First Report of Gladiolus Rust Caused by Uromyces transversalis in Michoacán, México

Invasions of gladiolus rust in North America are caused by a widely-distributed clone of Uromycestransversalis

Uromyces transversalis, the causal agent of Gladiolus rust, is an invasive plant pathogen in the United States and is regulated as a quarantine pathogen in Europe. The aim of this research was to: (i) determine the origin of introductions of U. transversalis to the United States, (ii) track the movement of genotypes, and (iii) understand the worldwide genetic diversity of the species. To develop molecular markers for genotyping, whole genome sequencing was performed on three isolates collected in the United States. Genomes were assembled de novo and searched for microsatellite regions. Primers were developed and tested on ten isolates from the United States resulting in the identification of 24 polymorphic markers. Among 92 isolates collected from Costa Rica, Mexico, New Zealand, Australia, and the United States there were polymorphisms within isolates with no genotypic diversity detected among isolates; however, missing data among the New Zealand and Australia isolates due to either poor amplification of degraded DNA or null alleles as a result of genetic differences made it difficult to generate conclusions about these populations. The microsatellite loci and flanking regions showed high diversity and two divergent genomes within dikaryotic individuals, yet no diversity among individuals, suggesting that the invasive U. transversalis populations from North America are strictly clonal.

Fungicides Used Alone, in Combinations, and in Rotations for Managing Gladiolus Rust in Mexico

Gladiolus rust, caused by Uromyces transversalis, is a quarantine-significant pathogen in the United States. However, the fungus is endemic to commercial gladiolus-producing areas in Mexico and has been intercepted frequently on gladiolus plants entering the United States for the cut-flower market. The present study assessed 15 fungicide active ingredients (five quinone outside inhibitors: azoxystrobin, fluoxastrobin, kresoxim-methyl, pyraclostrobin, and trifloxystrobin; six triazoles: cyproconazole, difenoconazole, epoxiconazole, myclobutanil, propiconazole, and tebuconazole; three succinate dehydrogenase inhibitors: boscalid, flutolanil, and oxycarboxin; and one broad-spectrum protectant: chlorothalonil) and one plant activator, acibenzolar-S-methyl, applied alone, in combinations, and in various rotations for efficacy against U. transversalis on field-grown gladiolus plants in Mexico. Experiments were conducted in 2010, 2011, and 2012 in commercial fields in Atlixco and Santa Isabel Cholula in Puebla and Cuautla and Tlayacapan in Morelos. Fungicides were applied at 2-week intervals starting when plants had three full leaves. Disease severity was recorded each week for at least 7 weeks after the first application. Under high disease pressure in 2010, fungicides were less effective than in 2011 and 2012, when disease pressure was not as high. In all 3 years, most fungicide treatments significantly reduced disease severity. Triazoles were more effective than quinone outside inhibitors when applied as individual products in 2010, and combinations of two fungicides in different mode-of-action groups were more effective than fungicides applied individually in 2011. In 2012, rotations of fungicides, either with individual products or with combinations of two products, provided excellent rust management. Reducing disease development by U. transversalis on commercial gladiolus plants in Mexico will reduce the potential for introducing this pathogen on cut flowers into the United States.