What is in this article?:
- Rust diseases cut into North Carolina wheat yields
- Very warm winter
- Overcame resistance
• Warm winter and spring temperatures were the main culprits for the increased rust pressure in wheat, but the size of the crop also provided more opportunities for disease to form.
USDA PLANT PATHOLOGIST Christina Cowger shows the results of rust damage to wheat in a North Carolina field.
Very warm winter
In North Carolina, the state recorded its eighth warmest winter on record. Across the United States, the winter of 2011-2012 was the warmest on record. Warm weather allowed rust and other diseases to over-winter farther north than usual and to get to areas like the Carolinas and Virginia earlier than usual.
Rust has to blow into the Carolinas from somewhere else. Typically, south Florida and south Texas are about as far north as wheat rust spores can over-winter. This year the disease causing organisms were able to survive the warmer winter months much farther north than typical.
In North Carolina, Cowger says stripe rust and leaf rust probably began establishing in January instead of March. In addition to being in place earlier, rust also got some cool nights and warm days that produce a heavy dew that is conducive to rust development.
Stripe rust typically thrives in temperatures that range from 55 and 75 degrees F, and it needs these temperatures to occur over a period of weeks. If temperatures get warmer, as they almost always do in the Carolinas in late spring, the disease starts to slow down, Cowger says.
Leaf rust, by comparison, is much better suited to the Carolina climate. It likes temperatures in the 65-85 degree F range, which is much more common during prime times for wheat development in the region.
Usually temperatures in the region turn too warm too quickly for stripe rust to get started.
“This year we had a warm winter and a long period of cool early to mid-spring weather, which allowed stripe rust to get going.”
Despite the obvious bright ‘hot spots’ that showed up in various Coastal Plains fields, Cowger says stripe rust caused relatively little damage compared to wheat leaf rust. Economic damage to wheat was limited to a few bad cases scattered throughout the region, she says.
However, the USDA scientist adds there is reason for concern about stripe rust damage to wheat. It occurs so rarely in the Upper Southeast that plant breeders have difficulty breeding for stripe rust resistance. As a result, many of the wheat varieties planted in the region are highly susceptible to the disease.
Leaf rust, by comparison, is commonly occurring in North Carolina and surrounding states, and there are a number of high yielding wheat varieties that have built-in resistance to the disease.
In some years, this genetically built-in resistance is enough to carry growers through the year without the need to spray fungicides to protect their crop, Cowger adds.
“This year every wheat variety in the book needed a fungicide application. We had such a huge population of rust spores and somewhere in that big cloud of spores there was a race that could overcome a lot of the resistance genes that normally protect our wheat,” she says.