Corn Disease Update: July 11, 2015

Southern corn rust.

Not a tremendous amount has changed since last week.  The diseases encountered throughout Mississippi continue to be the same.  However, southern rust continues to be one of the more talked about issues.  Even though more southern rust was encountered over the past week this does not mean that we have a lot of southern rust in any one location.

Northern corn leaf blight

In general, as a corn plant continues to mature, increased levels of NCLB will be observed throughout a field.  In most cases, a low number of lesions will be encountered in any single corn field.  Fields in continuous corn production may generally contain more NCLB.  But, there have been cases where first year corn fields planted to a susceptible hybrid have contained a good bit of NCLB.  In addition, as daytime temperatures increase from the mid-80s to the mid-90s, NCLB will typically struggle to reproduce and create more lesions.  Even though temperatures generally fall into the 70s at night, the number of hours where the fungus can grow and reproduce are much less than during the day.  Therefore, since we have reached the dent growth stages in most of our corn, NCLB will likely not result in a yield loss.  Moreover, for a fungicide application to be economically beneficial at this point in time a $20+ application will have to save 4-5+ bushels of corn at the current corn prices.

Lower canopy diseases

Over the past week I have had several conversations regarding southern corn leaf blight and gray leaf spot in the lower canopy.  Corn disease occurring on leaves below the ear leaf will not result in a yield loss.  With regards to southern corn leaf blight, the majority of the hybrids that are commercially available likely contain some level of tolerance to southern corn leaf blight.  In addition, some hybrids do an excellent job of keeping a disease such as gray leaf spot lower in the canopy.  Based on the current temperature the two diseases are likely happier deep in the corn canopy since most fungi cannot reproduce during the day time temperatures.  To help manage the diseases in the future plan on rotation or choosing a hybrid with known resistance to gray leaf spot if you have to economically continue to produce corn on the same acres.

Current (7/11/2015) distribution of southern corn rust in MS and beyond.

Southern corn rust

Even though southern rust has been observed in several counties (19 as of July 11, 2015) and numerous corn fields within each County, the level of southern rust in most fields is extremely low.  On Wednesday I observed southern rust in Grenada, Leflore and Tallahatchie counties.  In Grenada and Tallahatchie counties I was able to find less than 10 pustules on a single leaf in a single field in each County.  I spent almost an hour looking in Tallahatchie County and left the field after finding two open pustules and four pustules that were not yet open.  In all three counties the corn was beyond dent with the starch line beginning to move.  Corn that has reached dent with an observable starch line will likely outrun damage as a result of southern rust.

All commercially available corn hybrids are susceptible to southern corn rust.  At present, no genetic resistance exists within the hybrids planted in MS.  However, there may be slight levels of tolerance present in a few hybrids.  But, when I say “slight levels of tolerance” if you considered this on a scale of 0 to 9 whereby 0=no disease and 9=a lot of disease, the hybrids available would range between 7 and 9.  From a breeder’s standpoint that would be a difference between moderately susceptible and susceptible.  Several calls over the last week have all been regarding southern rust in the same hybrid.  The hybrid in question is planted on a substantial number of acres and does not mean that this particular hybrid is more susceptible than another.

Be mindful that over the past week several people have received text messages containing exaggerated information.  On Wednesday I read an internet post that suggested “each rust pustule contains billions of spores”.  While a southern rust pustule can contain a lot of spores a billion is likely an extreme exaggeration.  Southern rust pustules are small and likely contain on the order of thousands of spores per pustule rather than billions.  In addition, the same post suggested on the order of 45% yield loss.  I do agree that southern rust can be a damaging disease.  However, for the MS corn crop to realize a yield loss on the order of 45% the disease would have needed to have been here in early June instead of July.  I honestly do not think there is enough time in the corn season for corn farmers to realize a large yield loss (if any yield loss occurs at all) due to southern rust.  Most corn is approximately 14-21 days from black layer.  Southern corn rust will generally require 7-14 days from sporulation to infection to pustule formation to a new flush of spores.  Even though the corn crop will need to stand in the field beyond black layer I have not witnessed standability issues in the past associated with southern rust.  In 2010 I witnessed a heavy southern rust infection in Stoneville.  The corn survived in the field until moisture had dropped to 15% even though the level of infection increased in the corn from the time the disease was observed at dent+ (July 30) to harvest (September 2), or 34 days.  Lodging was one of the variables rated in that particular trial and was not observed to be greater than 1% in any of the plots.

Fungicides will potentially slow southern corn rust down.  However, fungicides will not stop the disease and in the situations I have observed over the past 8 years a fungicide application will observationally look like it did nothing on southern rust.  If you choose to make a fungicide application to prevent yield loss be mindful that 14-21 days following application there will be substantially more southern rust regardless of product selection.  The biology of this organism is extremely different than a rust disease such as soybean rust.  Most of the pustules that are observed following a fungicide application are there due to the infection process and biology of the organism in the corn leaf.  Corn at the dent growth stage, or approximately 2 weeks from physiological maturity (black layer) will likely not benefit economically from a fungicide application.  Scout the youngest fields of corn and assess the need for a fungicide application based on: presence of southern corn rust, yield potential, and growth stage of the corn.  Scout pivot irrigated fields more carefully since the overhead irrigation can simulate rainfall and increase the movement of spores within the field to non-infected plant material.  If you need help with fungicide product selection please feel free to call.