Girija Ganeshan

Pseudomonas fluorescens, a potential bacterial antagonist to control plant diseases

Abstract Fluorescent Pseudomonads belong to plant Growth Promoting Rhizobacteria (PGPR), the important group of bacteria that play a major role in the plant growth promotion, induced systemic resistance, biological control of pathogens etc. Many strains of Pseudomonas fluorescens are known to enhance plant growth promotion and reduce severity of various diseases. The efficacy of bacterial antagonists in controlling fungal diseases was often better as alone, and sometimes in combination with fungicides. The present review refers to occurrence, distribution, mechanism, growth requirements of P. fluorescens and diseases controlled by the bacterial antagonist in different agricultural and horticultural crops were discussed. The literature in this review helps in future research programmes that aim to promote P. fluorescens as a potential bio-pesticide for augmentative biological control of many diseases of agriculture and horticultural importance.

Antifungal Activity of Mucuna pruriens Seed Extractives and L-dopa

The seeds of Mucuna pruriens (L.) D.C. (Leguminosae) were extracted first with hexane, then with ethyl acetate, and finally with methanol. The solvents were removed to get the respective extractives. These were assessed for the mycelial growth inhibition of three phytopathogenic fungi—Colletotrichum gloeosporioides, Colletotrichum capsici, and Fusarium solani—by the poisoned-food technique. Methanol extractive showed highest activity against all the three test organisms. L-dopa [3-(3,4-Dihydroxyphenyl)-L-alanine], which was qualitatively detected in the ethyl acetate and methanol extractives and quantitatively assessed in the seeds, showed antifungal activity against all the three test organisms, showing that it is one of the active principles in these extractives.

Antifungal Activity of the Extractives of Coleus Forskohlii Roots and Forskolin

Hexane, ethyl acetate and methanol extractives of Coleus forskohlii roots exhibit mycelial growth inhibition of Colletotrichum gloeosporioides and spore germination inhibition of Alternaria solani. Forskolin, a diterpene present in C. forskohlii exhibit dose dependent spore germination inhibition of A. solani, this being the first report on the antifungal activity of this compound. Estimation of forskolin in the extractives by HPLC showed that while hexane extractive contained 3.9% forskolin and ethyl acetate extractive contained 4.6% forskolin, while the methanol extractive did not contain forskolin. The results clearly show that forskolin is one of the antifungal compounds present in the plant.

Transgenic Chili Possessing Baculovirus Chitinase Gene Exhibits in vitro Fungal Inhibition

Cultivation of chili (Capsicum annuum L.) is limited by several fungal diseases, the serious among them being anthracnose. In the absence of a source of resistance for anthracnose in C. annuum background, the transgenic approach, i.e., introduction of a gene encoding pathogen cell wall-degrading enzyme, viz., chitinase, into the host plant against the test fungus Alternaria alternata as well as anthracnose (Colletotrichum capsici) was explored. Chili being recalcitrant to in vitro regeneration, the regeneration protocol was optimized in cotyledonary explants using different cytokinins, viz., 6-benzyl amino purine (BAP), thidiazuron (TDZ), or zeatin, in combination with gibberellic acid (GA3) and/or auxins, viz., indole-3-acetic acid (IAA) or phenyl acetic acid (PAA). Agrobacterium-mediated transformation of cotyledonary explants with Autographa californica baculovirus chitinase gene under the control of 35S promoter resulted in seven putative transformants under kanamycin selection. The presence of transgene was confirmed in two of the seven primary transformants. One of the transformants exhibited enhanced inhibition of the fungus Alternaria alternata in in vitro bio-assays at T0 (primary transformant) stage. The two transgenic plants were then advanced to T1 generation (progenies of T0) and their fruits were examined for resistance to anthracnose. A few lines among the progenies of each of the primary transformants exhibited higher tolerance to anthracnose than the rest of the lines and untransformed control. Higher endochitinase activity in one of the primary transformants was correlated to better expression of anthracnose tolerance in its progenies, suggesting the possibility of chitinase gene being used for developing transgenic lines tolerant to anthracnose.

Evaluation of entomopathogenic fungus against Alternaria porri (Ellis) causing purple blotch disease of onion

Biological control through the use of antagonistic micro-organisms is a potential and non-chemical means of managing plant diseases. Alternaria porri causes purple blotch disease mostly in onion. In this study, three different entomopathogenic fungi, viz. Beauveria bassiana, Metarhizium anisopliae and Verticillium lecanii, were tested against A. porri. All the three entomopathogenic fungi showed inhibitory effect against A. porri subjected to in vitro studies under dual culture technique, spore (conidial) germination, food poisoning (mycelia germination) and seed germination. The percentage inhibition of mycelial growth (PIMG) of A. porri (69.24, 56.17, and 45.81%) and the percentage inhibition of conidial germination (CG) (97.81, 42.11, and 67.69%) were observed for B. bassiana, V. lecanii and M. anisopliae, respectively. Seed germination index was found to be maximum with M. anisopliae (5557 ± 107.7) and B. bassiana (4390 ± 129.9). B. bassiana exhibited efficient antagonism against A. porri showing the highest PIMG and CG with moderate seed germination index. Microscopic examination showed the disintegrated edge of conidia which had turned black and also stopped the growth of germ tube. Hence, these entomopathogenic fungi were found to be more effective antagonistic organisms and also moderately promote seed vigour index (4390 ± 129.9) as well as plant growth. The present study indicated that the entomopathogenic fungi evaluated have shown significant inhibition of A. porri. Thus, there is a possibility of exploiting them in the management of the disease.