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First Report of Aristastoma Leaf Spot on Desert Rose

In the spring of 1995, a species of Aristastoma was isolated from foliar lesions of Adenium obesum that originated in a commercial nursery in Dade County, FL, where 100% of the crop was affected. Plant foliage had irregular, oval to circular, rusty brown, necrotic lesions 5 to 15 mm in diameter. Large leaf spots developed tan centers. An undescribed species of Aristastoma was isolated consistently from symptomatic plant material. The identity of the fungus was confirmed by the International Mycological Institute, Egham, England. Both in vivo and in vitro, the fungus has immersed mycelium, pale brown to hyaline, branched and septate. Pycnidia are 160 to 300 μm in diameter with prominent setae 75 to 125 μm surrounding the ostiole, with hyaline, 1 septate, conidia that are 25 to 30 × 3 to 4 μm. Pathogenicity was tested on shadehouse-grown, 8-month-old desert rose plants by spraying plants with a conidial suspension containing 10⁴ conidia per ml. Inoculated plants developed foliar symptoms within 5 to 10 days that were consistent with those originally observed on the diseased plant samples. The pathogen was isolated from symptomatic tissue of inoculated plants. Species of Aristastoma are reported to cause foliar symptoms mostly on leguminous crops, and in Florida Aristastoma oeconomicum causes lesions on Vigna. This is the first report of this or any species of Aristastoma on Adenium obesum or any other member of the Apocynaceae.