Effects of Pre- and Postanthesis Applications of Demethylation Inhibitor Fungicides on Fusarium Head Blight and Deoxynivalenol in Spring and Winter Wheat.

Anthesis is generally recommended as the optimum growth stage for applying a foliar fungicide to manage Fusarium head blight (FHB) and the Fusarium-associated toxin deoxynivalenol (DON) in wheat. However, because it is not always possible to treat fields at anthesis, studies were conducted to evaluate pre- and postanthesis treatment options for managing FHB and DON in spring and winter wheat. Network meta-analytical models were fitted to data from 19 years of fungicide trials, and log response ratio ([Formula: see text]) and approximate percent control ([Formula: see text]) relative to a nontreated check were estimated as measures of the effects of six treatments on FHB index (IND: mean percentage of diseased spikelets per spike) and DON. The evaluated treatments consisted of either Caramba (metconazole) applied early (at heading [CE]), at anthesis (CA), or late (5 to 7 days after anthesis; CL), or Prosaro (prothioconazole + tebuconazole) applied at the same three times and referred to as PE, PA, and PL, respectively. All treatments reduced mean IND and DON relative to the nontreated check, but the magnitude of the effect varied with timing and wheat type. CA and PA resulted in the highest [Formula: see text] values for IND, 52.2 and 51.5%, respectively, compared with 45.9% for CL, 41.3% for PL, and less than 33% for CE and PE. Anthesis and postanthesis treatments reduced mean IND by 14.9 to 29.7% relative to preanthesis treatments. The estimated effect size was also statistically significant for comparisons between CA and CL and PA and PL; CA reduced IND by 11.7% relative to CL, whereas PA reduced the disease by 17.4% relative to PL. Differences in efficacy against IND between pairs of prothioconazole + tebuconazole and metconazole treatments applied at the same timing (CE versus PE, CA versus PA, and CL versus PL) were not statistically significant. However, CA and CL outperformed PA and PL by 7 and 12.8%, respectively, in terms of efficacy against DON. All application programs had comparable efficacy against IND between spring and winter wheat types, but efficacy against DON was 10 to 16% greater for spring than winter wheat for applications made at or after anthesis. All programs led to an increase in mean grain yield and test weight relative to the nontreated check.

Genetic Linkage Mapping of Economically Important Traits in Cultivated Tetraploid Potato (Solanum tuberosum L.).

The objective of this study was to construct a single nucleotide polymorphism (SNP)-based genetic map at the cultivated tetraploid level to locate quantitative trait loci (QTL) contributing to economically important traits in potato (Solanum tuberosum L.). The 156 F1 progeny and parents of a cross (MSL603) between "Jacqueline Lee" and "MSG227-2" were genotyped using the Infinium 8303 Potato Array. Furthermore, the progeny and parents were evaluated for foliar late blight reaction to isolates of the US-8 genotype of Phytophthora infestans (Mont.) de Bary and vine maturity. Linkage analyses and QTL mapping were performed using a novel approach that incorporates allele dosage information. The resulting genetic maps contained 1972 SNP markers with an average density of 1.36 marker per cM. QTL mapping identified the major source of late blight resistance in "Jacqueline Lee." The best SNP marker mapped ~0.54 Mb from a resistance hotspot on the long arm of chromosome 9. For vine maturity, the major-effect QTL was located on chromosome 5 with allelic effects from both parents. A candidate SNP marker for this trait mapped ~0.25 Mb from the StCDF1 gene, which is a candidate gene for the maturity trait. The identification of markers for P. infestans resistance will enable the introgression of multiple sources of resistance through marker-assisted selection. Moreover, the discovery of a QTL for late blight resistance not linked to the QTL for vine maturity provides the opportunity to use marker-assisted selection for resistance independent of the selection for vine maturity classifications.

Complete genomic sequence of a Tobacco rattle virus isolate from Michigan-grown potatoes.

Tobacco rattle virus (TRV) causes stem mottle on potato leaves and necrotic arcs and rings in potato tubers, known as corky ringspot disease. Recently, TRV was reported in Michigan potato tubers cv. FL1879 exhibiting corky ringspot disease. Sequence analysis of the RNA-1-encoded 16-kDa gene of the Michigan isolate, designated MI-1, revealed homology to TRV isolates from Florida and Washington. Here, we report the complete genomic sequence of RNA-1 (6,791 nt) and RNA-2 (3,685 nt) of TRV MI-1. RNA-1 is predicted to contain four open reading frames, and the genome structure and phylogenetic analyses of the RNA-1 nucleotide sequence revealed significant homologies to the known sequences of other TRV-1 isolates. The relationships based on the full-length nucleotide sequence were different from than those based on the 16-kDa gene encoded on genomic RNA-1 and reflect sequence variation within a 20-25-aa residue region of the 16-kDa protein. MI-1 RNA-2 is predicted to contain three ORFs, encoding the coat protein (CP), a 37.6-kDa protein (ORF 2b), and a 33.6-kDa protein (ORF 2c). In addition, it contains a region of similarity to the 3' terminus of RNA-1, including a truncated portion of the 16-kDa cistron. Phylogenetic analysis of RNA-2, based on a comparison of nucleotide sequences with other members of the genus Tobravirus, indicates that TRV MI-1 and other North American isolates cluster as a distinct group. TRV M1-1 is only the second North American isolate for which there is a complete sequence of the genome, and it is distinct from the North American isolate TRV ORY. The relationship of the TRV MI-1 isolate to other tobravirus isolates is discussed.

Heterokaryotic nuclear conditions and a heterogeneous nuclear population are observed by flow cytometry in Phytophthora infestans.

A simple and reliable method for preparation of whole nuclei of a common oomycete, Phytophthora infestans, is described for laser flow cytometry. The ease of preparation, the absence of detectable debris and aggregates, and the precision in determinations of DNA content per nucleus improve interpretation and understanding of the genetics of the organism. Phytophthora infestans is the pathogen that causes potato and tomato late blight. The genetic flexibility of P. infestans and other oomycete pathogens has complicated understanding of the mechanisms of variation contributing to shifts in race structure and virulence profiles on important agricultural crops. Significant phenotypic and genotypic changes are being reported in the apparent absence of sexual recombination in the field. Laser flow cytometry with propidium iodide is useful in investigating the nuclear condition of the somatic colony of field strains of P. infestans. The majority of the studied strains contain a single population of nuclei in nonreplicated diplophase. However, mean DNA content per nucleus varies considerably among isolates confirming the heterogeneity of the nuclear population in regard to C-value, for field isolates. Nuclear DNA content varies from 1.75x to 0.75x that of nuclei in a standard strain from central Mexico. Some strains contain two to three populations of nuclei with differing DNA contents in the mycelium and are heterokaryons. Such a range in DNA content suggests DNA-aneuploidy, but direct confirmation of aneuploidy will require microscopy of chromosomes. Heterokaryosis and populations of nuclei of differing DNA content necessarily confound standardized assays used worldwide in crop breeding programs for determination of race profiles and virulence phenotypes of this important pathogen.

Tolerance of Mycelium of Different Genotypes of Phytophthora infestans to Freezing Temperatures for Extended Periods.

ABSTRACT Mycelium of Phytophthora infestans, the causal agent of potato late blight, can initiate crop infections over successive years by overwintering in infected potato tubers that survive as seed in fields or within cull piles. This study used four different genotypes of P. infestans to evaluate the influence of freezing temperatures on survival of mycelium in vitro. Sporangium-free mycelium of P. infestans US1, US8, US11, and US14 growing on rye agar plates was exposed to temperatures ranging from -20 and 0 degrees C (experiment A) for different periods up to 24 h and from -5 and 0 degrees C (experiment B) for periods up to 5 days. Cultures were incubated at 12 degrees C after exposure, and survival of the cultures was estimated after 28 days by a digital image analysis technique that measured the average reflectance intensity (ARI) of images of the mycelium of temperature-treated cultures. The ARI values of treated cultures were compared with the growth of mycelium in negative controls (mycelium not present) and positive controls (mycelium exposed to 12 degrees C for an equivalent period), and determination of recovery was based on statistical differences from the controls. There were significant differences in ARI values among genotypes, temperature treatment, and exposure periods in all experiments. An index of recovery was calculated for each genotype at all treatment temperatures and exposure periods for both experiments. In experiment A, exposure of mycelium of P. infestans (all genotypes) to -20 and -10 degrees C proved lethal for exposure periods of more than 1 h. All genotypes showed some degree of recovery up to 24-h exposure at -5 and -3 degrees C. In both experiments, exposure of mycelium of P. infestans to 0 degrees C was not lethal to any genotype tested for any exposure period. In experiment B, all of the genotypes survived exposure up to 3 days at -3 degrees C to some degree, but at -5 degrees C, exposure of 1 day was lethal to all genotypes. Tolerance of freezing temperatures by mycelium of P. infestans may be an ecologically important survival mechanism and the increased tolerance of US8 and US14 may explain their predominance in cooler climates such as north-central United States.

Screening for Late Blight Susceptibility in Potato Tubers by Digital Analysis of Cut Tuber Surfaces.

A method for quantification of late blight (Phytophthora infestans) in potato tuber tissue using a digital scanner and image analysis software is presented. The average reflective intensity of light reflected from the cut surface of sample tubers measures the darkened, diseased potato tuber tissue amid lighter, late blight-free tissue. In the absence of disease, potato variety, tuber shape, and tuber size do not influence the scan results. While digital quantification of late blight in tubers under controlled inoculation conditions is consistent, the digital assessments of late blight did not correspond exactly with those from a conventional subjective visual method. Used together, the methods can provide complementary information regarding varietal susceptibility to P. infestans development on the tuber surface and internal tuber tissue. The method of image analysis presented may be used to determine susceptibility of potato tubers to late blight in varietal development programs, storage research programs, or other tuber research programs.