Because soybean rust tends to initially develop in the lower and mid canopy, thorough coverage of foliage, including penetration of spray into the canopy, is essential to a successful soybean rust spray program. Fungicides are best applied at higher gallons per acre, higher pressures, and with different nozzles than herbicides.

Fungicides require smaller droplets and higher spray volumes to achieve maximum coverage of the soybean canopy than herbicides. This may require either different spray nozzles or higher spray pressure to generate the smaller droplets. Some recommendations suggest that fine to medium-sized droplets (150 - 350 VMD) are appropriate for fungicides. (VMD = volume mean diameter and is measured in microns).

However, we want larger droplets during herbicide applications to reduced the risk of drift. Medium or coarse-sized droplets (250 - 450 VMD) are recommended for herbicides, which are less prone to drift. If the application of a herbicide-fungicide tank mixture is optimized with larger droplets to reduce herbicide drift, the performance of the fungicide may suffer. If smaller droplets are used to optimize the fungicide performance, herbicide drift is more likely.

For the best performance, fungicide applications by ground rigs should be made in a minimum of 15 gpa of water and 20 gpa may be preferred. If a herbicide-fungicide application was applied at the lower spray volumes (e.g. 10 - 15 gpa) typically used for many herbicides, spray coverage with the fungicide may be comprised as well.

View a powerpoint presentation on fungicide application techniques by Chris Boerboom, UW-Extension Weed Scientist»