Evaluation of Pathological Variations in<em> Alternaria</em> Species Infecting Oilseed Brassicas in Diverse Regions of India for Induction of Systemic Resistance

Research Article

J Bacteriol Mycol. 2016; 3(3): 1029.

Evaluation of Pathological Variations in Alternaria Species Infecting Oilseed Brassicas in Diverse Regions of India for Induction of Systemic Resistance

Aneja JK1, Agnihotri A1*, Awasthi RP2 and Kolte SJ2

1Amity Institute of Microbial Technology, Amity University Uttar Pradesh, India

2Former Professor, Department of Plant Pathology, G.B. Pant University of Agriculture and Technology, India

*Corresponding author: Agnihotri A, Amity Institute of Microbial Technology, Amity University Uttar Pradesh, India

Received: April 20, 2016; Accepted: July 08, 2016; Published: July 11, 2016


The 32 isolates of A. brassicae, 20 isolates of A. brassicicola and 3 isolates of A. alternata originating from diverse regions of North-West India, where Alternaria blight is highly prevalent, were investigated for their pathogenicity on various host species. The disease reaction varied with necrotic lesions of 0.4- 7.5 mm, mostly having brown to black coloured lesions. The isolates caused moderate to severe chlorosis on B. rapa varieties (YSH-401 and Pusa gold) as compared to the B. juncea varieties (Varuna, Rohini, Kranti and PHR-01). The mean disease index varied from 2.22 ± 1.17 to 7.32 ± 0.67 in A. brassicae isolates, 2.38 ± 0.92 to 5.89 ± 0.89 in A. brassicicola isolates and 2.49 ± 0.97 to 5.87 ± 0.91 in A. alternata isolates. Out of seven A. brassicae isolates that tested positive for non-aggressiveness on specific host species, five isolates, on prior inoculation, induced tolerance to highly aggressive strains of A. brassicae in the respective B. juncea and B. rapa varieties. The study thus opens up the possibility of deploying attenuated virulence as naturally occurring biological control for induction of systemic resistance/ tolerance in Brassicas against A. brassicae, one of the most destructive fungal pathogen.

Keywords: Alternaria blight; Oilseed Brassica; Pathogenicity; Attenuated virulence; Systemic resistance


PDA: Potato Dextrose Agar; SA: Salicylic Acid; JA: Jasmonic Acid; ISR: Induced Systemic Resistance; PR: Pathogenesis-Related; DI: Disease Index.


Pathogens react vigorously to their environment; even the slightest variations in the environment may greatly impact the pathogen and result in minute to massive adaptations in its population [1]. The extent and rate of such adaptations brought about in a pathogen shapes its morphological and physiological characteristics, and also changes its behavior towards the host population. Alternaria species have been extensively studied for their differential pathogenicity/ virulence on different host species. Variability in virulence towards host species have been reported in isolates of A. brassicae [2-9] A. brassicicola [3,10,11], A. alternata [12] and A. solani [4,13,14]. The availability of pathotypes with varying degrees of pathogenicity towards different Brassica species and sub-species acts as an important aspect in identification, breeding and exploitation of durable resistance genotypes.

Induction of resistance in otherwise susceptible host plants, without changing their basic genetic make-up, through use of biotic as well as abiotic agents has been studied by a few scientists [15,16]. The resistance/ defense related genes in the vulnerable plants can be activated by inoculating the plant either by an avirulent form of the pathogen or by limited inoculation with the pathogen [17]. Infecting avirulent pathogen triggers natural defense responses in the plant through the release of the elicitors which then result in the expression of novel anti-pathogenic proteins.

In order to harness the benefits of induced host resistance and build up a stable, long term resistance mechanism in the host plant against the pathogen, there is a need to identify the pathogen and understand its behavior under diversified conditions. However, only a few studies [7] have been undertaken to evaluate the pathological diversity of the Alternaria isolates and its characterization for its applicability in India.

Materials and Methods

Raising Brassica plants

Six varieties of oilseed Brassica – B. juncea (4 varieties - Varuna, Rohini, Kranti and PHR-01) and B. rapa (2 varieties – YSH-401 and Pusa gold) were selected for evaluating variations in virulence/ aggressiveness of different isolates. The seeds of the selected varieties were sown in agropeat: sterile soil mix and plants were raised under natural conditions in field during Rabi Season and controlled conditions at 22 ± 2°C with 16 hr light/ 8 hr dark photoperiod and a light intensity of 12klx.

Isolation and purification of fungal cultures

Samples of Alternaria blight infected leaves from various Brassica species were collected/ procured from Northern and North-western regions of India. The fungal cultures were isolated and purified on PDA (potato dextrose agar) medium under continuous diffused light conditions, at a temperature of 20 ± 2°C for 15 ± 2 days. The purified Alternaria isolates were observed microscopically under 40× optical microscope and identified at species level as per the available monograph [18].

Pathogenicity assay

The differential virulence/ aggressiveness of the isolates were determined using detached leaf according to the method of Vishwanath and Kolte [19]. The maintained isolates were used to prepare individual spore suspensions (1.5 × 104 spores per ml). Third/ fourth fully expanded leaf from base of 30-day old plants of each variety were detached, inoculated with 10 μl of individual spore suspensions and incubated for 7 days at 22 ± 2°C and 12 hour photoperiod in moist chambers.

The response of different isolates on the selected hosts was assessed on the basis of lesion number, size and colour; presence of ring/ dots in the lesion; chlorotic zone (yellow halo) and latent period of infection, in terms of Disease Index (DI) on a scale of 0-9 (Table 1). The isolates were subsequently characterized as highly aggressive, aggressive or non-aggressive on the host differentials and analyzed statistically for significant variations, if any. On the basis of nonaggressiveness shown by the isolates, three varieties of Brassica – B. juncea (2 varieties – Rohini and PHR-01), and B. rapa (1 variety – YSH-401) were selected for further evaluations.