Over the past few seasons numerous farmers have made fungicide applications during vegetative corn growth stages (typically V4-V7) in hopes of preventing such maladies as greensnap. Greensnap occurs more readily in some specific hybrids that have what can be termed “brittle stalks”, but can occur in almost all corn hybrids if a major wind event occurs prior to ear formation (pre-VT) following a period of rapid growth in the corn plant, typically between V10 and V12 (10 to 12 leaf stage). More specifically, greensnap is the term given to corn stalks that are completely broken off below where the developing ear emerges. Root lodging can also occur as a result of high winds; however, the two issues are completely different from one another and will produce observably different symptoms in the field (for additional information see: http://www.mississippi-crops.com/2012/05/27/corn-hail-damage-and-other-storm-issues/).
During 2012 the Mississippi Corn Promotion Board funded a project to determine the results of applying fungicides at vegetative growth stages and comparing the effect on yield at several other, important growth stage timing strategies (e.g., V4-V7; VT/R1; V4-V7 fb VT/R1; R1). One of the opportunities that arose during the 2012 season was a major straight line wind event (June 11, 2012) that resulted in severe greensnap at one of the trial locations, west of Greenwood, MS. Seven different fungicides were applied at the particular location at the V6 timing (3 strobilurin products (Evito (4 oz), Headline (6 oz), Quadris (6 oz)), 3 pre-mix (strobilurin + triazole) products (Headline AMP (10 oz), Quilt Xcel (10.5 oz), Stratego YLD (4 oz)), and a 1 triazole product (Tilt (4 oz))) representing the major fungicide products used in corn production systems throughout the Mid-southern U.S. Fungicides were applied both with and without glyphosate (22 fl oz/A of Roundup PowerMax) to serve as the adjuvant at the vegetative timing since the specific formulation is a “loaded” herbicide. All applications were made at a general application rate of 15 gallons per acre. The specific hybrid treated was Pioneer 1187, a hybrid that has been reported to have a brittle stalk.
Within each plot (59 total treatments × 4 replications = 236 plots) all of the corn stalks were counted in the middle two rows of each four row plot. However, many of the plots had not received a fungicide (VT/R1 timing), and the figure included below reports data from those plots that received the V6 application (n=60 for fungicide alone; n=64 for fungicide + glyphosate). To arrive at a percent greensnap per plot: the total number of stalks in each single row were counted separately and averaged to derive an average number of stalks per plot. Corn stalks exhibiting greensnap symptoms were counted separately on a per row basis to derive an average. The average number of stalks exhibiting greensnap was divided by the average number of total stalks and multiplied by 100% to arrive at a percent greensnap per plot.
Throughout the entire test area (all 236 plots), greensnap ranged from 0% per plot, to greater than 40% per plot (or approximately 28 plants/plot exhibiting symptoms of greensnap) in the plots that received fungicide at V6. Overall, the average greensnap in the entire test area was 14%. Plots were 30 feet in length and generally speaking, on average, had 67 total stalks per row. Based on the trial area observed, fungicides, regardless of active ingredient (strobilurin; strobilurin + triazole; triazole), did not consistently reduce percent greensnap compared to the control (Figure 1 Percent greensnap figure). In fact, the minor differences that did occur between the treated and untreated plots would relate to approximately 2 plants per plot (based on the calculation (2 stalks/67 total stalks per row)*100%)).
The most consistent response following a fungicide application can be expected when disease occurs. At the location west of Greenwood, MS, fungicide application at V6 did not consistently reduce greensnap compared to the untreated control. As has been stated numerous times by university extension and research faculty from throughout the U.S., “plant health” responses following a fungicide application have produced sporadic results at best. Foliar fungicide applications should be used to prevent yield losses as a result of foliar disease, not to provide physiological plant responses.