Elsevier

Biological Control

Volume 44, Issue 3, March 2008, Pages 349-361
Biological Control

Beneficial microorganism survival on seed, roots and in rhizosphere soil following application to seed during drum priming

https://doi.org/10.1016/j.biocontrol.2007.11.005Get rights and content

Abstract

Priming is a technique used to improve seedling establishment of direct-seeded crops such as onion and carrot, resulting in a quick and uniform emergence. This work investigated the application of four selected beneficial microorganisms (Pseudomonas chlororaphis MA342, Pseudomonas fluorescens CHA0, Clonostachys rosea IK726d11 and Trichoderma harzianum T22) to onion and carrot seed during drum priming, and their subsequent survival and establishment in the rhizosphere once the seed was planted. Different application rates of fungi (7 log10 cfu g−1 dry seed) and bacteria (6 log10 cfu g−1 dry seed) were required on onion to achieve the end target of 5 log10 cfu g−1 dry seed, whereas a lower rate (5 log10 cfu g−1 dry seed for both bacteria and fungi) was successful on carrot. Microorganism-treated seed was planted in soil in the glasshouse and root and rhizosphere soil samples were taken at 2, 4 and 8 weeks post-planting. All seed-applied microorganisms were recovered throughout the experiment, although differences in the survival patterns were seen. The bacterial isolates declined in number over time, with P. fluorescens CHA0 showing better overall survival than P. chlororaphis MA342, particularly on the roots and in the rhizosphere soil of carrot. In contrast to the bacteria, the fungal isolate C. rosea IK726d11 showed good survival on both onion and carrot, and increased significantly in number throughout the 8-week period. Trichoderma harzianum T22 remained relatively constant in number throughout the experiment, but showed better survival on carrot than onion roots. Similar results were found in three different soil-types.

Introduction

Direct-seeded crops such as carrots and onions can suffer from poor crop establishment due to uneven seed germination in the field. This can lead to variable plant spacing and lack of uniformity, and may have a negative impact on yield and marketability of the crop. In order to improve seedling establishment, seeds can be primed before planting (Rowse, 1996b, Halmer, 2004). This procedure involves the controlled addition of water to a seed batch to start the physiological germination process, but before the radicle emerges from the seed coat the procedure is stopped and seed is dried back to a low moisture content. Seed can then be stored or planted straight away. Priming ensures that the entire seed batch is at the same point in the germination process, so that once the seed is planted there is a rapid and more uniform emergence, particularly under cold or wet conditions (McQuilken et al., 1998, Halmer, 2004). Various priming techniques exist, including osmopriming or hydropriming (Caseiro et al., 2004), solid-matrix priming (Taylor et al., 1988), steeping priming (Halmer, 2004) and drum priming (Rowse, 1996a, Caseiro et al., 2004). The latter two are used commercially in the UK.

Primed seed can subsequently be pelletted or film-coated with pesticides, according to normal commercial practices. However, due to environmental concerns, there is an ongoing need to reduce the use of pesticides in agriculture and horticulture, and alternatives to chemicals are being sought to improve crop establishment and health. One option is the use of beneficial microorganisms or biocontrol agents applied to seed or roots, which may promote plant growth or provide disease control through a variety of mechanisms, including production of plant hormones, antibiotics or enzymes; induced systemic resistance; direct parasitism of plant pathogens or deleterious microorganisms; or competition with pathogens for space or nutrients (Whipps, 2001, Zahir et al., 2004). In particular, the application of beneficial microorganisms to seed is a niche that can be exploited commercially to reduce the use of seed-applied pesticides and, importantly, microbiological seed treatments also can be used by organic growers.

Applying beneficial microorganisms to seed during the priming process is commercially realistic, as microorganism suspensions can easily be incorporated into the water used for seed priming. Previous work has shown that selected fungal and bacterial isolates could survive and proliferate on carrot and parsnip seed during drum priming (Wright et al., 2003a). However, not all seed-type/microorganism combinations were successful, and applied fungal isolates could not be recovered from leek seed after priming (Wright et al., 2003a). Another crop that is primed is onion (Caseiro et al., 2004, Halmer, 2004), but no work has been done on the potential for applying beneficial microorganisms to onion seed during drum priming.

Successful application of selected microorganisms to seed in a commercially viable way is only the first step towards using beneficial microorganisms to improve crop health. It is equally important that the microorganisms remain viable and can colonise the developing roots and rhizosphere in order to continue improving plant growth and to potentially control disease. Seed-applied microorganisms have the potential to become established in the rhizosphere of plants, as they may transfer onto the developing root as it emerges from the seed (Harman, 1991). Much work has been done on applying microorganisms to seed using various techniques, including the use of suspensions, slurries, powders, peat carriers, or encapsulation in alginate (Fravel et al., 1998, McQuilken et al., 1998, Walker et al., 2004). However, the survival and establishment in the root zone of beneficial microorganisms applied to seed during drum priming has not been studied previously.

In order to investigate the possibility of delivering beneficial microorganisms to the root zone by application to seed during priming, work was carried out with two main aims: firstly to investigate the survival of four selected beneficial microorganisms on onion and carrot seed during drum priming, using a laboratory-scale system. These microorganisms were all known for their biocontrol or plant growth promoting (PGP) properties. The second aim was to conduct glasshouse experiments to determine whether the seed-applied microorganisms could subsequently become established on the roots and in the rhizosphere of onion and carrot, in three contrasting soil-types.

Section snippets

Microorganism storage and preparation

Four beneficial microorganisms were selected for use in this study, based on their known biocontrol or PGP properties, or availability within a commercial product. Pseudomonas chlororaphis MA342 is the active ingredient in Cedomon®, targeting cereal pathogens (Johnsson et al., 1998), and was obtained from Dr. M. Hökeberg, BioAgri, Uppsala, Sweden; Pseudomonas fluorescens CHA0 has activity against a wide range of soil-borne pathogens (Maurhofer et al., 1994) and was obtained from Prof G. Défago,

Onion seed

The two fungal isolates both dropped in number at the end of the hydration phase following application to onion seed at 7 log10 cfu g−1 dry seed (Fig. 1). This high initial inoculum was required to achieve the final target of at least 5 log10 cfu g−1 dry seed after drying back, as an overall decline in numbers occurred during the priming process. However, at the end of the incubation phase, microorganisms were always recovered in higher numbers than those found at the end of the hydration phase.

Discussion

Seed priming is used to improve emergence of direct-seeded crops, particularly under wet or cold conditions (Halmer, 2004). Previous work has shown that combining priming with the application of beneficial microorganisms can improve crop establishment and several methods have been used to achieve this (Harman and Taylor, 1988, Callan et al., 1990, Jensen et al., 2004). These have generally involved first coating the seed with a microorganism suspension, and then priming the seed using a variety

Acknowledgments

We acknowledge funding from the Horticulture LINK programme, Elsoms Seeds Ltd., Germains Technology Group UK, the Horticultural Development Council, and the Department for Environment, Food and Rural Affairs. We also thank Andrew Mead for help with the experimental designs and statistical analyses, and Dr. Emma Coventry for critically reviewing earlier drafts of the manuscript.

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