Southern Corn Rust has been confirmed in Wayne and Lenoir counties by NCSU faculty. Southern Corn Rust can be devastating to corn yield. Prevention of infection by foliar fungicide application is currently the best management option. However, there appears to be confusion regarding evaluation and treatment.
The first point of confusion appears to be susceptibility of plants to potential damage. All corn is susceptible to damage but corn that has reached late dough to dent stage is likely to suffer much less damage than corn less mature than these stages. Thus, management should target scouting fields to determine the stage of corn, not to simply look for the disease. (Scouting for the disease does have merit in determination of fungicide choices. Read further for more details). Like most rust, by the time the disease is discovered, much yield loss has already occurred. Thus, primary efforts should include identification of fields most at risk.
Scout fields based upon planting date and maturity. Plants with corn at milk stage or earlier are most susceptible. Also, as a general rule, plants reach dough stage about 25-35 days after the silking stage. If recollection of approximate silking is known, this too can serve as a reference for targeting most susceptible corn fields. A quick visual reference for corn growth stages can be found athttp://extension.entm.purdue.edu/fieldcropsipm/corn-stages.php. A more detailed presentation of growth stages with images can be found athttp://www.ipm.iastate.edu/ipm/files/03%20Corn%20Growth%20and%20Development.pdf.
Another primary point to consider is that one must simply make a decision to treat with a fungicide very soon or not to treat. Either decision presents risks. Treating will likely require application by airplane with a minimum of 5.0 GPA. This will add additional cost to production but will provide protection against infection. Avoiding treatment places emphasis on lack of potential yield loss based upon the assumption that this disease will not spread. Producers need to weigh potential yield, price of corn, production cost and other factors against pesticide application cost to make a decision that fits personal risk management. Having thus said, assuming a $12/acre aerial application cost + $12 per acre fungicide cost, the breakeven cost at $4/bu of corn means only saving 6 bushels per acre. Assume a yield of 100 bu/ac and a minimum yield loss of 10% and the cost of protective fungicide treatment is a much cheaper option. Assume the same yield with 30% yield loss from Southern Corn Rust and the favorable decision to treat becomes very obvious!
Lastly, consider fungicide choices. Three types of products are commonly available. Those with active ingredients known as strobilurins act as a preventative treatment. Thus, they are normally less expensive. However, they must be applied before any known infection exists. Triazoles products act as a curative type fungicide. Thus, they can be applied when the disease is discovered and normally provide good control. However, they are much more expensive. The third option are products that contain both strobilurin and triazole active ingredients. Products with this combination may provide both curative and preventative abilities. However, the percentage of each active ingredient within a product will determine the length of time for control. Too, these combination products tend to be more expensive. Registered products for corn classified by type is provided below.
Strobilurin Products – Quadris, Headline and Evito
Triazole Products – Tilt, Folicur and Domark
Combination Products – Quilt, Statego, StrategoYld, QuiltXcel and Fortix
A good strategy for our current situation within Craven County would be to apply a strobilurin type fungicide to corn at R1 (50% of corn ears with brown silks) to R3 (milk stage). This is a good strategy because no confirmation of the disease has been made within the county yet this type of treatment will provide 10-14 days of protection allowing corn to approach dent stage by the time the fungicide treatment begins to be less effective. Remember, loss of leaf area at dough or dent stage has much less impact on yield so further fungicides are not likely to be warranted.
Source: weathercentral