enzymes (peroxidase and polyphenol oxidase) and also phenol production, increasing the immunity of the host plant. In addition to plant protection, T. asperellum and T. harzianum enhanced shoot length & branch, root length and chlorophyll content of Bacopa plant. Mechanistic study showed that these biocontrol agents have capacity of IAA hormone and siderophore production in vitro and also of solubilization of phosphate in vitro and in vivo. In conclusion, this was the first report of successfully eco-friendly encountering of the leaf blight disease and growth promotion of Bacopa monnieri by T. asperellum or T. harzianum spore suspension (10⁸ spores / ml) by sapling treatment with dip method before plantation and also spraying of this spore suspension three times after disease initiation in field at 7 days of interval.