The putative center of origin of Plasmopara viticola, the causal agent of grape downy mildew, is eastern North America, where it has been described on several members of the family Vitaceae (e.g., Vitis spp., Parthenocissus spp., and Ampelopsis spp.). We have completed the first large-scale sampling of P. viticola isolates across a range of wild and cultivated host species distributed throughout the above region. Sequencing results of four partial genes indicated the presence of a new P. viticola species on Vitis vulpina in Virginia, adding to the four cryptic species of P. viticola recently recorded. The phylogenetic analysis also indicated that the P. viticola species found on Parthenocissus quinquefolia in North America is identical to Plasmopara muralis in Europe. The geographic distribution and host range of five pathogen species was determined through analysis of the internal transcribed spacer polymorphism of 896 isolates of P. viticola. Among three P. viticola species found on cultivated grape, one was restricted to Vitis interspecific hybrids within the northern part of eastern North America. A second species was recovered from V. vinifera and V. labrusca, and was distributed across most of the sampled region. A third species, although less abundant, was distributed across a larger geographical range, including the southern part of eastern North America. P. viticola clade aestivalis predominated (83% of isolates) in vineyards of the European winegrape V. vinifera within the sampled area, indicating that a single pathogen species may represent the primary threat to the European host species within eastern North America.
Of 20 Plasmopara viticola isolates collected in four locations in Virginia and northwest North Carolina in 2005, 16 were resistant to QoI fungicides. The resistance factor was over 100, and label rates of formulated azoxystrobin and pyraclostrobin provided little or no control of these isolates. Additional sampling in 2006 revealed at least 15 additional vineyards with QoI-resistant P. viticola in Virginia, Maryland, and Pennsylvania. Of 22 isolates of Erysiphe (Uncinula) necator collected in 2005 from five Virginia locations, 20 isolates from 4 locations showed resistance to QoI fungicides. The G143A mutation for resistance was detected in several isolates of both pathogens. This is the first detection of this type of resistance in P. viticola in North America, and the second North American report of QoI resistance in E. necator. Accepted for publication 26 November 2007. Published 11 February 2008.
Demethylation inhibitors (DMIs) have been an important tool in the management of grapevine powdery mildew caused by Erysiphe necator. Long-term, intensive use of DMIs has resulted in reduced sensitivity in field populations. To further characterize DMI resistance and understand resistance mechanisms in this pathogen, we investigated the cyp51 sequence of 24 single-spored isolates from Virginia and surrounding states and analyzed gene expression in isolates representing a wide range of sensitivity. Two cyp51 alleles were found with respect to the 136th codon of the predicted EnCYP51 sequence: the wild-type (TAT) and the mutant (TTT), which results in the known Y136F amino acid change. Some isolates possessed both alleles, demonstrating gene duplication or increased gene copy number and possibly a requirement for at least one mutant copy of CYP51 for resistance. Cyp51 was over-expressed 1.4- to 19-fold in Y136F-mutant isolates. However, the Y136F mutation was absent in one isolate with moderate to high resistance factor. Two additional synonymous mutations were detected as well, one of which, A1119C was present only in isolates with high cyp51 expression. Overall, our results indicate that at least two mechanisms, cyp51 over-expression and the known target-site mutation in CYP51, contribute to resistance in E. necator, and may be working in conjunction with each other.
Boxwood blight is an emerging disease of great concern for the ornamental horticulture industry, historic garden managers, landscapers, and homeowners. Controlled-environment experiments were conducted to determine the effects of conidial concentration, temperature, interrupted leaf wetness period, cultivar, and leaf age on infection of boxwood leaves by Calonectria pseudonaviculata. Boxwood blight incidence (BBI) increased with increasing concentration up to 2.0 × 104 spores/ml. BBI also increased as temperature increased from 18 to 25°C, then declined gradually to zero at 29°C. Similar infection effects of inoculum concentration were observed in an experiment with four boxwood cultivars (‘Justin Brouwers’, ‘John Baldwin’, ‘Green Mound’, and ‘Nana’) of various degrees of susceptibility. The hypothesis that younger leaves are more susceptible than older leaves was supported for Justin Brouwers and Nana but not for Green Mound; and younger leaves of John Baldwin were less susceptible than older leaves. When inoculated plants (‘Suffruticosa’) were exposed to dry interruptions of 3 h or longer between 5 or 8 h of initial wetness and 12 h of additional wetness, these plants had significantly lower BBI compared with those exposed to continuous wetness for 20 h, and similar or at most slightly more infection than plants exposed to only the 5- or 8-h initial wetness. Continuous wetness durations beyond 20 h did not increase infection in these experiments. These results advanced our understanding of the environmental requirements of the infection process in boxwood blight development and they are essential for refining disease forecasting models.
Colcol, J. F., Rallos, L. E., and Baudoin, A. B. 2012. Sensitivity of Erysiphe neeator to demethylation inhibitor fungicides in Virginia. Plant Dis. 96:111-116.Grape powdery mildew {Erysiphe necator) isolates were collected from 2(K)5 to 2007 from vineyards mostly in Virginia but also some in Maryland, North Carolina, and Pennsylvania. Using a leaf disc assay, the isolates were tested against five demethylation inhibitor (DMI) fungicides. Most isolates exhibited reduced sensitivity to the live DMIs when compared with a sensitive group (H = 12) and compared with unexposed populations reported from other areas. The median resistance factor (RF) was highest for tebucona/ole (RF = 399) and myclobutanil (RF = 378), followed by tritlumizole (RF = 70), triadimefon (RF = 62), and fenarimol (RF = 44). Tbe sensitive group used as the basis for comparison appears to have been more sensitive than unexposed isolates in New York and California. Our finding that the greatest resistance shift occurred with tebucona/ole and myclobutanil contrasts with earlier reports from New York and California, whore the greatest resistance shift was observed with triadinielbn or triadimenol. Sensitivities to all five DMI fungicides were strongly correlated (pairwise r values of 0.70 to 0.87) but our data suggest that some may retain greater utility than others.
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