Morphomolecular and cultural characteristics and host range of Lasiodiplodia theobromae causing stem canker disease in dragon fruit.

Dragon fruit (Hylocereus polyrhizus) is an economically promising fruit in Bangladesh. The cultivation of dragon fruit has increased fourfold within a decade due to its popularity. Recently, a new disease known as stem canker was reported in some plantations of dragon fruit in Bangladesh, which forced some farmers to abandon their cultivation. This study aimed to explore the morphological, molecular, and cultural characteristics as well as host range of the causal agent associated with this destructive disease. Morphologically similar eight fungal isolates were recovered from eight canker symptomatic dragon fruit stems. Among them, two isolates (namely BU-DLa 01 and BU-DLa 02) were used for a detailed study. Morphological parameters and phylogeny of sequence data of internal transcribed spacer (ITS1, 5.8S rRNA, and ITS2), ß-tubulin, and translation elongation factor 1-α identified the isolates as Lasiodiplodia theobromae. The cultural features were studied hinged on the growth of the two isolates on various media, temperature, and pH. Though the mycelial growth of the fungi was supported by all the media tested, potato dextrose agar was the most suitable one for both isolates. The fungi thrived well at a temperature of 25-35°C and 5.5-6.5 pH. Inoculation trials of dragon fruit stem ascertained Koch's postulate. In host range test, the isolates were found pathogenic toward mango, guava, banana, and the fruits of dragon fruit. These data will contribute not only to understanding the biology of L. theobromae as a newly recognized pathogen of H. polyrhizus but also will help in designing a proper management package against this pathogen.

Characterization of Pestalotiopsis sp. causing gray leaf spot in coconut (Cocos nucifera L.) in Bangladesh.

Coconut (Cocos nucifera L.) is one of the most important fruit trees in Bangladesh. This tree is susceptible to various pathogens. Among them, a fungus was consistently isolated from gray leaf spot symptom in coconut. This study aimed to isolate, characterize, and find the management strategies of the causal fungus of gray leaf spot disease in coconut. Both morphological and molecular characters identified the pathogen as Pestalotiopsis sp. for the first time in Bangladesh. Artificial inoculation of this fungus showed symptoms similar to those previously observed in the field. Cross-inoculation test suggests that Pestalotiopsis sp. has a wide host range. The infection process of Pestalotiopsis sp. started at 2 h after inoculation (hai) with the formation of germ tube followed by the formation of infection hyphae, which penetrated directly into the host at 6 hai. Gray leaf spot symptom was developed at 120 hai. Numerous conidia developed from the acervuli at 168 hai. These conidia acted as the source of inocula for secondary infection. The optimum temperature for the growth of Pestalotiopsis sp. was 25°C, however, the growth of Pestalotiopsis sp. ceased at 15°C and 35°C. This pathogen was completely inhibited by Autostin 50 WDG (carbendazim) at 100 ppm. Trichoderma viride (Pb-7) was found as the potential biocontrol agent against Pestalotiopsis sp. These findings could contribute to describing the disease cycle and epidemiology of Pestalotiopsis sp. that would ultimately require to undertake effective control measures against this pathogen.

First characterization of a newly emerging phytopathogen, Sclerotinia sclerotiorum causing white mold in pea.

Pea (Pisum sativum L.) is of global importance as a food crop for its edible pod and seed. A new disease causing the tan to light brown blighted stems and pods has occurred in pea (P. sativum L.) plants in Chapainawabganj district, Bangladesh. A fungus with white-appressed mycelia and large sclerotia was consistently isolated from symptomatic tissues. The fungus formed funnel-shaped apothecia with sac-like ascus and endogenously formed ascospores. Healthy pea plants inoculated with the fungus produced typical white mold symptoms. The internal transcribed spacer sequences of the fungus were 100% similar to Sclerotinia sclerotiorum, considering the fungus to be the causative agent of white mold disease in pea, which was the first record in Bangladesh. Mycelial growth and sclerotial development of S. sclerotiorum were favored at 20°C and pH 5.0. Glucose was the best carbon source to support hyphal growth and sclerotia formation. Bavistin and Amistar Top inhibited the radial growth of the fungus completely at the lowest concentration. In planta, foliar application of Amistar Top showed the considerable potential to control the disease at 1.0% concentration until 7 days after spraying, while Bavistin prevented infection significantly until 15 days after spraying. A large majority (70.93%) of genotypes, including tested released pea cultivars, were susceptible, while six genotypes (6.98%) appeared resistant to the disease. These results on identification, characterization, host resistance, and fungicidal control of white mold could be valuable to achieve improved management of a new disease problem for pea cultivation.

Isolation and Identification of Plant Growth Promoting Rhizobacteria from Cucumber Rhizosphere and Their Effect on Plant Growth Promotion and Disease Suppression.

Plant growth promoting rhizobacteria (PGPR) are the rhizosphere bacteria that may be utilized to augment plant growth and suppress plant diseases. The objectives of this study were to identify and characterize PGPR indigenous to cucumber rhizosphere in Bangladesh, and to evaluate their ability to suppress Phytophthora crown rot in cucumber. A total of 66 isolates were isolated, out of which 10 (PPB1, PPB2, PPB3, PPB4, PPB5, PPB8, PPB9, PPB10, PPB11, and PPB12) were selected based on their in vitro plant growth promoting attributes and antagonism of phytopathogens. Phylogenetic analysis of 16S rRNA sequences identified these isolates as new strains of Pseudomonas stutzeri, Bacillus subtilis, Stenotrophomonas maltophilia, and Bacillus amyloliquefaciens. The selected isolates produced high levels (26.78-51.28 µg mL(-1)) of indole-3-acetic acid, while significant acetylene reduction activities (1.79-4.9 µmole C2H4 mg(-1) protein h(-1)) were observed in eight isolates. Cucumber plants grown from seeds that were treated with these PGPR strains displayed significantly higher levels of germination, seedling vigour, growth, and N content in root and shoot tissue compared to non-treated control plants. All selected isolates were able to successfully colonize the cucumber roots. Moreover, treating cucumber seeds with these isolates significantly suppressed Phytophthora crown rot caused by Phytophthora capsici, and characteristic morphological alterations in P. capsici hyphae that grew toward PGPR colonies were observed. Since these PGPR inoculants exhibited multiple traits beneficial to the host plants, they may be applied in the development of new, safe, and effective seed treatments as an alternative to chemical fungicides.