Frequent rains, mild temperatures, and high humidity across Indiana have increased the risk for foliar disease development in corn. So far, disease observations have been limited, but with more rain and mild temperatures forecasted over the next week, more disease may begin to appear in Indiana corn. The two diseases that I have seen most frequently in Indiana this week are Holcus leaf spot and northern corn leaf blight.

Holcus leaf spot is very common in Indiana fields this year. I have received many emails and calls asking if these are early symptoms of northern corn leaf blight or gray leaf spot, and it is important not to confuse Holcus leaf spot with other foliar diseases. Holcus leaf spot is a disease caused by the bacterium Pseudomonas syringae pv. syringae, and is characterized by round, discrete lesions that are initially pale yellow to white and then enlarge and turn gray or brown. Lesions have a water-soaked halo and on certain hybrids, the margin of the lesion may appear brown or purple (Figure 1). Holcus leaf spot is seen sporadically in Indiana, and is not known to limit yield. Although the disease may cause concern based on symptom appearance, no in-season treatment is available or necessary.  Fungicide applications will not have efficacy against this bacterial disease.

Figure 1. Holcus leaf spot in corn.

Northern corn leaf blight (NCLB) is the more concerning disease present in corn.  NCLB is caused by the fungus Exserohilum turcicum, and is characterized by long, tan, cigar-shaped lesions on leaves (Figure 2). This disease caused yield loss in some areas in 2014, and it has already been observed in the lower canopy of a few Indiana fields this year. Currently we do not have thresholds available for determining when fungicide applications are needed for NCLB, but we follow similar guidelines as gray leaf spot, in that fungicides will typically be more beneficial when applied to susceptible hybrids in high-risk fields for disease development, such as those that are in minimum or no-till. Scouting fields around V14, or just prior to tassel emergence, can help determine the level of disease pressure in a field. When scouting, if 50 percent of the plants in a field have disease lesions present AND additional factors or conditions that favor disease development are in play (residue present, favorable weather conditions) a fungicide application may be warranted. Research in Indiana indicates that fungicides are most effective at preventing yield loss due to NCLB when applied at the tasseling to early silking (VT-R1) growth stage. For fungicide recommendations, please see the updated fungicide efficacy table for management of corn diseases, which is developed by the national Corn Disease Working Group: http://www.extension.purdue.edu/extmedia/BP/BP-160-W.pdf. For more information on northern corn leaf blight, please read Purdue Extension bulletin BP-084-W: http://www.extension.purdue.edu/extmedia/BP/BP-84-W.pdf.

Figure 2. Northern corn leaf blight lesions are long, tan, and cigar-shaped.

Before deciding on any in-season disease management strategy this year, keep this in mind: The condition of corn in Indiana varies widely, and farmers should think carefully before making decisions to add inputs into their crop. Consider expected crop price, crop condition, planting date, predicted weather conditions and what impact these factors may have on your economic return this year.

It is important to remember that a V4-V6 application of fungicide to corn will not protect the ear leaf or above from disease that develops around tasseling. Farmers may be concerned about foliar diseases like anthracnose leaf blight (caused by the fungus Colletotrichum graminicola), which can be present in young corn. Typically, symptoms of this disease are confined to lower leaves throughout the growing season, and do not typically require management.

Farmers considering applying foliar fungicides at early growth stages of corn to improve the health of stressed or damaged corn should consider that past research indicates that the most consistent yield advantage from a fungicide application occurs when fungicides are applied in response to a high risk of disease development at VT-R1.

For more information on the current state of NCLB this year, please read the articles by my plant pathology colleagues in surrounding states:

Paul Vincelli, University of Kentucky: https://kentuckypestnews.wordpress.com/2015/06/23/watch-for-northern-leaf-blight/

Alison Robertson, Iowa State University: http://www.extension.iastate.edu/CropNews/2015/0616Robertson.html

Continual saturated soils this spring have put soybean stands at risk to seedling blights. Symptoms of blights include wilting or dead seedlings, missing plants, and delayed emergence. There are four main types of seedling blights, they are Fusarium, Rhizoctonia, Phytophthora, and Pythium. They each are favored by unique environmental conditions, and this season has had them all. Unfortunately, it is difficult to diagnose specific blights in a field. Positive confirmation of one, or more, of these diseases is necessary for future management considerations with fungicidal seed treatments. The publication, Fungicide Efficacy for Control of Soybean Seedling Diseases (BP-163-W) will assist in selecting affective fungicides for the blight(s) in fields.

Heavy and numerous rainstorms have kept many Indiana farmers out of the field the last two weeks and likely delayed postemergence herbicide applications.  As some fields begin to dry out enough to allow sprayers to re-enter fields, producers may need to consider a couple of factors before spraying.

The delay has likely allowed weeds to grow well beyond their recommended postemergence application height of 4 to 6 inches.  Producers may have to consider adjusting their planned application to higher labeled rates to control those larger weeds.  In some situations producers may need to consider an alternate herbicide option to overcome the additional weed size caused by the wet conditions. 

During this multi-week delay not only have many of the weeds advanced past their optimal growth stage, but crops will also be approaching or exceeding growth stages in which herbicide applications may be restricted.  As crops approach their reproductive stages the chances of herbicide injury and yield loss is increased.  Two tables from the Ohio, Indiana, and Illinois Weed Control Guide are included in this article that outline the corn and soybean growth stage restrictions for many of the commonly used herbicides.

Producers also need to consider the replant intervals of postemergence herbicides to avoid herbicide residue carryover into next season as the month of July approaches.  For example the fomesafen containing products that are often used on tough to control amaranths and ragweeds in have a 10-month replant interval for corn.  Applications made in July versus June will have a higher probably of causing carryover problems for next years corn crop.

While it has already been a difficult season for many farmers, they will need to keep these considerations in mind as they return to the fields to make postemergence applications in order to further protect their current crop yield as well reduce the risk of future crop injury.

Table 8. Rainfast Intervals, Spray Additives, and Crop Size for Postemergence Corn Herbicides

This table shows the required time interval between herbicide application and rainfall and summarizes label recommendations for spray additives and maximum crop stage. Check herbicide labels for additive rates. Information in this table applies to field corn only.

Table 18. Harvest and Feeding Intervals for Soybean Herbicides

Due to extreme weather conditions the Palmer Amaranth Day is CANCELLED and will not be held on Tuesday, June 30th, at our Palmer Amaranth research site near Rensselaer, IN.

Please plan to attend the Purdue Weed Day, scheduled for Thursday, July 2nd at the Throckmorton Purdue Agricultural Center, 8343 US 231 South, Lafayette, IN 47909-9049. Registration will begin at 8:00 AM EDT, and the program will begin at 8:30. We will view the plots on the west side of highway 231 in the morning, and a waterhemp site about 1 mile east of the farm in the afternoon. The farm is located approximately 5 miles south of Lafayette on the corner of county road 800S and U.S. 231 South. For those attending the 2015 Purdue Weed Day at Throckmorton, we have applied for 3 CCH’s for category 1A.

Wk 1 = 4/2/15 - 4/8/15; Wk 2 = 4/9/15 - 4/15/15; Wk 3 = 4/16/15 - 4/22/15; Wk 4 = 4/23/15-4/29/15;
Wk 5 = 4/30/15-5/6/15; Wk 6 = 5/7/15-5/13/15; Wk 7 = 5/14/15-5/20/15; Wk 8 = 5/21/15-5/27/15;
Wk 9 = 5/28/15-6/3/15; Wk 10 = 6/4/15-6/10/15; Wk 11 = 6/11/15-6/17/15; Wk 12 = 6/18/15-6/24/15