Early development of Erysiphe pisi on the leaves of susceptible and resistant pea cultivars/lines and on some non-hosts was studied by light microscopy. E. pisi conidia germinated and formed appressoria on both resistant and susceptible leaves of pea. However, further development was stopped on the leaves of resistant cultivars/lines as mostly only one germ tube was produced at 48 h after inoculation. Though papillae formation was there at the site of penetration in both resistant and susceptible pea leaves, it was prominent in the susceptible pea leaves. Failure of development was mostly associated with hypersensitive death of the attacked cells. While E. pisi failed to produce multiple germ tubes on a number of non-hosts, less than 1 % of conidia produced multiple germ tubes (from 1-4, uni- and bipolarly) on leaves of a fern (Nephrolepis cordifolia).
Oxalic acid (OA) is an important pathogenic factor during early Sclerotinia sclerotiorum ‐host interaction and might work by reducing hydrogen peroxide production (H 2 O 2 ). In the present investigation, oxalic acid‐induced cell death in pea was studied. Pea plants treated with biocontrol agents (BCAs) viz., Pseudomonas aeruginosa PJHU15, Bacillus subtilis BHHU100, and Trichoderma harzianum TNHU27 either singly and/or in consortium acted on S. sclerotiorum indirectly by enabling plants to inhibit the OA‐mediated suppression of oxidative burst via induction of H 2 O 2 . Our results showed that BCA treated plants upon treatment with culture filtrate of the pathogen, conferred the resistance via. significantly decreasing relative cell death of pea against S. sclerotiorum compared to control plants without BCA treatment but treated with the culture filtrate of the pathogen. The results obtained from the present study indicate that the microbes especially in consortia play significant role in protection against S. sclerotiorum by modulating oxidative burst and partially enhancing tolerance by increasing the H 2 O 2 generation, which is otherwise suppressed by OA produced by the pathogen.
Abstract The aim of this study was to evaluate the potentiality of three compatible rhizosphere microbes, viz. fluorescent Pseudomonas aeruginosa ( PHU094 ), Trichoderma harzianum ( THU0816 ) and Mesorhizobium sp. ( RL091 ), in community to mobilise antioxidant mechanisms in chickpea under the challenge of Sclerotium rolfsii . The microbes were applied as seed treatment in different combinations in two sets and the pathogen was inoculated in one of the sets after 3 weeks of sowing. A comparative study was conducted on the effect of the microbial combinations on host antioxidant mechanisms between the two sets. In pathogen challenged plants host defence responses included higher accumulation of hydrogen peroxide ( H 2 O 2 ) at petiolar and interveinal regions of leaf and high activities of catalase ( CAT ), glutathione reductase ( GR ) and guaiacol peroxidase ( GPx ) compared to unchallenged plants. The antioxidant enzyme activities increased 1.8‐3.3 and 1.9‐3.1 folds at 48 and 72 h, respectively, in the triple microbe treated challenged plants compared to unchallenged ones. Although, ascorbate peroxidase ( APX ) activity was significantly low, ascorbic acid ( AA ) and chitinase accumulation was high in the pathogen challenged plants. Antioxidant flavonols associated with host defence namely myricetin, quercetin and kaempferol also accumulated in high amounts in pathogen challenged plants. Among the microbial treatments, the triple microbe combination induced the highest response in all parameters as compared to dual or single application of the same microbes. The triple microbe consortium modulated the chickpea antioxidant mechanisms more efficiently and modulation of oxidative stress was directly correlated with lower plant mortality, thus demonstrating the synergistic behaviour of the microbes in protecting chickpea from the pathogen.
The early detection and identification of plant pathogens are an integral part of successful disease management. Rapid identification of a plant pathogen provides appropriate control measures that could be applied prior to further spread of the disease or its introduction. The classical approach to plant disease diagnosis at the preliminary stage involved identification by visual symptoms followed by laboratory identification using selective media and microscopy to identify the infecting pathogens. But the conventional methods are a relatively slow process and often require skilled taxonomists for reliable identification of the pathogens. Therefore, in the past decade, major focus has shifted to the development of rapid, accurate and low cost methods with application in plant pathogen diagnosis. These methods include enzyme-linked immunosorbant assay (ELISA), the use of monoclonal antibodies, and DNA and PCR-based technologies which increase the sensitivity of pathogen detection. Rapid diagnosis and on-site quantification of phytopathogens and mapping them at locations of high disease incidence would enable the timely forecast of the advent of disease and would enable the farmers, agricultural authorities and research institutions to perform various management practices to control the disease. The present chapter deals with recent advances in molecular methods developed to detect and identify the four major classes of plant pathogens:viz. bacteria, fungi, nematodes and virus.
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