Development of a new crop protection system using yeast extracts

Abstract

To demonstrate that yeast extracts exhibiting phytoalexin elicitor activity can be used as a practical method to control mildew on barley.

This was approached in two ways:

1. Optimisation of the composition of the yeast extract to enhance its activity by monitoring its:

(a) ability to elicit phytoalexins in a model system

(b) control of mildew on detached barley leaves and glasshouse-grown plants

(c) effect on biochemical and physical resistance mechanisms of target hosts

2. Demonstration through field experimentation of the level of disease control and yield benefit achieved.

Laboratory studies

Yeast cell wall extracts were prepared in a number of ways and their ability to elicit phytoalexins was characterised in a quantitative assay using soybean cotyledons. This characteristic is correlated with their ability to protect detached barley leaves from subsequent infection by powdery mildew (Erysiphe graminis f.sp. hordei). Extracts with the highest elicitor activity conferred the greatest protection against mildew infection with some extracts giving up to 95% reduction in infection.

The speed of host response is a critical factor in achieving effective resistance expression. At a cellular level resistance to epidermal penetrating pathogens is characterised by production of papillae, ie physical structures involved in defence reactions. Their formation was faster on leaves treated with yeast extracts prior to inoculation. In addition a more rapid induction of phenylalanine ammonia-lyase (PAL)activity, a key enzyme for biosynthesis of compounds involved in resistance, was observed following treatment with yeast extracts.

Extracts were sent to BASF in Limbergerhof, Germany to be evaluated in their standard fungicide screening programme. Their data showed activity against several other pathogens but the activity was neither broad nor high enough for them to decide to pursue it further at present.

Field evaluation

Yeast extract applied to spring and winter barley increased disease control, yield and thousand grain weight, although there was some cultivar interaction. In highly susceptible cultivars such as Golden Promise and less susceptible cultivars such as Triumph all treatments reduced mildew infection. Treatments were most effective on the less susceptible cultivars where one extract reduced infection in Triumph by over 60%, and this extract and others either equalled or exceeded respective fungicide values for yield and thousand grain weight. Mildew control over winter was poor but yields were nevertheless enhanced.

In the glasshouse and/or detached leaf experiments levels of mildew reduction approaching those of the fungicide treatment were achieved on wheat and oats as on barley for several extracts.

BASF UK, the Arable Research Centre (ARC) and the Chemical Spraying Company (CSC) of Perth all carried out field trials with our elicitors demonstrating a wide range of efficacy depending upon environmental and operational factors. BASF reported up to 93% reduction in mildew and 73% in brown rust on barley in field trials.

Extracts have sometimes been disappointing in their field performance probably due to practical problems encountered, the hazard of mainly relying on one site, but principally lack of formulation, the importance of which was demonstrated in laboratory studies. No detrimental side effects of the yeast extracts towards plants have been observed.

Future prospects

Since the use of resistance elicitors should control a broad spectrum of pathogens it is hoped that they can be applied as a means of disease control in other commercially grown crops where residues may be a problem. Our data suggest that it is unlikely that total disease control will be achieved using resistance elicitors. However, we have demonstrated that disease can be significantly reduced and that the yeast-derived elicitors can become an additional tool to control diseases in the field. The integrated use of yeast-derived resistance elicitors in a spray regime or mixture with reduced rate fungicide may provide a viable means of reducing inputs in an integrated agricultural system.