On Monday, we received a wheat leaf sample collected by Dr. Janet Knodel and Sam Haugen (Plant Pathology graduate student) that was confirmed as stripe rust. On Tuesday, the IPM scouts in central (Kyle Aasand) and northeast (Jaime Lundquist) North Dakota also documented the presence of stripe rust. This information combined with the early development of the disease in states to the south, we can expect that stripe rust is probably widespread on susceptible varieties on the eastern half of the state. I have not received any reports from western side of the state, but now is a good time to start scouting for the prevalence of this disease.
How common is stripe rust?
Stripe rust is often considered to be the least common wheat rust in ND. The last widespread stripe rust event in the state occurred in 2012 and several fields had high levels of stripe rust that required a fungicide application.
What are the ideal conditions for disease development?
The sporadic occurrence of stripe rust is largely explained by the environment. Stripe rust development is favored by cool nighttime (50-60˚F) and daytime (below 80˚F) temperatures with frequent moisture events such as heavy dews and rain; conditions that were readily observed during May.
Where does stripe rust come from?
Like all other wheat rusts, the stripe rust pathogen survives on living plants in the south and produces spores that are carried by winds north along the Puccinia Pathway. The route of spore travel is similar to the Central Flyway used by migrating waterfowl. This year Kansas and Nebraska reported several fields of high stripe rust incidence and severity producing an ample spore source for the neighbors to the North. This spore source combined with the cool wet weather experienced in May has contributed to the early documentation of stripe rust in ND.
How do you differentiate between stripe rust, leaf rust and stem rust?
Both color and rust pustule shape can help differentiate between the wheat rusts. Mature stripe rust pustules are yellow to orange and appear in an elongated stripe on the leaf. In early stages of stripe rust development, the typical elongating stripe lesion is not common and color must be used to identify stripe rust (Figure 1). Leaf rust has oval shaped pustules that are reddish-brown on the leaf blade (Figure 2). Stem rust primarily occurs on the stems of susceptible plants, but can also be observed on the leaf blades as dark red-brown irregularly shaped pustules.
Most of the rust breeding efforts on spring wheat have been dedicated to leaf and stem rust, yet susceptibility differences to stripe rust were observed in 2012. For example, Prosper, Faller and Vantage were documented in having high stripe rust severities. Greenhouse screening of winter wheat varieties listed Ideal, WB Grainfield and Art as being resistant to stripe rust. Screening efforts are underway evaluating germplasm for resistance to all three wheat rusts.
Fungicides in the triazole and strobilurin classes offer very good to excellent management of stripe rust. These same classes of fungicides have similar efficacy against other fungal leaf spots as depicted in the 2015 NCERA-184 Wheat Fungicide Efficacy Table (Click on the Link and select 2015 Wheat Fungicide Efficacy Table). The timing of an application is the most critical and trickiest part in fungicide management of stripe rust. The key leaf to protect from stripe rust is the flag leaf. Remember that fungicides do not cure infected leaves and should be used in a protective manner. When deciding to make a fungicide application, consider the growth stage, level of rust incidence in a field and future weather conditions.
No need to hit the panic button yet, but scout and pay attention to the weather. When it comes to scouting, if you don't see stripe rust now, this does not necessarily mean the stripe rust pathogen is not in your field. Once a spore lands on a leaf, a new pustule will not be produced for about two weeks under ideal weather conditions. Therefore, frequent scouting is needed to document any changes in disease incidence. If nighttime temperatures stabilize in the upper 60s and several daytime highs are above 80˚F, the pathogen will go dormant. However, if ideal conditions continue in the next two weeks, the decision to make a flag leaf fungicide will come into question.