Corn Disease Update and Late Fungicide Suggestions: July 19, 2013

As the corn season winds down I am still receiving numerous phone calls regarding disease incidence as well as when to stop scouting and/or getting concerned about specific diseases.  Generally speaking, applying a fungicide at dent (essentially two weeks prior to black layer) is not economically feasible unless a heavy infection of southern rust has already been detected in the field.  At present, even though southern rust has been detected in the state, the levels of disease in fields where the disease has been detected are extremely low and generally on the order of one to three leaves infected with the fungus.

Northern corn leaf blight (NCLB)

I have observed more NCLB this season than any other since arriving in MS.  I expect that is the result of a cooler than normal spring and more continuous corn in some areas.  Determining whether or not a fungicide is necessary to prevent yield loss as a result of NCLB should be based on several important factors.  First of all, determine what particular hybrid is present in a field.  In our corn production system there are several excellent hybrids that will tend to keep NCLB low in the canopy and rarely will you observe lesions on the ear leaf or above.  Applying a fungicide to one of these “defensive hybrids” will likely not elicit an economical response in the presence of the disease.  However, hybrids with a poor disease package or a poor rating for NCLB should be monitored more closely, especially in continuous corn situations.  Based on the amount of NCLB in my fungicide trials in Stoneville this year we should get some good information regarding how fungicides protect yield in the presence of moderate NCLB.

Southern corn rust

Southern corn rust

Much has been made over the years about the yield loss potential of southern corn rust.  Generally speaking, this particular disease is a late entry in our production system.  When I say late entry I mean it typically isn’t observed until corn reaches dent.  Once corn reaches dent and minimal levels of southern rust are detected it will require several weeks for the disease to move through an entire corn field.  I typically consider dent to be a growth stage whereby our corn farmers are out of the woods from disease, especially if nothing is observed prior to that developmental stage.  At the sites where southern rust has been observed, most of the corn is at full dent, and only single leaves have been observed in a given field.  I hardly consider that to be a level of disease that needs addressing.  Therefore I am not suggesting that fungicides be applied on a large scale.  But, with that said, every field should be considered a different situation and there are cases where a field in a particular county has been sprayed for a given disease.  The youngest corn remaining on farms should be scouted intensively for the presence of southern corn rust.  As of July 4 I observed a few corn fields in the Clarksdale area that had not reached dent due to late planting.

Sun scald and leaf scorch can be easily confused with other diseases.

Sun scald/leaf scorch

Scouting fields this time of year can be made extra difficult by sun scald symptoms.  Normally, sun scald can be misdiagnosed as: grey leaf spot, northern corn leaf blight, or even Goss’s wilt if you are looking through one of the many pictorial guides of disease symptoms.  Keep in mind, the likelihood of Goss’s wilt being a problem in a 100 acre corn field, across the whole field is extremely unlikely.  Dryland corn can be more prone to experiencing sun scald since the plants are extra stressed due to a lack of moisture.  However, pivot irrigated as well as furrow irrigated fields can also experience sun scald when several days of hot, sunny weather occur between irrigation events.  In general, sun scald symptoms will develop a pattern on a leaf and in cases where sun scald has been present for an extended period of time the lesions that develop can be invaded by secondary pathogenic fungi and produce a dark fungal growth.

Late fungicide applications

Determining whether or not a fungicide is necessary to prevent yield loss from a given disease should be based on scouting and a field-by-field basis.  I am not a proponent of “plant health” applications in the absence of disease nor have I observed some of the stated benefits (e.g., increased green leaf tissue at the end of the season, increased stalk integrity, prevention of drought stress, increased yield benefit) in any of the plots that I’ve conducted in the past six years; at least not with any consistency.

Fungicide applications at late growth stages – beginning at dent and beyond – should be weighed carefully.  Unless a large amount of disease is present the application will typically not pay for itself.  In addition, in some parts of the state the corn crop has been an expensive one to raise so added inputs at this point in time should be based on:

-disease present and amount of disease present (if you are struggling to find lesions of a disease in a field, walking large distances between leaves that are infected then a fungicide is likely unnecessary at this point)

-whether or not the corn is irrigated and has a high yield potential

-specific growth stage, bearing in mind that there are still some “young” fields of corn in the state that likely tasseled recently, scout these fields intensively for the presence of diseases such as southern rust since early infection (prior to tassel) can in some rare cases result in lodging due to cannibalization by the ear

-keep in mind that corn does not “require” a fungicide to properly grow and that fungicides are best used to prevent yield losses when high levels of foliar disease are present in a susceptible corn hybrid