By Sam Markell
Extension Plant Pathologist, Broad-leaf Crops
Low levels of dry bean rust has been showing up in the state. This is not necessarily surprising, the frequent and heavy dews and moderate to warm temperatures have been favorable for rust development. For those that applied fungicides for white mold you have earned a secondary benefit; while many white mold products are not great on rust, they can delay the onset and severity of the disease. Just remember that the efficacy doesn’t last more than a couple weeks so it is important to keep an eye out for rust.
Dry bean rust will usually occur first as ‘hot spots’ in a field. Hot spots are localized areas where rust has heavily infected the lower to middle canopy. Hot spots are more likely along a shelter belt (where the shade and protection from the wind allow for longer dew periods), or near last year’s beans (where inoculum can overwinter). Once hot spots form and rust reaches the upper canopy it can rapidly spread to the rest of the field. Signs and symptoms of dry bean rust include a small pustules filled with dusty cinnamon-brown spores. The spores can easily rub off on your thumb. Rust can complete a cycle (from spore infection to new pustule creation and new spore release) in about 10-14 days; capable of causing epidemics quickly.
The best timing for a fungicide application to manage rust is shortly after it is first found. Strobilurin fungicides [FRAC 11] (Aproach, Headline, Quadris, etc.) and Triazole fungicides [FRAC 3] (Folicur and generics, Proline, etc.) have consistently been the most effective in our research trials.
Once pintos begin to stripe (or the equivalent growth stages in other beans) management is not necessary.