M FORBES-SMITH, J PATON, S MU and F GHAHRAMANI

CRC for Food Industry Innovation, Department of Food Science and Technology, University of New South Wales, Sydney 2052, Australia

Background and objectives
The citrus industry relies on synthetic fungicides to control post-harvest decay by Penicillium digitatum. However, the development of resistance in P. digitatum to existing fungicides, such as the benzimidazoles, and consumer concern about the use of man-made chemicals for health and the environment, have restrained or eliminated the use of these chemicals [1].

A practical alternative to post-harvest chemicals is the utilization of elicitors that cause induced resistance in horticultural commodities [1]. The aim of this paper is to evaluate the efficacy of biological and chemical substances that activate defence responses in harvested oranges and control decay by P. digitatum.

Materials and methods
Plant growth regulators, yeast extracts, metal salts and chitosan were used to induce resistance in oranges to P. digitatum. Fruit were dipped in elicitor treatments followed by a spore suspension of P. digitatum (ca 10⁶ spores/ml) 0.5 or 20 h after elicitor application. Oranges were held at 20°C for up to 28 days and examined for decay. In vitro activity of the compounds was determined using agar plate diffusion tests.

Methanolic extracts of rind bordering P. digitatum decay were analysed for the phytoalexins scoparone, scopoletin and umbelliferone. HPLC was performed using a 150x4.6-mm ID reverse-phase C¹⁸ column with the mobile phase 70% water, 10% methanol and 20% tetrahydrofuran.

Results and conclusions
The most active elicitors that reduced Penicillium decay of oranges were chitosan and potassium phosphonate. Yeast glucans, calcium chloride, potassium phosphate, and plant growth regulators such as indole acetic acid, had no significant effect in controlling P. digitatum. No elicitor treatments were as efficacious as the fungicide imazazil.

As chitosan and phosphonate treatments produced very little in vitro activity to P. digitatum, the decrease in mould levels was attributed to the stimulation of host defence mechanisms within the rind of the fruit. Resistance of oranges to P. digitatum was more pronounced when infection occurred 20 h after application of chitosan and potassium phosphonate. This may be because longer durations between elicitor application and infection permit adequate development of host-resistance systems. HPLC analysis of extracts of fruit showed that potassium phosphonate and chitosan treatments triggered rapid production of scoparone, but not scopoletin and umbelliferone. The concentration of scoparone in control (water-dipped) oranges, and oranges treated with chitosan and potassium phosphonate, was 13.2, 69.8 and 335.0 µg/g tissue, respectively, 6 days after treatment.

Current work is focused on determining whether pathogenesis-related proteins are activated in response to elicitors, and optimizing elicitor treatments so that disease spoilage is controlled in oranges previously infected with P. digitatum.

References
1. Wilson CL, El Ghaouth A, Chalutz E et al., 1994. Plant Disease 78, 837-844.