Evaluation of Oxathiapiprolin for the Management of Sunflower Downy Mildew.

Downy mildew is a yield-limiting disease of sunflower, caused by the pathogen Plasmopara halstedii. Zoospore infection of root tissue shortly after planting results in systemic infection, causing postemergence damping off or severe stunting and head sterility. Although fungicide-applied seed treatments can be an effective management tool, the pathogen is resistant to phenylamide fungicides in many growing regions, and other available fungicides have limited efficacy. Oxathiapiprolin, the first member of the piperidinyl thiazole isoxazoline fungicides, was evaluated for efficacy on downy mildew in field trials conducted from 2011 to 2015 in North Dakota. Throughout the course of the study, the rate range was narrowed from active ingredient (a.i.) at 0.45 to 116.0 µg a.i. seed-1 to an optimal effective rate of 9.37 to 18.75 µg a.i. seed-1. Within that optimal range, the downy mildew incidence of sunflower planted with oxathiapiprolin-treated seed was significantly lower than the incidence in the nontreated sunflower in all 11 trials with disease pressure. Additionally, downy mildew incidence of sunflower planted with oxathiapiprolin-treated seed was significantly lower than sunflower planted with competitive commercially available fungicide-treated seed in 10 of those 11 trials. The use of oxathiapiprolin by sunflower growers is likely to reduce disease incidence and subsequent yield loss to downy mildew.

Genetic and morphological evidence that Phoma sclerotioides, causal agent of brown root rot of alfalfa, is composed of a species complex.

Phoma sclerotioides, causal agent of brown root rot of alfalfa, causes severe root and crown lesions on alfalfa and other perennial forage legumes in regions with harsh winters. Isolates of P. sclerotioides exhibit diverse cultural morphologies on potato dextrose agar (PDA), suggesting that they may exhibit a high degree of genetic diversity. To investigate the genetic relatedness of P. sclerotioides isolates, 154 isolates from North America were sequenced at 10 loci. Maximum parsimony and maximum likelihood analyses of the complete 10-locus data set placed isolates into multiple strongly supported clades, and analyses of gene-jackknife and single-gene partitions of the data set indicated robust support for six major clades and three subclades. Genetic differences corresponded closely to differences in conidial size and septation, pycnidial neck length, mycelial pigmentation, and growth rate in axenic culture at 18 and 25°C. Isolates exhibited morphologies broadly consistent with the species description of P. sclerotioides, and new species were not designated. On the basis of genetic and morphological differences, we propose establishing seven infraspecific varieties within P. sclerotioides: P. sclerotioides var. sclerotioides, champlainii, viridis, obscurus, steubenii, macrospora, and saskatchewanii. All varieties of P. sclerotioides caused brown root rot of alfalfa and grew well at low temperatures.

Characterization of Fusarium oxysporum f. sp. loti Forma Specialis nov., a Monophyletic Pathogen Causing Vascular Wilt of Birdsfoot Trefoil.

Fusarium wilt, a vascular wilt caused by Fusarium oxysporum, has been a serious problem for birdsfoot trefoil (Lotus corniculatus) production in parts of New York and Vermont since the 1970s, causing wilt, severe root necrosis, and rapid plant death. Analysis of F. oxysporum isolates causing this disease indicated that the pathogen has a unique host range relative to previously designated F. oxysporum formae speciales and is monophyletic. Pathogenic isolates from New York and Vermont caused severe vascular wilt of trefoil and moderate vascular wilt of pea but no disease on alfalfa, red clover, soybean, or dry bean. The host range of trefoil isolates was distinct from F. oxysporum isolates pathogenic to other legumes. F. oxysporum isolates pathogenic to trefoil belonged to a single vegetative compatibility group separate from nonpathogenic isolates and shared identical mitochondrial small subunit rDNA, translation elongation factor 1-alpha, and nuclear rDNA intergenic spacer haplotypes. Phylogenetic analysis of the concatenated sequence data assigned isolates pathogenic to trefoil to a single, well-supported clade distinct from other pathogenic F. oxysporum. We propose designating the fungus Fusarium oxysporum Schlechtendahl emend. Snyder & Hansen f. sp. loti forma specialis nova.