A PDF version of this article contains images: printer-friendly pdf

In southern areas of Minnesota, wheat crop growth stages are approaching head emergence. This is a crucial time to scout fields regularly. Diseases such as leaf and/or stripe rust can injure tender flag leaves in a matter of days if the (1) variety is susceptible, (2) weather promotes disease, and (3) rusts are established in the lower and middle plant canopies. It may be necessary to apply fungicide to protect flag leaves.

Leaf rust

Rust spores produced in southern states are transported by northerly winds to our area each year. Early this spring, a substantial amount of leaf rust infection occurred on susceptible winter wheats in Texas and Oklahoma – the primary states that serve as sources of inoculum for Minnesota. With an influx of available spores, this could be a ‘rusty’ year if weather conditions promote disease development.

The fungus that causes leaf rust of wheat needs warm days and nights (60-80 oF) with six to eight hours of leaf moisture. Leaves in the lower canopy are generally the first to become infected since they stay wet longer. Under the right environmental conditions, masses of spores capable of spreading disease can be produced one week after plants become infected.

Plant symptoms differ depending on disease resistance levels of varieties. Bright orange to reddish spots, called pustules, tear leaf tissues as spores multiply inside the leaves and eventually erupt (Fig 1). Overall, susceptible wheat plants produce larger pustules with more spores than resistant plants. Yield loss often occurs if infection takes place before the crop flowers. Disease development is promoted when spores are abundant and the weather remains warm and wet. During 2004, wheat rusts were a production issue in the southern to central areas of Minnesota.

Leaf Rust Control

If leaf rust is well established in the lower to mid-canopy of a susceptible variety and the weather is forecasted to be warm and wet, a fungicide application may be necessary. The flag leaf (the last leaf produced prior to the head) should be targeted for protection. When the flag leaf is fully emerged, but has few rust pustules present , apply a labeled fungicide product for control (see Table 1, below). If the disease is present, but conditions aren’t right for rapid disease development, a fungicide application at the early-flowering growth stage (Feekes 10.51) for Fusarium head blight (scab) control will also help in controlling leaf rust.

Stripe rust

Much like leaf rust, stripe rust spores are produced on susceptible winter wheats in southern states and are transported to Minnesota by wind.

This pathogen causes disease when air temperatures are cooler. Stripe rust disease development can progress rapidly when temperatures are 50-60 oF and plant leaves remain wet for extended periods. This rust pathogen can also infect tissues on heads of susceptible plants (Figs. 2, 3).

If stripe rust is established in the plant canopy and the forecasted weather conditions promote disease development, fungicide application may be needed to protect the flag leaf. Many spring wheat varieties have some level of resistance to stripe rust, however others have been observed as susceptible. Resistance ratings have not yet been established for spring wheats since the disease was rare in Minnesota prior to the spring of 2003. Hot and dry weather stops disease development.

Stripe Rust Control

If leaf rust is well established within the plant canopy and the weather is forecasted to be cool and wet, fungicide application may be necessary. When the flag leaf is fully emerged, but has very few rust pustules, apply a labeled fungicide product for control. If possible, waiting until the early-flowering growth stage, when a fungicide application for Fusarium head blight (scab) can be made is more desirable than making two fungicide applications on a crop just a few days apart.

After plant heads emerge, another decision will be needed as to whether plants should be protected from Fusarium head blight (scab). Unfortunately, symptoms of scab aren’t present until later, when kernels are developing. Producers must stay informed about the potential for disease in their area and protect their wheat crop, if needed, at the early-flowering growth stage.

Fusarium head blight (scab)

Fusarium head blight can result in the premature death of one or more spikelets on a susceptible spring wheat head. Diseased spikelets often appear tan-to-pinkish in color, and can be described as ‘bleached’ compared to the normal green color of healthy plant tissues ( Fig. 4). In a worst-case scenario, the majority of spikelets on heads of susceptible varieties may contain few kernels. The level of disease severity is determined in part by the plant’s level of disease resistance, growth stage when infected, and weather conditions prior to, during, and after the flowering growth stage. If a plant is infected early, the flower is aborted and the spikelet dies prematurely. If infection occurs later, normal seed filling processes are interrupted and kernels turn white-to-tanish in color and become shriveled. These collapsed, shriveled kernels are called ‘tombstones’. Delayed infection can result in plumper kernels that appear to be healthy - but contain a toxin produced by the fungus. When levels of the deoxynivalenol ( DON) toxin become too high (at or above 5 ppm), elevator personnel may refuse to accept the grain.

Decision aid for fungicide application

An experimental scab epidemic forecasting model is available online during 2005. The model, supported by the Minnesota Wheat Research and Promotion Council, is based on weather factors known to favor Fusarium spore production (e.g., temperature, relative humidity, and duration of rainfall). It estimates the risk that a scab epidemic will occur using locally and regionally available weather data beginning seven days prior to flowering. Growers are encouraged to use the experimental model as another management tool, but are cautioned that it isn’t 100% accurate. If a prediction is in question, consult local and regional extension educators and other agriculture professionals for additional information. Minnesota’s experimental forecasting model can be found on the Internet at http://mawg.cropdisease.com.

Scab Control

Fungicide application during the early-flowering growth stage (Feekes 10.51) is recommended for susceptible spring wheat if weather conditions favor Fusarium spore production, and the cost of the treatment is economically justified. Unlike other diseases, complete FHB control isn’t possible with today’s fungicides. Timely application of fungicide can increase yield and reduce DON levels in infected grain. The recommended wheat crop growth stage for fungicide application to control Fusarium head blight (scab) is during early-flowering. Because Folicur (Bayer CropScience) remains the most effective product for controlling scab across environments and years, another Section 18 has been approved by the U.S. Environmental Protection Agency (EPA) (See Table 1). The 2005 Folicur Section 18 label has not yet been developed. It will be posted on the www.nwroc.umn.edu and www.smallgrains.org websites when it becomes available. Applicators must possess a copy of the label during application. Strobilurin and strobilurin/triazole mixed chemistries (e.g., Headline, Quadris, Quilt, and Stratego) are not recommended for late growth stage applications for control of scab due to incidences of increased DON levels following application.

Table 1. Foliar and head-applied fungicides for application on small grain crops in Minnesota. Updated May 2005.