RESUMO
Big-leaf mahogany (Swietenia macrophylla) is valued for its high-quality wood and use in urban landscapes in Mexico. During surveys of mango-producing areas in the central western region of Mexico, symptoms of malformation, the most important disease of mango in the area, were observed on big-leaf mahogany trees. The objectives of this research were to describe this new disease and determine its cause. Symptoms on big-leaf mahogany at four sites in Michoacán, Mexico resembled those of the vegetative phase of mango malformation, including compact, bunched growth of apical and lateral buds, with greatly shortened internodes and small leaves that curved back toward the supporting stem. Of 163 isolates that were recovered from symptomatic tissues, most were identified as Fusarium pseudocircinatum (n = 121) and F. mexicanum (n = 39) using molecular systematic data; two isolates represented unnamed phylospecies within the F. incarnatum-equiseti species complex (FIESC 20-d and FIESC 37-a) and another was in the F. solani species complex (FSSC 25-m). However, only F. mexicanum and F. pseudocircinatum induced malformation symptoms on 14-day-old seedlings of big-leaf mahogany. The results indicate that F. mexicanum and F. pseudocircinatum, previously reported in Mexico as causal agents of mango malformation disease, also affect big-leaf mahogany. This is the first report of this new disease and the first time that F. mexicanum was shown to affect a host other than mango.
Assuntos
Fusarium/isolamento & purificação , Fusarium/patogenicidade , Meliaceae/microbiologia , Doenças das Plantas/microbiologia , DNA Fúngico/genética , Fusarium/genética , México , Tipagem de Sequências Multilocus , Filogenia , Plântula/microbiologiaRESUMO
Carrot (Daucus carota L. subsp. sativus (Hoffm.) Arcang.) is planted as a home-grown vegetable in the central region of Michoacan, Mexico. Powdery mildew was observed on carrot plants cv. Nantesa at several locations near Morelia, Michoacan during March 2009. Affected plants had abundant, white, superficial conidia and mycelium on leaves and stems. All plants at each of five locations surveyed had powdery mildew symptoms with percent foliage coverage ranging from 50 to 80%. Mycelial growth was amphigenous, mainly on the upper leaf surface, covering the whole leaf and with irregular patches on inflorescences and stems. Hyphae were ectophytic with lobed appressoria. Conidiophores presented foot cells 22.5 to 35 (30) × 5.75 to 7 (6.3) µm followed by two cells, one shorter and one longer than the foot cell. Conidia were produced singly, most subcylindric to cylindric, lacked fibrosin bodies, and measured 31.2 to 42 (36.2) × 8.7 to 11.2 (10.5) µm. The teleomorph was not observed. Genomic DNA was extracted from infected leaves; sequences of the internal transcribed spacers (ITS) inclusive of 5.8S rDNA were amplified using previously described primers specific for Erysiphales (3). The ITS sequences shared 100% homology to Erysiphe heraclei specimen VPRI41227 from carrot in Australia (GenBank Accession No. EU371725). On the basis of the morphological characteristics observed and the ITS rDNA sequences, the pathogen was identified as E. heraclei DC. The ITS sequence was deposited in NCBI as Accession No. GU252368. Pathogenicity tests were conducted twice on a total of 10 healthy 8-week-old carrot plants cv. Nantesa. Infected plants were placed in close proximity to healthy plants and maintained in a greenhouse at 27 ± 5°C. Initial signs and symptoms were observed 3 weeks after inoculation and appeared as small, white colonies, which later coalesced and covered most of the foliage. Microscopic examination of the conidia and mycelial morphology matched the originally described pathogen, E. heraclei. Powdery mildew caused by this pathogen has been extensively reported on diverse species and genera of the Apiaceae in Europe and remains one of the most important diseases of carrot (2). The appearance of E. heraclei in diverse regions on a variety of umbelliferous crops indicates that formae speciales have spread, infecting different and specific hosts (1-3). Recently, E. heraclei has been reported on parsley in Puebla, Mexico (4). To our knowledge, this is the first report of E. heraclei causing powdery mildew on carrot in Michoacan, Mexico. This pathogen should be considered as a threat to commercial carrot crops in Mexico. Other crops in the Apiaceae may not be at risk in this area if this powdery mildew is specific for carrots. References: (1) B. J. Aegerter. Page 22 in: Compendium of Umbelliferous Crop Diseases. The American Phytopathological Society, St. Paul, MN, 2002. (2) U. Braun. The Powdery Mildew (Erysiphales) of Europe. Gustav Fischer-Verlag. Jena, Germany, 1995. (3) J. H. Cunnington et al. Australas. Plant Pathol. 32:421, 2003. (4) M. J. Yáñez-Morales et al. Schlechtendalia 19:47, 2009.