USDA PLANT PATHOLOGIST Christina Cowger shows the results of rust damage to wheat in a North Carolina field

USDA PLANT PATHOLOGIST Christina Cowger shows the results of rust damage to wheat in a North Carolina field.

Rust diseases cut into North Carolina wheat yields

• 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.

Stripe rust is rare in the Carolinas and Virginia, but this year hot spots of the disease created sporadic problems for growers in the region and contributed to a nearly 10 bushel per acre yield drop in North Carolina, says USDA Plant Pathologist Christina Cowger.

“The last time we’ve seen stripe rust in wheat at damaging levels was 2005. This year growers began seeing yellow, rust-infected patches of wheat, called ‘hot spots.’ These hot spots were scattered throughout the coastal plain of North Carolina,” she says.

Oat crown rust is another disease that rarely occurs in North Carolina. This year it was abundant and is typical of unusual or rarely seen rust diseases on a wide range of wheat and grass crops in the state, according to Cowger.

Leaf rust is the primary rust disease found in wheat in the Carolinas and Virginia, and it occurs to some degree in most cropping seasons. This past season, the disease was much more widespread than usual and caused significant damage to the wheat crop in the region.

It forms reddish pustules or red dots on the leaves of wheat. It’s a fungus that needs living tissue to survive, so it’s happy when the wheat is rapidly growing, and it causes the most problem while wheat plants are still green, Cowger says.

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.

In North Carolina wheat producers harvested more than 770,000 acres. Though average yield was down by 10 bushels from the 2011 record of 68 bushels per acre, total wheat production set an all-time state record of 44.7 million bushels.

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.

Overcame resistance

“Unfortunately, the abundance of rust spores meant that a number of strains could overcome built in genetic protection. With the perfect weather conditions, these rare rust strains took a toll on some of the varieties that normally have good protection against leaf rust.”

Fortunately, a number of strobilurin and triazole fungicides provide very good protection to wheat from leaf rust and stripe rust damage.

Strobilurin fungicides tend to build up in the cuticle of the small grain plant and tend to be best against the spores. Once the spores germinate and start to grow, this family of fungicides is less effective. So, in general the strobilurin fungicides are used as a preventative.

Triazole-based fungicides have more systemic activity and move more inside the plant. So, these fungicides have some activity even after infections have been established and are somewhat more effective in managing diseases after they occur on the plant.

There are also combinations of strobilurins and triazoles that work extremely well to manage wheat diseases. However, regardless of the fungicide treatment used, Cowger says it is critical to catch a rust epidemic before it goes too far. It’s always going to be best to apply fungicides before the leaf rust has gotten going on a big scale.

To move around, disease spores depend on wind to move from place to place. In the Upper Southeast, there was plenty of wind to go along with warm temperatures in the winter and spring. All in all, rust had a near perfect environment to grow, and it had a negative impact on wheat yields in the region.

A late spring freeze didn’t help and likely cut yields as much or more than the heavy rust outbreak in some areas of the Upper Southeast.

The combination of the freeze and the otherwise warm weather that produced excessive, early wheat plant growth contributed significantly to reduced wheat yields statewide, according to Extension specialists from Maryland to South Carolina.

Throw the heavy rust year into the mix and growers had a battle on their hands to keep what at one time looked like a record wheat crop in terms of yield, and keep it a good wheat yield.

Despite the setbacks on yield, North Carolina State Small Grains Specialist Randy Weisz says it appears growers are gearing up to plant even more wheat this fall.

“Seed for some of the more popular wheat varieties in the Upper Southeast may be in short supply this fall,” Weisz says.

He points out that growers would be ahead of the game to look at North Carolina State’s recently released variety performance reports and determine which varieties would work best under a number of different scenarios, including varieties with rust and mildew resistance.

The North Carolina Small Grain Growers Association recently released a video and posted it on social media in which Weisz gives an overview of wheat varieties and provides some valuable insights on how growers may go about choosing varieties based on performance, maturity date, and resistance to diseases.

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