A new fungal parasite of the reniform nematode has been observed parasitizing nematode populations that have increased on cotton in a sandy loam field soil in the greenhouse. When enumerated, 46% of the stock reniform nematode population was colonized by this fungus. Egg, vermiform, and adult stages of the reniform nematode were observed with light microscopy and scanning electron micrography (SEM). The nematophagous fungus, Catenaria auxiliaries, was identified morphologically. There are no sequences on the GenBank to achieve a molecular identification. This nematophagous fungus has been previously reported on the beet cyst nematode in Europe (1,2); however, to our knowledge there are no reports of this fungus parasitizing the reniform nematode. In vermiform life stages of the nematode, rhizomycelium is observed in the initial phase of infection and is characterized by ovoid cells, 9.5 to 13.5 × 17.0 to 24.5 μm in diameter, separated by septa. Usually 10 to 15 ovoid cells lacking intercellular hyphal filaments are produced within each vermiform body. Rhizoids 3.5 to 4.0 μm wide develop from the rhizomycelium. Mature swollen cells produce precursor sporangia that may mature into resting spores or zoosporangia. Resting spores are yellow-to-cream, 20 to 40 μm in diameter with a reticulate appearance, and are common in the vermiform nematode life stages. Zoosporangia are ovoid, 9.5 to 13.5 × 17.0 to 24.5 μm, and will erupt from the cuticle of the vermiform nematode releasing zoospores via papillae. Zoospores are 2.9 to 4.9 μm with visible globules in the anterior region and single flagella that are 9 to 11 μm long. The zoospores swim short distances, maneuvering in the direction of the flagellum. Adult reniform females observed through SEM exhibit zoospores encysted in the metacorpus region of the nematode. Parasitized eggs are internally colonized with zoosporangia that are 20 to 25 μm in diameter. In advanced stages of infection, the eggs darken in color and zoosporangia erupt through the cuticle of the egg. Reniform nematodes visibly colonized with zoosporangia and resting spores were placed on corn meal, water, and potato dextrose agars. None of these media supported growth of the fungus, supporting our theory that this organism appears to be an obligate parasite of the nematode. Koch's postulates was completed when eggs colonized with rhizomycelium and resting spores or zoosporangia were added to cotton plants in sterile soil previously inoculated with 2,000 healthy vermiform reniform life stages. Plants were allowed to grow for 30 days in the greenhouse after which the next generation of vermiform nematodes were extracted from the soil and examined under the microscope. Rhizomycelium, resting spores, and zoosporangia were present in 42% of the reniform vermiform life stages. Morphological comparisons of the rhizomycelium, resting spores or zoosporangia, and zoospores colonizing the reniform nematodes were similar to the initial observations. Thus to our knowledge, this is the first report of Catenaria auxiliaries parasitizing the reniform nematode.
References: (1) B. Kerry. J. Nematol. 12:253, 1980. (2) H. T. Tribe. Trans. Br. Mycol. Soc. 69:367, 1977.
