Combining in a Year with Heavy Southern Rust and Leaf Diseases

By Doug Houser

Some years test both our crops and our patience. This season, with southern rust and other leaf diseases showing up across fields, harvest won’t 

be business as usual. While these diseases have already trimmed yield potential during the growing season, they’ll also create challenges when you pull into the field with the combine.

The key this fall is to make the right adjustments in the cab so you protect grain quality, manage residue, and avoid problems that will carry into spring.

What Leaf Diseases Mean at Harvest

• Lighter test weights – Diseased leaves cut photosynthesis short, which often means shallower kernels and more brittle grain.
• Moisture swings – Some plants are already dead-dry while others are still green, leading to wide moisture variation.
• Weak stalks and down corn – Disease stress makes stalks soft and more prone to lodging, especially after late winds or rain.
• Heavy, brittle residue – Diseased plants leave behind more leaf material that tends to bunch and not spread correctly.

Each of these issues has a direct effect on how you set and run your combine.

Combine Settings to Consider

1. Rotor/Cylinder Speed

• Problem: Diseased kernels are softer and crack more easily.
• Adjustment: Run rotor/cylinder as slow as possible while still threshing clean. Start ~250–300 RPM (rotor) or 350–450 RPM (cylinder) depending on your machine, and back off if you see cracked kernels in the tank.

2. Concave Clearance

• Problem: Fragile grain breaks if squeezed too tight.
• Adjustment: Open concave slightly wider than normal. Check behind the combine if you’re seeing whole cobs with kernels left, tighten a touch.