Corn Disease Update: July 4, 2015, Southern Rust Observed in Five Counties (UPDATED @ noon)

Diplodia ear rot.

Diplodia ear rot

Over the past two season I have received numerous calls regarding ear leaves that appear extremely water-soaked or brown and “dead”.  The ear itself will also develop a slightly water-soaked appearance or the entire shuck cover will turn brown.  Upon removal of the ear from the plant and the shuck the ear itself will be covered with a white fungus.  Even though fungicides are labeled for Diplodia ear rot, making a fungicide application as a result of observing the ear rot phase of the disease will not be beneficial.  Once the disease has reached a level whereby ears are covered with the fungus the fungicide will not provide a benefit to those ears.  In addition, for non-infected ears to become infected to that level of disease will require several weeks and the corn plant will more than likely outrun the disease.

Southern rust. Note more numerous pustules, the slight yellow halo around the pustules, and the light orange sporulation erupting through the leaf’s epidermis.

Southern rust

As of late last week southern rust had been observed on one leaf in Coahoma County, MS in a non-irrigated field by a consultant.  Yesterday (July 3, 2015), additional southern rust was observed in corn at the dent growth stage in Adams (just north of Natchez, MS) and Claiborne counties (just north of Port Gibson).  On July 4 southern rust was observed in Forrest and Stone counties.  Southern rust was observed on a few scattered leaves in what could be considered a small hot-spot.  Typically southern rust is a late-season entry into MS (late July/early August).  However, with the general southern flow of air currents we have received since the spring this is no surprise to find southern rust in a few scattered locations in early July.  Regionally southern rust has also been observed in scattered locations in Louisiana (Rapides Parish, LA).  Be mindful that southern corn rust will have a slight yellow halo around the lesions and pustules will generally only occur on the upper leaf surface.  Pustules of southern rust are typically more numerous than common rust and the sporulation that erupts through the leaf surface is light orange in color compared to common rust that is more brown or darker orange in color.

Scouting for southern rust can be a difficult task.  When I scout for the disease I typically observe numerous leaves as I slowly walk through a sizeable area in the field moving over several rows every one hundred to two hundred feet.  When I identify a single lesion of southern rust I will typically slow down and more carefully observe leaves within the general area of the first lesion observed.  I will move over a few rows and continue to scout for the disease.  Once I find the first lesion I typically want to see if there is a larger hot-spot within the area as the disease spreads out from a point of initial infection.  If you are struggling to find the disease in a corn field that means the disease is present at super low levels or not at all and corn will likely outrun the disease.  Oftentimes, scouting for southern rust in situations where the poly-pipe runs between portions of a field can produce better results.  The slight difference in environment in areas where the irrigation water may reduce temperatures can increase the level of southern rust infection.

Comparison between common rust (left) and southern rust (right). The coloration of the sporulation is a helpful diagnostic feature.

Almost annually the myths regarding southern rust far outweigh the facts.  Plant diseases will typically not take a 200 bushel/acre corn crop to zero bushels.  Two myths are typically discussed and almost always begin telephone conversations when I receive calls regarding southern rust.  Firstly, with regards to lodging as a result of southern rust.  Lodging as a result of southern rust will ONLY occur if the disease causes a heavy level of infection in pre-tassel corn.  A heavy infection would be every leaf on every plant in a field.  The only time I have seen a level of infection that equaled the above scenario was during 2008/2009 when corn was planted following a wheat crop.  Late planted corn is more at risk from southern rust simply due to the time of year when the crop matures.  Secondly, southern corn rust will not kill a plant in 7 days.  In 2010 I was able to conduct one really good fungicide trial with southern rust.  The corn was planted in Stoneville, MS just late enough (likely May that year) to receive a heavy level of southern rust infection (on a scale of 0-9 where 0 = no infection and 9 = heavy infection, the field was on average a 3.5) by the time dent occurred.  In that situation, fungicide treatment was economically beneficial.  Last year, we conducted a large plot fungicide trial outside of West Point, MS.  The corn was at the dent growth stage at the time of application.  Stratego YLD was applied at 4 oz with 0.25% of non-ionic surfactant by air in a field that was heavily infected with southern rust in five replicated strips.  Yield collected at the end of the season suggested the application was not economical (244.5 bushels/acre non-treated compared to 232.3 bushels/acre treated) at that particular growth stage.

Keep in mind, that if you do choose to make a fungicide application as a result of southern corn rust that regardless of product or the amount of southern rust in the field the disease will appear to get worse following the fungicide application.  Fungicides protect yield by reducing additional infection and slowing down the epidemic.  But, at this time, I hardly consider southern rust to be a tremendous threat.  Fields should be scouted carefully before making a decision to apply a fungicide.

For current information regarding the location of observations of southern rust stay tuned to the MS Crop Situation, the Disease Monitoring maps associated with this webpage, or the Southern Corn Rust IPMPIPE at: http://scr.ipmpipe.org/cgi-bin/sbr/public.cgi.

Northern corn leaf blight

Additional NCLB has been observed throughout MS.  Annually, as the corn crop matures and reaches advanced reproductive growth stages, NCLB can appear more regularly.   I realize that NCLB lesions are long and oftentimes appear to be quite damaging.  But, as corn reaches dent the potential yield loss as a result of NCLB is low.  Several years ago LSU conducted large strip trials using fungicides on maturing corn with NCLB infection.  The results of the trials indicated fungicides were not beneficial as corn reached advanced growth stages.  Diseases do not have treatment thresholds since so many factors come together for disease to occur.  As plants mature naturally, and send nutrients from leaves to the developing ear, the leaves become more susceptible to infection.  Typically I receive calls where someone is finding “a lot” of NCLB.  I have regularly gone to look at some of these fields only to find a lesion on one leaf and then I have to hunt for additional lesions.  As plants move closer to physiological maturity more NCLB will typically be observed on corn leaves.  Keep in mind that over the past few years a few particular hybrids have appeared to have more NCLB than others.  Scout those particular hybrids more thoroughly and be mindful that one lesion per leaf per plant is far less than 1% of the total plant’s leaf surface area and should not be considered a trigger for fungicide application.

Southern corn leaf blight. Note that lesions typically cross the veins and do not have parallel margins as occurs with gray leaf spot.

Southern corn leaf blight

Southern corn leaf blight (SCLB) is typically a disease of no consequence observed in the lower canopy.  Most hybrids have some level of tolerance to SCLB and at this time the most virulent race (race T) of the fungus is not present in the MS corn production area.  SCLB can be easily confused with gray leaf spot as the appearance of the two lesions can be quite similar.  While gray leaf spot produces a lesion that has parallel margins in between the leaf veins, SCLB will produce a lesion that crosses the veins and does not have parallel margins.  In general, SCLB will be observed in the lower plant canopy and will not move above the ear leaf.  The lowest leaves on the corn plant are typically nutrient stressed and shaded out once the canopy closes.  In the years I have been in MS I have not observed SCLB at the ear leaf or above.

Banded or aggregated sheath blight.

Banded or aggregated sheath blight

Over the past three weeks I have received several calls regarding “stalk rots”.  In these instances, the disease in question was banded or aggregated sheath blight.  The symptoms will be most often observed on the lowest portion of the stalk and appear as water-soaked areas that produce a grayish-brown area on the leaf wrapped around the stalk.  Typically, when stalk rots are observed the disease will produce lodging similar to green snap.  The fungus that causes banded sheath blight, Rhizoctonia solani, is the same fungus that causes sheath blight of rice.  In the most advanced stages of the disease, the fungus will move up the plant and infect the ear.  However, this season I have only observed the disease in some hybrid trials and had a few associated calls whereby the consultant was concerned about stalk rot.  Some hybrids are clearly more susceptible to this particular disease than others.  Fungicide application would not reach the location on the plant where the product would be needed.  The fungus is soilborne and starts at the base of the plant and moves up the stalk from the soil line.