Corn Disease Update and Tassel Fungicide Decisions: May 15, 2012

Tom Allen, Extension Plant Pathologist
By Tom Allen, Extension Plant Pathologist May 15, 2012 17:39

Corn Disease Update and Tassel Fungicide Decisions: May 15, 2012

Common rust of corn. Note darker, cinnamon-colored spore masses that have emerged through the ruptured pustules.

The corn crop appears to be much earlier this year than it has over the recent past.  I’ve driven by numerous fields from south Washington County to north Bolivar County where almost 100% of the field has reached the tasseling stage (VT).  To date the level of foliar disease observed in the corn crop has been low and I suspect some of that has to do with the general environmental conditions that have to less rainfall than what we encountered earlier in the season.

Common rust – At present, the only disease that I have observed has been common rust.  More often than not, and particularly at this time of the season, common rust can be present on leaves from the bottom of the plant, to the top of the plant but in general are present at low levels.  More often than not, the appearance of common rust on the lowest leaves can be confused with southern rust simply due to the general number of pustules present on the leaf and appearance on the lowest leaves.  Typically, common rust does not present a risk to yield and is likely present at low levels in most (if not all) corn fields throughout the corn belt by the end of the season.  In addition, the pustules formed by common rust are generally larger (than southern rust) and the spores that are released through the ruptured pustule are darker and more cinnamon in coloration than southern rust spores.  Also, and this may be more difficult to observe in some cases, common rust pustules appear larger and the leaf tissue looks to be more “shredded” but much like things in the English language there can be exceptions.  BUT the comparison I’m suggesting here is much easier when both diseases are present rather than attempting to make a judgment based on the presence of only common rust. 

Comparing spore shape, size, and color at this time of the year is important to determine which rust may be present.  I have kept a southern rust sample from several years ago so I can compare spores prior to making a report as to the presence of southern rust.  In some cases, a photograph is enough to determine the specific rust present at the field level but sometimes a more in depth analysis of the sample is required.

Neighboring states, specifically Alabama and Louisiana, are presently reporting low levels of northern corn leaf blight as well as southern rust in isolated areas of the state but more specifically in southern locations.  I suspect some of this has to do with the general earliness of the corn crop in AL and LA and at least in LA the more regular rain showers they have received over the past month. 

To stay current on the presence of southern rust throughout the U.S. the information can be observed at http://scr.ipmpipe.org/cgi-bin/sbr/public.cgi .  Much like the data uploaded regarding the presence of soybean rust, plant pathologists throughout the U.S. upload data and/or specific management scenarios in response to southern corn rust.

Tassel Fungicide Applications

I continue to receive a number of telephone calls regarding the tassel (VT) fungicide application in corn.  The greater body of research conducted in MS still suggests that a fungicide application (in corn) is most beneficial when the product is applied in response to the presence of a particular disease; especially foliar diseases that are considered to be yield-limiting (e.g. gray leaf spot, southern corn rust).  For more information on the topic please consider reading the 5 part series on fungicide use in corn as it appeared on the blog during in 2011.

The corn fungicide dilemma: when should a fungicide be applied? Part I of V, yield in the absence of measureable disease

The corn fungicide dilemma: when should a fungicide be applied? Part II of V, irrigated versus non-irrigated situations

The corn fungicide dilemma: when should a fungicide be applied? Part III of V, green leaf material as measured by percent chlorophyll

The corn fungicide dilemma: when should a fungicide be applied? Part IV of V, standability/lodging

The corn fungicide dilemma: when should a fungicide be applied? Part V of V, preventing yield loss from foliar disease

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Tom Allen, Extension Plant Pathologist
By Tom Allen, Extension Plant Pathologist May 15, 2012 17:39
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