Corn Holcus Leaf Spot Occurring in the Central Delta

Holcus spot of corn, notice two lesions attempting to coalesce. Observe the yellow halo around the 3 well developed lesions.

The appearance of Holcus spot in corn this year is much earlier than in the recent past.  However, at this point in the season it can be difficult to distinguish Holcus spot from other leaf injuries, especially those associated with herbicide drift (e.g. Gramaxone, Halex).  Frankly, there is no easy way to identify Holcus spot in the field; save for observing the types of lesions present on the corn leaves themselves and then comparing those to any potential lesions on weeds around the field.  However, with that said, there are some additional hosts of the bacterium that causes Holcus spot including Johnson grass, sorghum, Sudan grass, and wheat.  Last week (4/10/12), I received several calls regarding round, white, non-descript leaf spots on young (V2 through V5) corn plants. Leaves were returned to the laboratory to confirm the presence of the bacterium by observing bacterial streaming from the cream to white lesions.  In addition, Friday (4/20/12) numerous calls from the Hollandale, Indianola, and Shaw areas confirmed the presence of Holcus leaf spot in most corn fields throughout the central Delta.  I was able to scout corn fields from east of Winona to the Washington County line.  Fields around Winona were observed to have very little Holcus spot and when present the general size of the leaf spots was quite small.  However, once I was north of Itta Bena I could easily detect symptoms of Holcus spot on corn and Johnson grass along field edges.  In addition, the general size of the lesions was quite large; larger than what I have observed over the past few years.

Note watersoaked lesion-type, indicative of infection, at the end of the arrow.

Diagnostically speaking, and to aid in scouting for the presence of the disease, Holcus spot will begin as dark green, water soaked areas and more generally appear as a greenish halo when the light catches the leaf.  When the lesions age, the characteristic cream to white appearance we are most familiar with will be the predominant symptom.  Generally speaking, the Holcus leaf spots are 2 to 10 mm in size (or can be as large as ¼ of an inch in size; see attached images).  In the past I have commonly observed Holcus spot lesions that were generally 5 mm in size.  However, this year in particular lesions appear to be much larger and on many plants the lesions have coalesced and don’t appear to be round but are more oblong.  If halos are present around Holcus spot lesions, the halo will typically be a yellowish color on mature lesions.  Keep in mind, the major difference between Holcus spot and spots produced by herbicide drift or injury will have oftentimes have a colored halo present around the lesion or (depending on the type of herbicide injury; see photo) may produce an elongated lesion.  Compare the yellow halo to the halo coloration that is present when herbicides either drift onto a corn leaf or are sprayed directly over the top of the crop by accident (see photo caption(s)).  Herbicide injury in the early stages and depending upon chemical can also develop a water-soaked appearance.  Scouting the field over a period of several days is likely the best way to observe the lesions.

Note size variation between Holcus spot lesions. In some cases, the lesions in corn around Indianola, MS may be larger than the lesions in this particular photo.

More often than not, Holcus spot is not severe and will not result in a yield loss.  In addition, based on hail injury models a yield loss will not result from leaf area loss (due to disease or other injury) typically unless the corn is beyond V6.  Moreover, even if you were to add up all of the associated area where the leaf spots are present we would still be talking about a total leaf surface area less than 10%.

Typically, the spotting associated with Holcus spot will be associated with warm (high-70s to mid-80s), rainy, windy, or humid weather.  Generally, the appearance of Holcus spot will follow rain showers.

Based on the information regarding Holcus leaf spot in the literature, there is little if any information available regarding whether or not specific hybrids are either susceptible or resistant to infection from the bacterium.  With that in mind, I suggest that all hybrids are susceptible but the reaction of the plant may differ in some cases.  While rotation and management of debris are suggested to aid in the management of the disease, most of the fields with symptoms are in their first year of corn and it is more likely that the disease occurred following the rain that occurred across much of the affected area this week.

Severe herbicide injury of young corn. Note jagged edges of most lesions.

Once Holcus spot is observed there are no management options available.  Remember, Holcus spot is caused by a bacterium, so the application of a fungicide in response to the leaf spots will not result in an economical benefit.