SYSTEMIC ACQUIRED RESISTANCE IN COTTON AGAINST ALTERNIA MACROSPORA
ES COLSON1, BJ DEVERALL1 and SJ ALLEN2
1Department of Crop Sciences, University of Sydney, NSW 2006, Australia; 2Australian Cotton Research Institute, Narrabri, NSW 2390, AustraliaBackground 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 ß-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  (Bion WG 50 from Novartis) were also tested for effectiveness in causing systemic resistance.
Materials and methods
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 ß-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 -80°C in individual bags before assays for ß-1,3 glucanase were performed as in , except that 0.1 M sodium phosphate buffer at pH 6.2 was used and the reaction at 30°C was stopped after 10 min.
Results and conclusions
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.