R. J. Holguín-Peña,
L. G. Hernández-Montiel, and
H. Latisnere, Laboratorio de Fitopatología, Centro de Investigaciones Biológicas del Noroeste, La Paz, B.C.S. 23096, Mexico; and
E. O. Rueda-Puente, Universidad de Sonora, Santa Ana, Sonora 84600, Mexico
Giant cardon (Pachycereus pringlei ((S.Watson) Britton & Rose) is the most common cactus in northwestern Mexico and is endemic to the Baja California Peninsula and Sonora Desert. A large part of the peninsula (El Vizcaino Biosphere Reserve and Gulf of California) now consists of protected areas and is classified as a World Heritage site by UNESCO (http://whc.unesco.org/en/list/1182). Cardon cactus is an important ecological resource for indigenous people and is used as feed for range cattle. Since 2000, in the central and southern part of the State of Baja California Sur, an apical stem rot has spread to ~17% of the natural cardon population around San Pedro (23°29′N, 110°12′W), La Paz (24°08′N, 110°18′W), and El Comitán (24°05′N, 110°21′W). Affected cacti display necrosis of apical branches, dry rot, cracks in the stem and branches, bronzing of mature spines surrounding the affected area, and reddish brown gummy exudate. Thirty samples from the edges of symptomatic lesions were surface disinfected for 2 min in 0.8% (wt/vol) NaOCl and ethanol (70%), rinsed in sterile, distilled water, and grown on potato dextrose agar at 27°C. A cottony, brownish fungus was consistently isolated from affected tissues. Koch's postulates were performed in pots of 10 cm in diameter with 5-year-old cacti inoculated (9-day-old mycelia) and incubated (15 days) at room temperature (26°C). The rough, dry, brownish, circular lesions that appeared were the same as those observed in the field. Healthy cacti inoculated with potato dextrose agar plugs were symptomless. The fungus was always reisolated from infected cacti and morphological examinations (2) were performed: one-septate, olive-green, smooth, ellipsoidal conidium and two-celled ascospores (15 to 20 × 5 to 7 μm) were present. Also present were conidial masses from monomorphic, penicillate conidiophores in sporodochia. Cottony and white-to-light yellow PDA colonies were observed. Genomic DNA was extracted from lyophilized hyphae using the method described by O'Donnell (1) or with a DNeasy Plant Mini Kit (Qiagen, Hilden, Germany). The internal transcribed spacer (ITS) regions 1 and 2 of the 5.8, 18, and 28S ribosomal RNA genes were amplified with the primer pairs ITS1 and ITS4 (3). The expected amplicon of 571 bp was sequenced and compared with fungal sequences available from the GenBank-EMBL database using the BlastN and CLUSTAL programs (MegAlign, DNASTAR, Madison, WI). The closest nucleotide similarity had 99% identity with a Bionectria sp. (GenBank Accession No. HM849058.1). To our knowledge, on the basis of morphological characteristics, DNA comparisons, and pathogenicity tests, this is the first report of a Bionectria sp. causing an apical stem rot disease in cardon cacti in Mexico. Since there are no control measures in Mexico there is a permanent risk that the disease will spread to healthy areas.
References: (1) K. O'Donell et al. Mycologia 92:919, 2000. (2) H. J. Schroers. Stud. Mycol. 46:1, 2001. (3) T. J. White et al. PCR Protocols: A Guide to Methods and Applications. Academic Press, San Diego, 1990.