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Stripe rust: A new challenge to wheat yield in New York

Gary C. Bergstrom and Michael R. Fulcher
School of Integrative Plant Science, Plant Pathology and Plant-Microbe Biology Section, Cornell University

Stripe rust, known in many parts of the world as ‘yellow rust’ for its yellow-orange urediniospores, is a relatively new problem for wheat production in the eastern U.S.  However, the disease has been steadily increasing in severity and geographic range in the central and eastern U.S. over the past decade.  It has been observed sporadically in New York State for several years, but occurred at epidemic levels associated with potential yield losses for the first time in New York in 2016.  As of early June 2017, it has already been found in 24 states and three Canadian provinces.  It has been observed in several wheat fields in the Finger Lakes and western New York and may become widespread in New York before the crop matures.

Figure 1. Characteristic yellow-orange pustules of stripe rust urediniospores running in stripes or lines along veins on the wheat leaf surface. Smaller, circular, cinnamon-brown pustules of leaf (brown) rust of wheat may be seen on the same leaf. Photo by Gary Bergstrom. Inset photo (by Kent Loeffler) shows close-up of stripe rust pustules.

Stripe rust of wheat is caused by the fungus Puccinia striiformis f. sp. tritici. Stripe rust is identified by its telltale, yellow-orange spore pustules arranged in stripes along the leaves in contrast to the smaller, cinnamon brown pustules of leaf (brown) rust (Figure 1). Stripe rust isolates currently found in the Eastern U.S. do not attack barley.  Like other cereal rusts, the stripe rust fungus only survives between growing seasons on living wheat plants.  Therefore, stripe rust survives the winter primarily on winter wheat in frost-free areas of the southern U.S. Spores become airborne, move long distances in the atmosphere, and are deposited on green wheat plants in northern states each spring/summer.  Occasionally stripe rust may overwinter on wheat plants in New York during mild winters or under snow cover, resulting in an earlier spring epidemic. Once infection begins in a field, new generations of rust spores can be spawned as quickly as every 10 days under mild temperatures and moist conditions, thus magnifying disease in individual fields and providing new spores to be blown to both nearby and distant fields.  Significant yield losses can result when rust attacks the upper leaves of wheat during the critical first weeks of grain filling.

The best way to manage rust diseases is to plant resistant varieties.  However, rust pathogens are tricky, and fungal populations can evolve new races that attack once-resistant wheat varieties.  We are just beginning to understand the susceptibility of regional wheat varieties to stripe rust.  We learned in 2016 that certain widely grown soft red and white winter wheat varieties were particularly susceptible to stripe rust (Figure 2).  There are several foliar fungicides labeled for stripe rust control in New York and these will be very useful to utilize on susceptible varieties in years when there is a significant risk of stripe rust infection.  Rust epidemics observed in 2016 developed primarily following head emergence of wheat.  We found that a flowering time (Feekes stage 10.51) application of either Caramba or Prosaro fungicides for Fusarium head blight suppression provided complete protection of flag leaves against late-developing stripe rust. However, in future years when epidemics are initiated at earlier growth stages, it is likely that we will need to apply protectant fungicides at jointing to flag leaf emergence stages if scouting reveals the early presence of rust.

Figure 2. Relative severity of stripe rust observed on soft red and soft white winter wheat varieties compared over four New York nursery locations in June 2016. The boxplot midlines are median values, and the diamonds mark the average severity.

Since stripe rust is still fairly new to New York, we are tracking its progress and making collections of the fungus to determine races and genetic variation.  Please contact your Cornell Cooperative Extension Field Crop Educator or the Cornell Field Crops Pathology Program if you find stripe rust in your wheat over the next few weeks. You can help us learn more about this new yield robber and how we can minimize the risk.

Acknowledgements:
This work is supported in part by funding from USDA-NIFA Hatch grant NYC153436 and USDA-NIFA Smith-Lever grant NYC153652.

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