What Should You do about Late Southern Corn Rust?

Southern rust of corn.

We continue to receive a high frequency of phone calls regarding southern rust observations in corn fields at advanced growth stages.  Southern rust can be an explosive disease given a conducive environment.  However, in most cases the corn fields throughout MS are likely safe when it comes to a major (or likely even a minimal) yield loss as a result of southern rust based on the general age of the corn crop throughout the greater part of the MS.  As a general statement, most corn fields likely have some southern rust present at varying degrees of infection that in some cases range from a few pustules present per leaf to some large hot spots of the disease to a few fields that have been reported to be thoroughly covered.  Some fields in east MS have been observed to contain heavy southern rust infection; however, to a lesser degree, corn fields in the Delta appear to have less southern rust infection present.

Be mindful that a general disease threshold does not exist whereby a specific level (concentration) of infected plants in an area are required to trigger a fungicide application.  When weighing a fungicide application for the purposes of preventing yield loss from a foliar disease one should consider several important factors, one of which would be whether or not an economic benefit would be achieved as a result of the fungicide applied.  Economically speaking, a fungicide will not likely provide an economical benefit (cost of fungicide, adjuvant plus aerial application = minimum $20 depending on product) when an application is made as late as two weeks prior to physiological maturity (≈ R5, dent).  Since corn commodity prices have dropped (≈ $3.60 to $3.98 depending on date of delivery) the likelihood of an economic benefit resulting from a late fungicide application is decreased substantially since a greater number of bushels would be required to offset the cost of the fungicide application.

From a product selection standpoint, even though products such as propiconazole can be purchased quite cheaply, the specific products that contain propiconazole as a stand-alone fungicide product (e.g., Bumper, Tilt) are not the preferred products for southern rust management.  In general, products that contain a strobilurin as a stand-alone active ingredient or a pre-mix or tank-mix option that would contain a strobilurin and a triazole (or carboximide) would be preferable since they will act to dry the rust pustules down as well as prevent additional spread of the fungus to leaf material that has not previously been infected.

Our general thoughts regarding late growth stage (beyond R3/R4) fungicide applications:

-As corn maturity advances past the dent stage (R5), the amount of grain yield loss resulting from premature leaf death or senescence which may caused by heavy Southern rust infection quickly diminishes. Thus, it would take extraordinary circumstances to produce an economic return from a fungicide application very long after full dent stage. Corn grown in Mississippi progresses from full dent to physiological maturity in about 20 days.

-Standability or lodging which may result from heavy southern rust infection is a relatively minor and secondary consequence dependent upon premature leaf death. Lodging should not result from Southern rust infection unless substantial yield reduction occurs. Stalk integrity will only be jeopardized, if leaves die or senesce prematurely. Of course, once plants attain physiological maturity, Southern rust should have little, if any effect on stalk integrity. After all, corn leaves naturally senescence shortly following physiological maturity.

-Corn fields should be scouted for the presence of southern rust prior to applying a fungicide.  Corn fields receiving overhead irrigation should be scouted more intensively since the simulated rain splash will act to spread spores throughout the field and infect additional leaf material.

-Over the past several years I have heard field personnel suggest that a corn plant will die from southern rust infection in as little as 7 days.  In all the fields we have scouted throughout the years we have not observed this to be an issue, nor have our counterparts in other states in our region (AL, AR, GA).