R. R. Pandey

Antagonistic interactions between fungal pathogens and phylloplane fungi of guava

Dominant phylloplane fungi of guava (Psidium guajava L.) were screened for their antagonistic activities against the two pathogens,Colletotrichum gloeosporioides andPestalotia psidii, bothin vitro andin vivo. Culture filtrates ofAspergillus niger, Fusarium oxysporum andPenicillium citrinum caused more than 50% growth inhibition ofC. gloeosporioides. Filtrates ofCephalosporium roseo-griseum andF. oxysporum were most effective in reducing the growth ofP. psidii. Volatiles produced from the cultures ofA. niger, F. oxysporum, P. citrinum andP. oxalicum inhibited the growth ofC. gloeosporioides, whereas volatiles fromC. roseo-griseum, F. oxysporum andTrichoderma harzianum inhibited the growth ofP. psidii. The inhibitory effect of volatiles decreased with increase in incubation time. In general, the maximum effect of volatiles was noticed after 48 h incubation. Different grades of colony interactions in dual cultures were recognised between the two pathogens and the phylloplane fungi examined. Maximum inhibition ofC. gloeosporioides was caused byAureobasidium pullulans, Cladosporium cladosporioides, epicoccum purpurascens, F. oxysporum andMyrothecium roridum, whereasAspergillus terreus, C. roseo-griseum andP. oxalicum significantly reduced the growth ofP. psidii. Application of a spore suspension of each test fungus inhibited lesion development of guava leaves caused by the test pathogensin vitro. Inhibition was more pronounced when the spore concentration was increased.A. pullulans, C. cladosporioides, E. purpurascens, F. oxysporum, andT. harzianum were found to be strongly antagonistic toC. gloeosporioides. A. niger, A. terreus, C. roseo-grisem andT. harzianum were strongly antagonistic toP. psidii.

Diversity of potential microbial parasites colonizing sclerotia of Macrophomina phaseolina in soil

The colonization of Macrophomina phaseolina sclerotia by microbial parasites was evaluated in unsterilized field soil at different levels of soil moisture (0,-5, and-10 kPa) and temperature (20, 30, and 40°C). The maximum colonization of sclerotia was recorded in soil held at-5 or-10 kPa at 30–40°C. Trichoderma harzianum isolate 25–92 and Pseudomonas fluorescens isolate 4–92 were recorded as potential sclerotial parasites, and they significantly (P=0.05) reduced the germination of sclerotia by 60–63%. Cells of P. fluorescens and buffer-washed conidia of T. harzianum were completely agglutinated at 28°C with crude agglutinin of M. phaseolina. The ability of different antagonists to parasitize the sclerotia were correlated with the agglutination ability of the antagonists.

Effect of environmental conditions and inoculum density on infection of guava fruits by Colletotrichum glososporioides.

The influence of environmental factors (temperature and humidity), inoculum density on infection by Colletotrichum glososporioides and development of anthracnose lesions were determined on uninjured, sand-injured and punctured fruits. The optical temperature for severe infection was 30 °C, whereas the disease incidence was less at 20 and 35 °C. Inoculated guavas that received 1–60 h of continuous free moisture developed lesions, but the disease was minimal (0–7%) after 1–6 h free moisture. Infection rates of uninjured, sand-injured and punctured fruits receiving 60 h of free moisture were 34, 70 and 100%, respectively. Disease incidence increased as inoculum density increased from 101 to 106 conidia/ml. In field conditions, the development of anthracnose lesions was greater on punctured guavas than on uninjured or sand-injured ones, in both rainy and winter seasons. In general, the number of lesions was highest in sand-injured fruits, followed by punctured and uninjured fruits. In rainy season the number of lesions on injured and uninjured fruits was greater than similarly treated guavas in winter.

Role of Arbuscular Mycorrhizal Fungi in Alleviation of Acidity Stress on Plant Growth

Soil acidity is a major constraint for crop production worldwide as nearly half of the potential arable lands are acidic. Plant productivity in acid soils is limited by toxic levels of aluminum (Al), manganese (Mn), and iron (Fe) as well as deficiencies of plant available nutrients, especially phosphorus (P). Plants have developed several morphological, biochemical, and physiological adaptations against acidity stress. Among these, symbiosis with arbuscular mycorrhizal (AM) fungi is a strategy plants have evolved to survive and thrive in acid soils. The AM symbiosis increases plant growth in acid soils through enhanced uptake of nutrients. A reduction in Al and Mn phytotoxicities also occurs in response to AM symbiosis through a number of mechanisms such as binding of the toxic ions by the fungal hyphae, vesicles or auxiliary cells, exudation of organic acids, phosphatases, and production of glomalin. However, like plants, AM fungal species and ecotypes also vary to a great extent in their tolerance and ability to impart plant growth benefits in acid soils. It is, therefore, essential to identify suitable AM fungi that could confer tolerance and render maximum benefits to crops in acid soils.

Succession of microfungi on leaves of Psidium guajava L.

PANDEY, R. R. (Centre of Advanced Study in Botany, Banaras Hindu University, Varanasi221005, India). Succession of microfungi on leaves of Psidium guajava L. Bull. Torrey Bot. Club 117: 153-162. 1990.-The phylloplane mycoflora of guava (Psidium guajava L.) was studied from the bud stage to leaf fall stage in summer, rainy and winter seasons using a combination of cultural methods. In general, the number of fungi/cm-2 on leaves increased from the time of bud opening to senescence, but declined at leaf fall. The bud (unexpanded leaves) and newly expanded leaves were colonized by a limited range of fungi such as Alternaria alternata, Aureobasidium pullulans, Aspergillus niger, Cladosporium cladosporioides, Epicoccum purpurascens, pink yeasts and white yeasts. The number of fungal species was maximal at the senescent leaf stage in each season. Qualitatively, three categories of fungi, viz., early leaf fungi, mature leaf fungi, and dying and dead leaf fungi were recognized. On average, more fungal species were found in the rainy season and fewer in the summer. Some species were common to all seasons. Three main seasonal groups of fungi were observed: season specific, season sensitive, and season insensitive. Four potentially pathogenic species (Colletotrichum gloeosporioides, Fusarium oxysporum f. sp. psidii, Pestalotia psidii, and Phoma psidii) were consistently present during all seasons. Most fungi were isolated by dilution plating, followed by the moist chamber method, and the fewest by the washed disk technique.

Arbuscular mycorrhizal and dark septate fungal associations in shallot (Allium cepa L. var. aggregatum) under conventional agriculture

Arbuscular mycorrhizal and dark septate fungal associations in shallot (Allium cepa L. var. aggregatum) under conventional agriculture We examined roots of the shallot (Allium cepa L. var. aggregatum), one of the most popular cultivated crops of the family Aliaceae, cultivated under conventional agriculture for arbuscular mycorrhizal fungal (AMF) and dark septate fungal endophyte (DSE) associations. All the plants had dual colonization of both AMF and DSE associations. The intermediate-type AMF morphology in the shallot is the first report of this AMF type for the family Aliaceae. The extents of total AMF and DSE colonization ranged from 20.7 to 67.3% and 3.6 to 35.3% respectively and varied significantly among fields. Though no significant relationship existed between total AMF and DSE variables, they were correlated to the soil variables. Significant correlations existed between soil P and microscelerotia and also between soils N and K and AMF spore numbers. A total of six AMF spore morphotype belonging to Glomus and Scutellospora were identified. Scutellospora calospora was the most dominant morphotype in the studied fields.

Growth enhancement of Sesamum indicum L. by rhizosphere-competent Azotobacter chroococcum AZO2 and its antagonistic activity against Macrophomina phaseolina

Indigenous strains isolated from rhizosphere may contain highly competent genotypes to enhance the plant growth and often perform better than the introduced isolates. The present study deals with the characterisation of plant growth-promoting (PGP) attributes and antagonistic activity of Azotobacter chroococcum AZO2 against Macrophomina phaseolina causing charcoal rot disease and their effect on the growth of sesame (Sesamum indicum L.). Eight strains of Azotobacter were isolated from sesame rhizosphere on nitrogen-free medium, which exhibited significant PGP parameters such as phosphate solubilisation, indole acetic acid and siderophore production. The strain A. chroococcum AZO2 (EU274299) was characterised by 16S rDNA gene sequencing. Amplification of 781 bp nif H gene confirms nitrogenase activity of all the strains. A. chroococcum AZO2 exhibited strong antagonistic activities against M. phaseolina causing 81% colony growth inhibition and resulted in hyphal perforations, empty cell (halo) formation, hyphal twisting, shrinking and lysis of fungal mycelia along with degeneration of sclerotia. A. chroococcum AZO2 produced chitinase that caused degradation and digestion of the cell wall component of M. phaseolina. Different vegetative and reproductive parameters of sesame were found to be enhanced significantly upon application of A. chroococcum AZO2 + half doses of chemical fertilisers. A. chroococcum AZO2 was also found to be an effective root coloniser, plant growth promoter and potential antagonistic bacterium. It can be concluded that A. chroococcum AZO2 strain bears the characteristics of technological applications for inoculant preparation and growth enhancement of sesame besides being utilised as a better PGP bacterium as well as an effective agent for biocontrol of M. phaseolina.