RESUMO
In Mexico City, the Canary Island date palm (Phoenix canariensis Chabaud) is an important plant forming part of its landscape identity. In February 2022, pink rot disease symptoms were observed on 16 P. canariensis plants in Mexico City (19°25'43.98"N, 99° 9'49.41"W). The incidence was 27%, while the severity 12%. External symptoms included necrotic lesions that advanced from the petiole towards the rachis. Internal symptoms were rotted, dark brown discoloration in bud, petiole, and rachis. Abundant conidial masses were developed on the infected tissues. Pieces of diseased tissues (5-mm cubes) were surface sterilized for 2 min in 3% sodium hypochlorite, rinsed with sterilized distilled water, plated onto potato dextrose agar (PDA), and incubated at 24°C and 12-h photoperiod, 20 pink fungal colonies were developed with sparse aerial mycelia on PDA. Conidiophores were hyaline, dimorphic, penicillate, and Acremonium-like. Conidia were dimorphic, typically with somewhat truncated ends, 4.5 to 5.7 × 1.9 to 2.3 µm (mean 4.99 × 2.15, n = 100), borne in long chains on penicillate conidiophores; on Acremonium-like conidiophores conidia were cylindrical, straight, and slightly curved, 4.55 to 10.1 × 1.2 to 2.35 µm (mean 8.2 × 1.7, n = 100). These morphological characteristics resembled those of Nalanthamala vermoesenii (Biourge) Schroers (Schroers et al. 2005). Genomic DNA was extracted from the mycelia of a representative isolate CP-SP53. The internal transcribed spacer (ITS) region and the large subunit of ribosomal ribonucleic acid (LSU) were amplified and sequenced. The sequences were deposited in GenBank with accession numbers OQ581472 (ITS) and OQ581465 (LSU). Phylogenetic trees based on ITS and LSU sequences of Nalanthamala species were reconstructed using maximum likelihood and Bayesian inference methods. Isolate CP-SP53 was placed in the clade of Nalanthamala vermoesenii. The pathogenicity test was carried out twice with isolate CP-SP53 on five 3-year-old P. canariensis plants. Four petioles per plant were surface disinfected with 75% ethanol, and wounded with a sterilized scalpel (shallow cuts 0.5 cm wide). A mycelial plug (5 mm in diam.) of a 1-week-old PDA culture was placed on each wounded site. Sterile PDA plugs were used for five non-inoculated control plants. All plants were maintained at 22 ± 2°C and a 12-h photoperiod. Twenty-five days after inoculation (dai), wounded petioles showed the same symptoms observed in the field, whereas control plants remained healthy. Forty-five dai, all inoculated plants died. Pink conidial masses developed on symptomatic tissues. To fulfill Koch's postulates, the pathogen was reisolated by placing the pink conidial masses onto PDA. The colony characteristics and morphometric measurements were identical to those of isolate CP-SP53. Nalanthamala vermoesenii has been reported on P. canariensis in Greece and United States (Feather et al. 1979; Ligoxigakis et al. 2013) and Syagrus romanzoffiana in Egypt (Mohamed et al. 2016). To our knowledge, this is the first report of Nalanthamala vermoesenii as the causal agent of pink rot on P. canariensis in Mexico. This plant is the most commonly planted ornamental palm in Mexico City. The spread of N. vermoesenii could be a threat for the estimated 15 thousand palms, therefore dramatically change the urban landscape.
RESUMO
The genetic structure of a sample of isolates of the oomycete plant pathogen Phytophthora cinnamomi from natural and agricultural outbreaks and the long-distance movement of individual genotypes were studied using four microsatellite markers to genotype 159 isolates of Californian, Mexican, and worldwide origins. Allelic profiles identified 75 multilocus genotypes. A STRUCTURE analysis placed them in three groups characterized by different geographic and host ranges, different genic and genotypic diversity, and different reproductive modes. When relationships among genotypes were visualized on a minimum spanning network (MSN), genotypes belonging to the same STRUCTURE group were contiguous, with rare exceptions. A putatively ancestral group 1 had high genic diversity, included all A1 mating type isolates and all Papuan isolates in the sample, was rarely isolated from natural settings in California and Mexico, and was positioned at the center of the MSN. Putatively younger groups 2 and 3 had lower genic diversity, were both neighbors to group 1 but formed two distinct peripherical sectors of the MSN, and were equally present in agricultural commodities and natural settings in Mexico and California. A few genotypes, especially in groups 2 and 3, were isolated multiple times in different locations and settings. The presence of identical genotypes from the same hosts in different continents indicated that long-distance human-mediated movement of P. cinnamomi had occurred. The presence of identical genotypes at high frequencies in neighboring wildlands and agricultural settings suggest that specific commodities may have been the source of recent wild infestations caused by novel invasive genotypes.
