Plant Pest and Disease Science Section, The Royal Horticultural Society's Garden, Wisley, Woking, Surrey GU23 6QB, UK
1 Present address: Horticultural Research International, Stockbridge House, Cawood, Selby, North Yorkshire Y08 OTZ, UK

Background and objectives
Systemic acquired resistance (SAR) has the potential to control diseases effectively by enhancing mechanisms of resistance naturally present in plants. There have been previous reports that methyl jasmonate can induce SAR in various host/pathogen systems [1,2]. This paper reports on the effect of methyl jasmonate application on resistance against the pathogen Puccinia antirrhini on Antirrhinum plants both in glasshouse studies and field trials.

Materials and methods
Antirrhinum Trumpet Serenade was used in both experiments. Aqueous methyl jasmonate solutions were applied to the upper leaf surface using an aerosol spray unit. In a glasshouse experiment, a suspension of rust spores was applied to the upper set of leaves 2 days later. The field trial, laid out as a randomized block design, in 1997 contained three treatments: methyl jasmonate (1000 g/ml), pesticide (bupirimate and triforine; Nimrod T) and a control (water). Plants inoculated with Antirrhinum rust were placed in the centre of each plot to introduce the disease, and the levels of disease were assessed by recording the percentage leaf covered in pustules throughout the duration of the trial. Other measurements on plant growth and development were also recorded.

Results and conclusions
In the glasshouse studies, application of methyl jasmonate to lower leaves prior to inoculation of the rust on the upper leaves significantly reduced disease levels on the upper leaves. Concentrations of methyl jasmonate between 100 and 5000 g/ml reduced levels of rust by up to 74% (P>0.01). Increasing control was achieved as the concentration of methyl jasmonate was increased. Methyl jasmonate (1000 g/ml) also reduced disease levels when applied up to 10 days prior to inoculation of Antirrhinum rust by 37% (P>0.01) of control. No chlorosis was evident on the plant foliage after application of methyl jasmonate, although there was some necrosis of the leaf tillers at the highest concentration (5000 g/ml).

In the field, methyl jasmonate significantly reduced disease levels by up to 67.3% (P>0.05) of an untreated control, compared to 52% (P>0.001) using the chemical treatment. As the trial progressed and the level of rust inoculum increased, the effect of methyl jasmonate on disease was reduced. Bupirimate and triforine application reduced rust levels significantly for most of the length of the trial. Methyl jasmonate reduced the height and size of Antirrhinum plants significantly compared to both the control and chemical treatment. However, on termination of the trial there was no difference in dry weight measurements among the treatments. The poster discusses the results from these trials, comparing them to previous studies on methyl jasmonate and disease control. This is the first published work indicating the potential value of SAR for disease control in horticultural crops in the field situation. Other, similar trials also carried out by the author screening methyl jasmonate against Uromyces viciae-fabae and Botrytis fabae on broad beans recorded larger and more consistent reductions in disease levels. Further work is required on the Antirrhinum rust host-pathogen system to confirm these reductions in disease levels in the field and optimize application rates and frequencies. More detailed information on the mode of action of methyl jasmonate in controlling disease may allow further improvements to be made in disease control.

1. Lyon GD, Forrest RS, Newton AC, 1996. Proceedings Brighton Crop Protection Conference: Pests and Diseases, pp. 939-946.
2. Cohen Y, Gisi U, Niderman T, 1993. Phytopathology 83, 1054-1062.