The results are not finalized, but Texas AgriLife Extension Service wheat specialists and Texas AgriLife Research wheat breeders believe the crop is being damaged this year by a new or different race of stripe rust.
Because this winter and early spring were cooler and wetter than normal, conditions were prime for stripe rust, said Dr. Ron French, Texas AgriLife Extension Service state small grains pathology specialist from Amarillo.
“The problem is, producers may be facing a new race of stripe rust that attacks varieties previously thought to be resistant," French said.
"It's attacking some varieties that normally don't see a problem," he said. "We first saw it in the College Station wheat nursery, then in North Texas on a soft red winter wheat. The thought among the wheat scientific community is that it is a new race."
French said samples of stripe rust infected leaf tissue have been sent to a U.S. Department of Agriculture-Agricultural Research Service wheat genetics lab in Washington State, where characterization studies of this rust will be done to determine if it is a new race.
"What we know is that some of the wheat that we thought had good resistance is showing a potential breakdown in resistance," said Dr. Rob Duncan, AgriLife Extension state small grains specialist. "This could well have been accentuated by weather conditions that are favorable for both the pathogen and disease development."
So far no economic value has been assessed on the losses, but French said stripe rust can drop yields substantially, especially if the flag leaf is compromised. In studies conducted in Australia, losses in a susceptible variety could amount to 75 percent if stripe rust attacks when the flag leaf is fully emerged. However, in a resistant variety, losses were only 5 percent.
"We will have to wait until harvest to determine the economic impact," he said. "This problem occurred in pockets around the state. The good thing in Texas is that we do not depend on just a few varieties; we have a diversity of varieties planted.
"Plus, our wheat breeders have been doing a great job staying a step ahead of fungal threats by developing suitable varieties with different genetic backgrounds that are not only in demand in Texas, but in other states as well," French said.
This stripe rust infection in 2010 is important to keep in mind, he said, because while the losses may have been relatively low this year, it could be a warning for next year as fungal inoculum may have built up this year and the surviving inoculum may be present in higher numbers next year if conditions are right.
AgriLife Extension personnel are conducting trials this season to look at the potential impact of fungicide applications on stripe rust on several varieties of importance to Texas wheat producers, French said.
This year, the possibility of a stripe rust epidemic was apparent in North Texas by the first week of April, with widespread infection levels, he said. Some of the most susceptible varieties were some that had exhibited good resistance to stripe rust last year.
"Since there has been an apparent race change, resistance in previously resistant varieties should not be taken for granted," French said. "There is no substitute for field scouting."
Duncan agreed scouting is the key. Some producers have ended up spraying certain varieties they heard were being affected without actually finding the disease first. Each field should be treated as a separate case; don't just spray without considering the variety resistance, yield potential and fungicide costs.
There have been pockets and hot spots, but with the warmer weather, things should begin to calm down, he said. However, cool and wet weather could cause it to flare up again and move to other susceptible fields. Also, now leaf rust is starting to show up in similar hot spots and producers will need to be scouting for it as well, as it can have a negative impact on yield.
Stripe rust can survive between seasons on alternative host plants, but the fungus, Puccinia striiformis, needs a living host to thrive, French said. Symptoms on wheat first appear as narrow orange-yellow stripes of fungal growth that resemble powdery stitches on the leaves.
Early cases of stripe rust started showing up in March in the College Station area on Jagger, Jagalene, TAM 203, TAM 401 and other previously resistant varieties, both specialists said. It has not shown up in TAM 111, Fannin and Doans so far.
Producers, as well as AgriLife Extension agents and specialists, have been monitoring the situation in the past few months and fungicide treatments may have been warranted in some cases when the crop was compromised, he said.
Factors considered when determining if spraying would be feasible include: yield and profit potential, weather, variety resistance and fungicide cost. The key is to protect the flag leaf, which is the top contributor for grain fill, he said.
The only regions that may see potential damage that might not have been treated yet are the Rolling Plains and the High Plains, French said. However, the crop may be at a stage when fungicides may not be warranted. If a variety of wheat is resistant, there should be no reason to spray.
"One has to look at the PHI (pre-harvest intervals) for the fungicide being used and the growth stage of wheat when these fungicides can be applied," he said. "Some fungicides can be applied as far along as the beginning of flowering."
"Scout your fields on a regular basis, know what is going around your area, know what resistance is being reported for stripe rust and other diseases in that variety, be aware of weather conditions and consult with AgriLife Extension personnel in your area," French advised.
If the stripe rust comes after fungicides can no longer be applied, and there is no resistance to stripe rust, the worst-case scenario could result in losses between 40 percent and 50 percent of the yield in affected fields, he said. That is why plant resistance, not fungicides, is the first line of defense against stripe rust.
As of late April, French said, stripe rust has been observed throughout Texas except for the Panhandle. "We are just too dry, but if we get wet and keep cool (in the 50s and 60s) some wheat may still be at a vulnerable stage."