Sechium edule (Jacq.) Sw., also known as mirliton or chayote, is a perennial, monoecious, cucurbitaceous plant native to Mexico and Central America. It is cultivated worldwide for a variety of uses (4). Mirliton fruit is rich in carbohydrates, has 16 amino acids, and is a traditional staple in New Orleans, LA. During the spring of 2009, the LSU AgCenter's Plant Disease Diagnostic Clinic received diseased mirliton plants from a small commercial grower in New Orleans. Symptoms included yellow, irregular spots on both surfaces of the leaves. Microscopic examination revealed the presence of powdery mildew conidia and conidiophores. Initially, white, cottony mycelial colonies were present on the abaxial surface, but as the disease progressed, white, cottony colonies developed on the adaxial surface, the spots coalesced, and the entire leaf turned yellow and necrotic. Conidia were hyaline, ovoid, borne in long chains with crenate edges, and measured 25.6 to 36.6 μm long (mean = 31.2) × 14.6 to 18.3 μm wide (mean = 17.1). Conidia contained fibrosin bodies and produced a lateral germ tube with a simple appressorium. Conidiophores were erect, simple, unbranched, and measured 54.9 to 76.9 μm long (mean = 66.4) × 11.0 to 14.6 μm wide (mean = 12.9). The cylindrical foot cell had a simple base with basal septum adjacent to the mycelium. No teleomorph was observed. Morphologically, this powdery mildew fits either Podosphaera fusca or P. xanthii so DNA analysis was conducted. We designed Podosphaera-specific primers PFITS-F (5′-CCAACTCGTGCTGTGAGTGT-3′) and PF5.8-R (5′-TGTTGGTTTCTTTTCCTCCG-3′) to amplify and sequence the internal transcribed spacer region (ITS) of the nuclear rDNA. The 331-bp sequence (GenBank Accession No. GQ902939) was identical with haplotype 27 of P. fusca (GenBank Accession No. AB040324) (3), which is now called P. xanthii (1). Pathogenicity tests were conducted by pressing infected leaves against healthy leaves of two vines. A noninoculated vine served as a control. Plants were maintained in a greenhouse at 30°C. Five days after inoculation, yellow, irregular spots appeared on the inoculated vines and white, powdery mildew colonies appeared on the abaxial surface. Spots coalesced and the entire leaf turned yellow 8 days after inoculation and necrotic 12 days after inoculation. No symptoms developed on the controls. On the basis of DNA sequence data, this powdery mildew is identified as P. xanthii sensu (1). Erysiphe cichoracearum has been previously reported to cause powdery mildew on mirlitons in Florida and Hawaii (2). To our knowledge, this is the first report of powdery mildew caused by P. xanthii on mirliton in the United States. A voucher specimen has been deposited in the Bernard Lowy Mycological Herbarium (LSUM 185359).
References: (1) U. Braun and S. Takamatsu. Schlechtendalia 4:31, 2000. (2) D. F. Farr and A. Y. Rossman. Fungal Databases. Systematic Mycology and Microbiology Laboratory, SMML, Online publication. USDA-ARS, 2009. (3) T. Hirata et al. Can. J. Bot. 78:1521, 2000. (4) M. Janssens et al. Tropical Crops. ARTS; Field and Vegetable Crops, PTS 130. Bonn, Germany, 2002/03.