Effect of Eight Anti-bacterial Chemicals on the In Vitro Growth of Pseudomonas syringae pv lachrymans
Universidad del Valle de Guatemala (Guatemala)
Laura S. Vergara, Guillermo Sanchez (Guatemala); Stephen Weller (US)
Angular leaf spot, caused by Pseudomonas syringae pv lachrymans (PSL) has become a serious disease threatening the production of Guatemalan melons exported to the USA. Melons are grown in the Motagua River valley located in eastern Guatemala, where land is subjected to intensive annual plantings of this crop and environmental conditions are favorable for disease development during the rainy season. Growers, who obtain 2 harvests (rainy and dry season cycles) in an 8-month period, rely mainly on the spraying of antibiotic and copper-based chemicals for the management of the disease, therefore increasing the risk of acquired resistance by PSL to the commonly used bactericides. Recently, chemical distribution companies have begun promoting the use of new bactericides based on different active ingredients (iodine, ammonium and citrus extract). For these reasons, 8 different bactericides were tested in vitro to determine their effect on the growth of PSL using the disc diffusion sensitivity assay (DDS).
ANOVA and Duncan's multiple range test showed that the three antibiotics were the most effective inhibitors, followed by Timsen, Vanodine and Kilol all of them statistically different to each other. Under the conditions of the DDS assay, the copper compounds showed variable or no growth inhibition capacity of the PSL colonies. The bacterial isolate was also a significant factor in the inhibitory capacity of the bactericides as three separate statistical groups, showing varying levels of sensitivity, were detected. In conclusion, the antibiotic-based bactericides are the most effective inhibitors, followed by the inorganic compounds and finally the copper-based products. The differential response exhibited by the 5 bacterial isolates warrant further studies to determine whether resistance to the most commonly used bactericides has developed. The positive in vitro inhibitory effect shown by ammonium and iodine justify further in vivo tests to determine their possible inclusion in the phytosanitary programs utilized by melon growers as part of a rotational spraying program against angular leaf spot.