With Tar Spot confirmed early this year, there were initial fears multiple pass fungicide applications might be needed. As summer progressed, however, Tar Spot didn’t take off as quickly as we first thought it might, allowing most fungicide applications to be delayed until our typical VT/R1 growth stages.
Recent weather events, however, may have added another disease to the mix to consider: Southern Rust.
This map is a great reminder of how diseases like Southern Rust arrive each year from the south – making their arrival difficult to predict. Guess no longer: Southern Rust MAY be in a field near you and that means continued scouting – or starting scouting again – is necessary.
There are two nice reads in the most recent KSU Agronomy eUpdate if you are a) still waiting to see if Tar Spot becomes an issue and b) if you’re concerned about Southern Rust. In them, K-State Research & Extension Row Crops Pathologist Dr. Rodrigo Onofre answers many of the common questions you’ll likely have about these two diseases, including:
- where either/both of these diseases been confirmed
- when/IF a fungicide should be applied
- how to choose a fungicide product
TwoTar Spot blog posts below provide additional resources if you are interested – so I’ll add just a few things on Southern Rust to Dr. Onofre’s article:
Know What Disease You Have
There are other things that mimic Tar Spot. Insect feces are the major ‘copy cat’. For Southern Rust, Common Rust can be a confusing pathogen. A few differences in rust species (Common v. Southern) include:
Common Rust:
- Pustules appear on upper AND lower leaf surfaces
- Typically brown/brownish red, elongated, and scattered on leaf surfaces
- Prefers cooler weather (61-77°F)
- Takes heavy disease pressure to cause losses (Wisconsin estimates indicate it takes 162 common rust pustules/leaf to reach 1 percent severity).
Southern Rust
- Pustules appear primarily on the upper leaf surface
- Orange to light brown, round, and densely packed on the leaf surface
- Prefers warmer weather (77-82°F.)
- Can reach heavy pressure levels quickly requiring vigilant scouting for making potential fungicide application decisions.
Both rust fungi require only short periods of leaf wetness for infection – 6 hours of dew is usually enough – and mixed infections of both fungi can occur on the same plant.
Know What a Fungicide – Even One Previously Applied – Will Do
With any luck, a single fungicide pass will provide the protection needed against any/all fungal pathogens – but they don’t last indefinitely. If you’ve held off on a fungicide, scout now to determine if one is necessary (it may not be!). If you have already made a fungicide pass, scouting should restart 14-21 days post application as fungicide efficacy starts to wane.
How late can a fungicide work? While the R5 (dent) growth stage typically signals the end of any potential benefits of a fungicide application, it’s difficult to see much return on investment from applications even at R4 (dough stage – approximately 30 days post-silking). Applications from the late R2/R3 growth stages and beyond need to be carefully considered to evaluate the potential for a return on investment.
