Southern Rust: So Far So Good, But What IF?

Dr. Kemerait, UGA Plant Pathologist, as of last week, tells us that southern corn rust has been found in Decatur Co., Jeff Davis Co., Bacon Co., and Wayne Co.  That is good news (in a weird way).  I have talked to many growers, consultants, seed reps, and agents and, so far, no one else is reporting having seen any other southern rust. However, it doesn’t mean we need to stop looking.  The more I thought about how widely varied our crop is as it relates to growth stage, the more I felt it was important to discuss a “What if?” scenario.  What do I mean?  First, let’s consider the variation in growth stages. Some of our corn fields are quite young (V7-V8), practically all in north Georgia and some entering R5 (dent), all in South Georgia.  So, we have corn in many stages of growth which means that southern rust can still significantly hurt our crop (especially before VT/R1) in many areas of the state if the weather starts favoring its infection. What if southern rust suddenly spikes in South Georgia and begins to move northward?  Are we still in danger?  For a lot of growers, the answer is yes.

Depending on the environment, time of year and spore load, rust infections generally occur throughout the grain development phases of corn and certainly undermine stalk integrity, cob integrity, kernel development and test weight.  So, it is important then to recognize the reproductive stage of the ear/kernel when infections occur to determine the potential threat to yield and the best ROI (return on investment) when considering control measures.  The following table is the approximate days to each of the developmental stages of a kernel from the time of first silking.

Kernel Dev. Days from R1 R1 begins the first signs of silk
R1 to R2 10-12 Kernels begin to look like blisters, ivory color with clear liquids inside.
R1 to R3 18-20 Kernels are milky and pop easily under slight pressure.
R1 to R4 24-26 Kernel interior is similar to dough, more pressure is needed to “pop”.
R1 to R5 31-33 Kernels are beginning to dent. Can see “milk lines”.
R1 to R6 64-66 Kernels are physiologically mature.  At maximum dry matter.

Let’s say you just entered the R3 phase where kernels are milky and you can pop the kernels & no dough is present.  You have 6 to 8 days before reaching R4 and 13 to 15 days to R5. Should I spray if I find southern rust? Given that in general, we have an average to above average crop, the answer is yes.  The loss of leaf tissue would significantly reduce carbohydrate production and undermine yield potential and increase harvest losses.  Let’s look at it from a different perspective.  Your corn is nearing the dough stage (R4) and you sprayed a multi-mode fungicide at the beginning of R1 (silking) and you have found some rust.  The efficacy of these fungicides generally last 21 days.  R4 should begin about 24 to 25 days after silking which is now outside the expected efficacy window of your fungicide. If you routinely scout, then watch closely over the next couple of days and if infections increase, then I would certainly spray again.  That application should protect your crop well into the dent stage. If you haven’t been scouting and choose not to scout, I would recommend an application then.

Today, we have a solid line-up of fungicide choices with single to multi-modes of actions.  Fungicides with multi-modes of actions provide slightly longer efficacy but are also more expensive.  Plus, they have a broader spectrum against other diseases such as NCLB, SCLB, etc.  The earlier the infections the more prone I am to using the broader spectrum fungicides as they provide the longest efficacy against multiple diseases.  If you are scouting and find southern rust and hear that southern rust has become wide spread and you are at late R4 and early R5, I would consider a single mode of action fungicide such as a tebuconazole because you have at least 30 days to black layer.  It would be the least expensive fungicide, provide protection for roughly 14 days and get you closer to maximum dry weight.

Hopefully, none of this will happen and southern rust will continue to move very slowly or ultimately cease any infections.  The prudent action is to continue to thoroughly scout ALL fields for its presence and be prepared to protect your yield potential.   The 2020 corn crop is looking good in most places and with good rainfall, we can expect good yields if we are diligent to protect it.

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