Seeing spots? Not to worry with Holcus Leaf Spot

Holcus leaf spot is a foliar disease that affects corn caused by the bacterium Pseudomonas syringae. The disease is usually cosmetic and rarely causes an impact on yield potential.

Identification and Symptoms: The disease is characterized by small, round to oval spots, that are typically light tan or cream colored, have a water-soaked appearance, and usually appear on lower leaves first. If conditions remain favorable for development, the tan spots enlarge and have a distinct halo which gives them a translucent appearance (Figure 1). The spots are scattered randomly across the leaf surface and can be mistaken for other diseases or damage from some contact foliar herbicides.

Causes and Conditions: The bacterium thrives in cool, wet conditions and is spread by rain splash and windy storm events. The bacteria can survive on plant debris in the soil and infect new plants through wounds or natural openings in the leaves.

Impact on Corn: Holcus leaf spot is generally considered a minor corn disease as loss in yield potential is rare. If the lesions become prominent, the primary effect of the disease may be a reduction in photosynthetic area due to the leaf spots. The disease is often more of a cosmetic issue than a major agronomic concern.

Management and Control: Managing holcus leaf spot primarily involves cultural practices such as residue management which helps to reduce the bacterial inoculum in the field. Because this is a bacterial disease, fungicides are of no benefit in management.

Conclusion: Holcus leaf spot is a relatively minor but sometimes noticeable corn disease. It is characterized by light-colored spots on the leaves that can reduce the photosynthetic area of the plant. While generally not a major threat to corn yield potential, effective management through cultural practices can help minimize its impact.

Channel Agronomist
Todd Vagts

Sources

Robertson, A. 2008. Unusual foliar diseases showing up in Iowa corn. ICM News. Integrated Crop Management. Iowa State University Extension and Outreach. https://crops.extension.iastate.edu/cropnews/2008/07/unusual-foliar-diseases-showing-iowa-corn

Malvick, D. 2018. Holcus spot in corn. University of Minnesota Extension. https://extension.umn.edu/corn-pest-management/holcus-spot-corn

Holcus leaf spot of corn. 2019. Crop Protection Network. A Product of Land Grant Universities. https://cropprotectionnetwork.org/encyclopedia/holcus-leaf-spot-of-orn#:~:text=Holcus%20leaf%20spot%20is%20caused,have%20a%20light%20brown%20border.

Wise, K., Mehl, K., and Bradley, C.A. 2017. Holcus leaf spot of corn. PPFS-AG-C-06. University of Kentucky. https://plantpathology.ca.uky.edu/files/ppfs-ag-c-06.pdf

Web sources verified 6/29/24. 1110_430837