1Department of Crop Sciences, University of Sydney, NSW 2006, Australia; 2Australian Cotton Research Institute, Narrabri, NSW 2390, Australia

Background and objectives
Alternaria leaf spot is one of the most common foliar diseases of cotton, occurring in most regions of the world. Experiments were carried out first to assess the abilities of the activator 2, 6- dichloroisonicotinic acid (INA) formulated by Ciba-Geigy as CGA41396 and its formulation material (WP) systemically to induce resistance against Alternaria macrospora and to enhance activity of the pathogenesis-related protein -1,3 glucanase in cotton seedlings. In the light of the results obtained, some of the components of the WP material and the newly available activator benzothiadiazole [1] (Bion WG 50 from Novartis) were also tested for effectiveness in causing systemic resistance.

Materials and methods
Gossypium hirsutum cv. Siokra 1-4 plants were grown in a glasshouse with successive 12-h diurnal periods of 18 and 30C for inoculation and in growth cabinets with successive 16 h 32C light periods followed by 8 h 25C dark periods for -glucanase assays. Alternative suspensions or solutions of CGA41396 (20 g INA as a.i. and 60 g WP/ml), WP formulation material (60 g WP/ml), INA (20 g/ml), kaolin (9.8 g/ml) or silicic acid (16 g/ml) or Bion WG 50 (70 g/ml) were applied to the cotyledons to run-off using an atomizer, with kaolin applied using a fine camel-hair brush.

For inoculation, 10-14-day-old cultures of A. macrospora grown on V8 agar were sources of washed conidial suspensions, applied to the first or second leaves at 5x105 ml using a fine camel-hair brush. Four to six days later, the number of lesions on the upper sides of leaves were counted.

For -glucanase studies, all treatments were applied to the cotyledons to run-off using an atomizer, except for kaolin which was applied using a fine camel-hair brush. First leaves were harvested 72 or 120 h later, fresh weighed and then frozen at -80C in individual bags before assays for -1,3 glucanase were performed as in [2], except that 0.1 M sodium phosphate buffer at pH 6.2 was used and the reaction at 30C was stopped after 10 min.

Results and conclusions
Plants treated on the cotyledons with CGA41396 or the WP formulation material had significantly (P<0.05) fewer lesions on first and second leaves than water controls. Glucanase activities were elevated in the first leaves of uninoculated seedlings following application of any of CGA41396, INA, the WP formulation material, silicic acid or kaolin to the cotyledons compared with water controls.

Significantly (P<0.05) fewer lesions developed on the first and second leaves of seedlings that had received cotyledonary treatments with kaolin or silicic acid than with water. Bion WG 50 also had a significant (P<0.05) effect in decreasing lesion numbers on the first and second leaves of cotton seedlings after application to cotyledons. Seedlings treated with Bion WG 50 on the cotyledons had a 43-87% reduction in lesion numbers on the first leaves compared with water controls in three separate experiments.

The results show that cotton seedlings are responsive to chemical activators in developing systemic resistance to the leaf spot disease caused byA. macrospora.

1. Gorlach J, Voirath S, Knauf-Beiter G et al., 1996. Plant Cell 8, 629-643.
2. Dann EK, Meuwiy P, Metraux, J-P, Deverall BJ, 1996. Physiological and Molecular Plant Pathology 49, 307-319.