Identification of the Causal Agent of a Foliar Disease Affecting Melons in “La Fragua” Valley in Zacapa, Guatemala
Universidad del Valle de Guatemala (Guatemala)
Cynthia Ralda, Guillermo E. Sanchez (Guatemala); Stephen Weller (US)
Some areas of the Motagua River valley, in the eastern Department of Zacapa in Guatemala have been continuosly planted with melon for the last 10 years, increasing the risk of augmented pest pressure over the crop. In addition, the environmental conditions during the rainy season have favored the appearance of new diseases in the last 2 years, causing significant losses. Growers suspected a new bacterial pathogen to be the cause of the main foliar symptoms that suddenly appeared in melon fields. This situation motivated the melon growers to search for technical support in studies focusing on the identification of the causal agent(s) promoting the foliar symptoms. The IPM CRSP-Universidad del Valle (UVG) team responded by conducting a series of trips to the area and meetings with the field technicians in charge of the melon field operations, where a strategy for the identification of the causal agent was designed. The most severe outbreaks in the field were associated to angular leaf spots on cantaloupe melons and watermelons. During these field trips symptom-containing samples from 5 participating companies were collected and taken to UVG laboratories for identification purposes. Additional samples were collected and transported to UVG by field technicians working for the growing companies. The laboratory procedures included (1) symptom recording, (2) observations under light microscopy, (3) gram and flagella staining, (4) growth on selective media, (5) biochemical reactions and nutrient utilization, (6) hypersensitive response on tobacco and, (7) serology (Enzyme linked immunosorbent assay), where applicable.
Out of 58 samples, 48 bacterial colonies with features typical of pathogenic bacteria were detected. Of these, 19 were identified as Pseudomonas syringae pv lachrymans, causal agent of the angular leaf spot in cucurbits. Seventeen isolates were identified as Erwinia, belonging to the herbicola or quercina species. Five bacterial colonies were identified as Xanthomonas (associated to leaf spots in melon) and 7 other colonies belonging to different Pseudomonas sp. (pseudoalcaligenes, putida, cepacea). Results show that the main cause of the bacterial outbreaks is Pseudomonas syringae pv lachrymans, although Xanthomonas was also associated with the symptoms. Bacterial leafspots caused by Xanthomonas campestris pv cucurbita have been detected in other cucurbits but not melons, however our results suggest that, in Guatemala, Xanthomonas may be a pathogen of cantaloupe melons. Further studies into this possibility are recommended.