First report of Pythium deliense as a pathogen of melon in Honduras and Guatemala

In 2004 and 2005, several soil surveys were conducted in the main melon (Cucumis melo) production areas of Honduras and Guatemala, all areas with a history of vine decline. Fifty-nine samples from twenty different farms were collected from the rhizosphere zone of wilted plants. Samples were analyzed by a baiting method consisting of sowing melon seeds in a mixture of each rhizosphere sample and vermiculite (1:6 vol/vol). Thirty-six melon plants per sample were placed in a growth chamber with a 16-hour photoperiod and temperatures of 23 to 25°C. Diseased plants started to appear when plants showed one or two true leaves. The first symptom observed was girdling of the lower stem, followed by leaf chlorosis and wilting. Plants died in less than three days after wilting. Affected plants exhibited necrotic and poor root systems. Washed and dried crown and root pieces from wilted plants were placed on malt-extract agar. Colonies of Pythium species were transferred to potato-carrot agar and into dishes of sterile distilled water with immature carnation petals, or to pond water with grass. Ten isolates from rhizospheric soil from ten different fields (nine from Honduras and one from Guatemala) were identified by morphological characters as P. deliense (Van der Plaats-Niterink, 1981). The isolates produced inflated filamentous sporangia with swollen lateral branches. The oogonia were typically terminal with oogonial stalks bending to the antheridia. The antheridia were terminal or intercalary and the oospores were aplerotic.

The identification of two of the isolates was confirmed by sequencing of amplicons of the internal transcribed spacer region (ITS) rDNA using universal primers ITS1 and ITS4 (White et al., 1990). The pathogenicity of all ten isolates was confirmed by inoculation of melon plants in a growth chamber (16 hour photoperiod, 23- 25°C). Ten plants of cultivar Amarillo Canario, grown on sterilized vermiculite, were inoculated at the three true leaf stage by drenching pots with 100 ml of a suspension of each isolate (1 x 10³ CFU mL^-1). Non-inoculated plants served as controls. There were three replicates per isolate. Plants were maintained in the growth chambers for 30 days after inoculation. The first dead plants appeared seven days after inoculation, showing the same symptoms as described above. Incidence of the disease (measured as number of dead plants compared to the total number of inoculated plants) reached an average of 90%. All isolates were reisolated from dead plants' crowns and roots. P. deliense has been previously reported in Oman associated with melon vine decline, but pathogenicity tests were not published (Deadman et al., 2007). Van der plaats-Niterink (1981) also referred to a work where P. deliense was reported as a pathogen on melon, but this appeared to be an erroneous citation. To our knowledge, this is the first confirmed report of P. deliense as a pathogen on melon plants.