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Allen, T.

The corn fungicide dilemma: when should a fungicide be applied? Part V of V, preventing yield loss from foliar disease

Over the past 4 updates I’ve attempted to present data that suggests the best time to apply a fungicide is in response to a foliar disease that could potentially reduce yield.  In many situations particular fungicide products are being touted as having the ability to reduce drought stress (topic of Part II), increase yield in the absence of disease (topic of Part I), increase green leaf material (topic of Part III), and prevent lodging by increasing corn standability (topic of Part IV).  The Mississippi research project, from 2007 to 2009 funded by the Mississippi Corn Promotion Board, determined that: i) yield was not increased in the absence of disease when a fungicide was applied at tassel (VT), ii) fungicides did not increase yield in non-irrigated situations, iii) percent chlorophyll was not increased following a fungicide application, and iv) lodging still occurred post-fungicide application following a straight-line wind event that occurred at two specific locations.  However, based on the data from the three year study we believe that few differences occurred between fungicide treated and untreated plots because little if any measureable foliar disease occurred.

During 2010 a severe outbreak of southern corn rust occurred almost statewide.  Typically, southern rust is more of an issue late in the season and generally isn’t identified until July or even August.  However, early observations of southern rust occurred in June 2010.  Southern rust prefers high temperatures and much of the 2010 summer could be characterized as warmer than normal.  To determine if yield loss could be prevented and disease severity decreased, a fungicide trial was initiated in a field of second year corn in Stoneville, MS (corn was Pioneer 31D59).  Late fungicide applications were made at approximately R5 (dent).  Typically, fungicide suggestions have stated that an economic benefit from the fungicide might not be expected if applications were made 2 weeks prior to black layer.  The dent application ended up being 34 days prior to harvest.  Fungicides were applied at the labeled rate and included a crop oil concentrate at 1.0% v/v.  Applications were made by ground equipment in 10 gallons of water per acre.  Prior to fungicide application, plots were rated for the presence of southern corn rust.  Rust was the ONLY disease rated in plots since this particular disease can be aggressive and lead to significant yield reductions.  However, potential yield reductions will depend on the growth stage timing of infection and the specific environment following infection.  Southern rust was rated on the Horsfall-Barrett scale (1 to 12, where 1 = 0% and 12 = 100%) on the date of application and two weeks post-application.  In addition, plots were rated for incidence and severity of lodging at the time of harvest.  One particular question during 2010 had to deal with whether or not corn would lodge if infected with southern corn rust.  I spent a tremendous amount of time in this particular corn field during 2010 and believe southern rust was first visible at low levels at approximately brown silk.

Southern rust infection was consistent across the field and was observed at between 6 and 12% severity within plots prior to fungicide application (see Table).  Two weeks post-application southern rust severity ranged from 25 to 87%.  In some cases there were significant reductions in the overall amount of southern rust between treated and untreated plots.  Quadris at 6 & 9 oz/A, Quilt at 10 & 14 oz/A, Stratego at 12 oz/A, the Stratego + Tilt combination and Stratego YLD at 5 oz/A all significantly reduced observable southern rust 14 days post-treatment.

Lodging within plots was rated based on the overall appearance of the plot.  However, 31D59 has good stalk strength so widespread lodging did not occur.  But, with the level of southern rust that was present late in the season the plants were judged to not be at risk to lodging.  Many people within the corn industry suggest that unless a severe foliar disease (such as gray leaf spot, southern corn rust) occurs prior to tasseling than lodging will not occur as a result of disease.  Lodging occurred infrequently in the field and in most cases was a single plant or at most 3 plants in a plot.  Plots were rated on a scale of 0-9, so if a single plant was lodged than a score of 1 was assessed since lodging was “present”.  Scores less than 1 mean lodging as a result of southern corn rust was not a problem even when treated and the untreated plots were compared.

With regards to yield, there were not only mathematical differences, but significant differences between treated and untreated plots as well as between treatments.  Keep in mind, this was a one year study in a single location so don’t assume that the best products in this situation will respond the same across numerous locations.  In addition, strobilurin chemistries should be applied in a more preventive fashion (i.e. prior to disease) rather than during an epidemic.  In this situation Headline at 6 oz/A, Quilt + Quadris, Stratego at 12 oz/A, Stratego + Tilt, and Stratego YLD at 7 oz/A all significantly increased yield compared to the untreated check.  More research is necessary to determine the most efficacious fungicides in situations where moderate to severe southern rust occurs as well as the most advantageous growth stage timings.

Based on the results from the 3 year study to determine if a “plant health” benefit resulted following an at tassel (VT) application of a strobilurin fungicide and the results from this single location our suggestion continues to be: apply a fungicide if and when a yield-limiting disease is detected in a corn field.  However, with that said, there is still a tremendous amount of research to be done to determine specific timings when fungicides would still be economically beneficial.  Moreover, pre-harvest intervals may restrict the use of specific fungicides at late, dent-type timings; specifically triazoles (see column in above table on PHI).

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