By: Dave Pike
Southern Rust on corn is regularly listed as one of the top ten worst fungal diseases in corn. Although southern rust commonly causes yield reductions of 5 to 25% along the gulf coast states, it seldom makes front page news across the corn belt. There are two reasons for this: One, the fungal spores are not capable of surviving the colder winters in the midwest, and two, the spores find it difficult to remain viable while making the arduous journey from southern areas.
Although southern rust is a serious disease, the fact that it tends to be more regional in its impact means that it has not been researched as thoroughly as some other crop diseases. However, it does share some similarities in its transport and infection processes with other fungal diseases, which allows us to make some inferences about where it comes from and how it survived the trip to midwestern fields in 2016.
The scientific name for southern rust is Puccinia polysora. Like other fungal spores that aren't able to overwinter in the midwest, it must be transported via winds and storm systems from areas where it is able to overwinter, such as southern Texas, Mexico, and, at times, other gulf coast states. Common rust on corn is also transported in a similar manner.
So, why was 2016 a bad year for southern rust?
First, conditions have to be good for transporting a sufficient number of viable spores. That means that in southern states, a substantial reservoir of spores (inocula) must have developed on infected corn crops. The weather and winds over those crops also have to be of sufficient force to pull the spores up out of the canopy and into the air.
The spores then must be transported many miles into their eventual destination in an expedited manner, lest UV light or temperature conditions in the atmosphere degrade the spores and render them unviable. Optimal conditions for transport usually include warm and cloudy weather that protects the spores from the UV light.
Finally, the spores must be deposited on green corn leaf tissue. This can happen in a dry or wet deposition; wet deposition is usually associated with rain from the same clouds that protected the spores in transit. So, the reality is that a storm system that had sufficient turbulence to pick up the spores from the south was probably the same system that deposited the spores on fields in the midwest.
Late planted corn was affected much more seriously than early planted corn, because it had more juvenile leaves at the time southern rust spores were experiencing optimal temperatures for germination (76 to 82F). The early juveniles of corn (v1 through v5 or v6) of all hybrids, regardless of hybrid 'resistance' to rust, are more susceptible to infection than are the later emerging adult leaves (v8+). This is due to the differences in cuticle thickness, leaf hairs, leaf waxes, and cell walls. So, late planted corn is more susceptible than early planted corn because it has more juvenile leaves in the whorl, where moisture is highest, when optimal temperatures for spore germination are occurring. This makes corn more susceptible to initial infections. These initial infections then develop the large number of inocula necessary for the secondary (and more yield robbing) infections which continue to take place in the warm and humid conditions of late summer.
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