Updated Analysis On Fungicides And Alfalfa

DR. PAUL VINCELLI           

   Last year, Headline® fungicide received a federal label for control of foliar diseases of alfalfa for use on seed, for hay, or for silage (with a 14-day pre-harvest interval in all cases). See additional label restrictions.
   The April 9, 2013, issue of Kentucky Pest News included an article on the overall performance of Headline® fungicide based on public field research.  The present article includes updated tables of field performance.  These tables include all the data from the previous article plus two additional sources:
   • UK field research conducted this past summer;
   • A paper1 on fungicide performance recently published by researchers in Wisconsin.
   Updated summary of public research
   The results of our analyses are shown in the table below, and key points are summarized here:
   • In almost all comparisons, applications of strobilurin fungicide reduced damage from foliar/stem diseases.
   • In spite of the reduction in disease damage, yield increase was observed in only 11 of 42 comparisons. Most of those yield increases occurred in the first or second cuttings, consistent with manufacturer reports. When they occurred, yield increases were almost always in the range of one- to three-tenths of a ton of dry matter per acre.
   • Forage quality (measured in various ways; see table) was improved in very few comparisons. It is important to note that there were also a number of instances where quality was slightly reduced in the fungicide-treated alfalfa (data not shown, available on request).
   • The physiological “greening” effect of strobilurin fungicides was not reported in alfalfa in experiments where such observations were included in the report.
   *Each strobilurin fungicide in each cutting = a discrete comparison
   **“Beneficial responses” refers to examples of statistically significant improvement in agronomic performance. Statistical discrimination among treatments at 10 percent error rate (P<0.1)

   It seems clear that application of a strobilurin fungicide very commonly reduces levels of foliar disease in alfalfa in humid regions of the USA. However, in research thus far, strobilurin fungicide was associated with increased forage yield in only about 25 percent of comparisons. Yield increases, when they occurred, almost always were in the first or second cutting. This may be because the fungus that causes spring black stem and leaf spot, which is most active in these earlier cuttings, is highly sensitive to Headline®. In any case, if considering the use of fungicide, best results will be obtained by applying during the growth phase of the forage for the first or second cutting.
   Forage quality has been minimally improved by strobilurin fungicides in research thus far. This latter finding surprises us, because leaves represent high-quality forage, and we would expect that improved leaf health would translate to improved forage quality. Continuing research is advisable, since several of the growing seasons of these tests were unusually hot and dry; results economically favorable to fungicide use could occur in wet growing seasons.
   1Samac et al, 2013. Evaluating Headline Fungicide on Alfalfa Production and Sensitivity of Pathogens to Pyraclostrobin. Online. Plant Health Progress doi:10.1094/PHP-2013-0917-01-RS.∆
DR. PAUL VINCELLI: Extension Plant Pathologist, University of Kentucky
DR. RAY SMITH: Extension Forage Agronomist, University of Kentucky

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