Colletotrichum siamense leaf blight/spot: Characterization of a newly identified disease on Cinnamomum tamala.

Cinnamomum tamala (bay leaf) is widely used for culinary and medicinal purposes in South Asia. A leaf blight/spot disease was first discovered on nearly 90% of C. tamala plants with a mean severity of 48% to 74.4% in Gazipur and Bogura, Bangladesh, in 2019. The present study identified and characterized the causal organism and formulated the optimum growth conditions and effective fungicides for the chemical control of the pathogen. The characteristic symptoms on the infected leaves appeared circular to oval reddish-brown spots with raised margins and often developed in tear-stain patterns. Severe infection of C. tamala sapling resulted in dieback symptoms with leaf defoliation. A fungus with floccose, dense, white colonies with well-differentiated acervuli was isolated from the infected leaves. Combined cultural, morphological, and molecular characteristics identified the pathogen as Colletotrichum siamense. Inculcating healthy leaves and 1-year-old saplings of C. tamala with a conidial suspension of the fungus reproduced the same symptoms observed in the bay leaf orchard. The highest mycelial growth was recorded on V-8 Juice Agar media, while the maximum radial mycelial growth and level of sporulation of the fungus were significantly higher in incubation temperature 30°C. Fungicide trials showed that carbendazim 50 WP, azoxystrobin, mancozeb, and trifloxystrobin, either singly or in combination, successfully reduced fungal mycelial growth in vitro. Therefore, disease management strategies should be opted to halt the further spread of this issue. To our knowledge, this is the first study to document the incidence of Colletotrichum leaf blight on C. tamala in Bangladesh and even in the world.

Cosmos white rot: First characterization, physiology, host range, disease resistance, and chemical control.

A new disease of Cosmos sulphureus Cav. causing external and internal stem discoloration, premature death, and wilting was observed in 27.8% of plants with an average disease severity rating of 4.4 in Gazipur, Bangladesh. Morphological, pathological, and molecular analyses identified the isolated fungus as Sclerotinia sclerotiorum (Lib) de Bary, the causative agent of white rot disease. The optimum growth and sclerotium formation of S. sclerotiorum occurred at 20°C and pH 5.0, while glucose, peptone, yeast extract, casein, and ascorbic acid were the appropriate nutrient sources. Furthermore, mycelial growth and sclerotial development were favored in media containing potassium, magnesium, calcium, and sodium. As many as 20 plant species of 10 families; Calendula officinalisi, Chrysanthemum indicum, Catharanthus roseus, Solanum tuberosum, S. lycopersicum, S. melongena, Capsicum annum, Lablab purpureus, Phaseolus vulgari, Lens culinaris, Vigna radiata, Vigna mungo, Daucus carota, Raphanus sativus, Brassica juncea, Punica granatum, Spinacia oleracea, Ipomoea batatas, Ipomoea aquatica, and Elaeocarpus serratus were identified as the new hosts of the pathogen in Bangladesh. None of the C. sulphureus and Cosmos bipinnatus germplasms screened were genetically resistant to the pathogen. Among the tested fungicides, Autostin 50 WDG (carbendazim) and Rovral (Dicarboxamide) were most inhibitory to the fungus, while Autostin 50 WDG provided an efficient control of the pathogen in vivo up to 15 days after spray. The acquired results on characterization, physiology, host range, resistance, and fungicidal control of the pathogen could be valuable for effectively managing cosmos white rot in the field.