When should I start scouting for soybean rust?
Follow the development of soybean rust in the south to gauge when to start scouting for soybean rust in Iowa. When the risk of soybean rust is elevated, local scouting can be done to determine if individual fields have rust. To identify low incidence of rust, leaves need to be incubated for 24-48 hours and looked at with a dissecting microscope.  “Submitting Soybean Samples” has additional information on submitting samples to the Plant and Insect Diagnostic Clinic.

Are there any soybean varieties on the market that are resistant to soybean rust?
Soybean varieties with some resistance to soybean rust have been developed in Brazil, China and Taiwan. At this time there are no commercially available varieties in the United States. Researchers are looking for resistant varieties for U.S. growers, but current estimates indicate that those varieties will not be available for six to 10 years.

What is a Section 18 fungicide and what restrictions are associated with these fungicides?
Section 18 fungicides are pesticides temporarily exempt from registration under the Federal Insecticide, Fungicide and Rodenticide Act based on an emergency condition that may cause a significant economic loss by a particular pest problem.

Section 18 fungicides for soybean rust must be used for the management of soybean rust only. That is, applications of Section 18 fungicides should not be made to control other foliar diseases or “plant health” benefits.

The Section 18 label always takes precedence over the Section 3 label (the label on the container). The applicator is responsible for complying with both labels. When applying Section 18 fungicides the applicator is required to have a copy of the Section 18 label in their possession.

When should I spray fungicides to manage soybean rust?
Fungicide applications should be made before soybean rust is detected in the field or when incidence of rust is very low. Since it is difficult to identify low levels of soybean rust in the field, many fungicide applications will be made when the risk of rust is high, but before rust is actually confirmed in the field.

Can I spray fungicides too early for soybean rust management?
Depending on the active ingredient, formulation and environmental conditions, fungicides for rust management do persist for some time (never more than three weeks for the best products), but the effective life of a treatment varies.  Preventative fungicides applied too far in advance of rust arrival may lose effectiveness by the time the pathogen first infects the field. 

What is the latest stage in soybean plant development that a fungicide should be applied for managing soybean rust?
Soybean varieties in Iowa are indeterminate so they will continue to accumulate biomass and set vegetative nodes until growth stage R5.5 (just before R6). This typically occurs around mid-August in Iowa. After the R6 growth stage, when seeds are fully developed in the cavity of the pod, the yield has been determined and fungicides will not increase yield.

There are legal restrictions as well. Some fungicides may only be sprayed until R5 growth stage, according to the label. Also, some fungicides have a 30-day pre-harvest interval, so you need to be careful not to apply fungicide too close to harvest.

Should I be concerned about the soybean rust fungus developing resistance to the fungicides?
The development of fungicide resistance in soybean rust populations is a major concern, particularly for strobilurin and triazole fungicides. Resistance concerns are based on the unique modes of action represented by the strobilurins and triazoles.

Fungicide resistance to soybean rust has not been reported yet but has been reported on similar pathogens in parts of the world, including the United States. Protecting these fungicide groups is especially important because they currently are the primary line of defense against soybean rust.

Four things you can do to minimize fungicide resistance developing are:

• Only use fungicide when it is needed.
• Alternate subsequent applications of fungicides with another product that has a different mode of action.
• Use a premix product that combines a preventative (strobilurin) and curative (triazole) fungicide.
• Monitor treated fields for signs of resistance development. If soybean rust continues to develop after application, contact your chemical representative or extension personnel. Scouting fields for soybean rust both prior to fungicide application and after application is critical.

What are the recommendations for additives or adjuvants?
There are different recommendations specific to given products and how they are applied – aerial or ground. Some product labels recommend additives; the most common are non-ionic surfactant (NIS) or crop oils. Some products do not require any additives. The best recommendations are included in the fungicide’s labeling.

Can I tank mix rust fungicides with herbicides or insecticides?
Legally, unless the label specifically prohibits it, you can mix herbicides or insecticides with fungicides. Simultaneous applications of different pesticides seem like an efficient approach to reduce cost but it is often impractical because of timing and application issues. Fungicide applications for management of soybean rust are probably not going to be made when weeds need to be sprayed. These fungicide applications may overlap with late-season insecticide sprays. Fungicides and insecticides require small droplet sizes and high spray volumes to ensure good overage and maximum efficacy. Larger droplet sizes are required of most herbicide applications to reduce drift.