While certainly unwelcome, the arrival of Asian soybean rust has had a peripheral benefit for row crops.
“ASR has led to more fungicides being introduced and developed in the United States,” said Rick Cartwright, Arkansas Extension plant pathologist at the recent Southeast Branch Station field day in Rohwer, Ark. “We haven't had a lot of good ones until relatively recently.”
Some of the newer products will likely be available for rice as well as soybeans. To get a “head start,” Cartwright and fellow plant pathologist Cliff Coker have many tests set up around the state. The studies are checking not only the new products' effectiveness but also any phytotoxicity.
“One of the things we use in disease management for all crops is genetic resistance. It may not offer total control of a disease, but it's pretty handy. Even a bit of genetic resistance can make other management practices, including fungicides, perform much better
“We do these evaluations both on-station and off in cooperation with our county Extension agents. We pay special attention to those at this time of year to get a feel for what the new varieties offer — not only in yield and quality potential but in disease resistance potential.”
The major Arkansas rice diseases, especially in the southeast, tend to be…
Sheath blight on the semi-dwarfs and long-grains;
Blast on various varieties, especially in areas where water management is difficult. “Blast ‘holes’ can occur on sandy soils, percolating soils, areas surrounded by trees.”
Smuts have turned into a major problem. “Some of the high-yielding rice varieties released in the 1980s and 1990s are managed with a lot of nitrogen and are very susceptible to the smuts. That disease became major trouble in the 1990s.”
Genetic resistance is the primary weapon in the battle with blast.
“Right now, one of our highest yielding varieties is Francis. On good soils, Francis is hard to beat. Because it's very susceptible on blast soils, you must be careful where it's planted. We are using Francis very successfully on 10 percent, or so, of the acres in the state.”
Another variety similar to Francis, Banks, was released several years ago. Banks illustrates well what can happen when there's only a single resistance gene in a plant.
“It provided good control of blast races that were common at the time of its release. But it's also easy for the fungus to change. Over time, that fungus finds a way to do damage.
“So Banks started out resistant and now it's susceptible. Remember that. Even though Banks was (a University of Arkansas release), I'm tired of looking at neck blast on it.”
Other varieties resistant to blast include several from LSU like Clearfield 131 and CL161, Cheniere, Cocodrie and Trenasse. The University of Arkansas also has Cybonnet.
“Of all those, Cybonnet has the best blast resistance, by far. It has multi-gene blast resistance but it's very susceptible to sheath blight, so there's a trade-off.
All the LSU varieties behave similarly to blast in the field.
“If you look, you'll find some leaf lesions, but the disease doesn't progress to neck rot. I have confidence in the LSU varieties. With good management, I believe you can grow them even in blast situations without much fear.”
However, as a general rule, “if you've got a blast problem the hybrid rice is best. If you can afford them and you've got sandy soils, they're the best choice. Cybonnet is next.”
With respect to sheath blight, all semi-dwarf long grains are susceptible. One that seems less so is Cheniere.
“I don't know what the reason is, but with normal management, Cheniere seems to have fewer problems with sheath blight. We still have to spray it, but it's rated between Cocodrie and some of the taller varieties. There are always exceptions, but normally we can use fewer fungicides on Cheniere.”
Over the last few years, growers have learned the hard way that highly susceptible semi-dwarf long grains need to be scouted closely for sheath blight from mid-season on.
“If you let sheath blight blow out of the top of those rices, you've lost 15 to 20 bushels you won't get back. That means those varieties need earlier and more frequent scouting. They also potentially need to be treated earlier than taller varieties.”
Cartwright hasn't seen any results from spraying hybrids with a fungicide.
“Some folks spray them a little. Frankly, I've yet to measure a yield response from a fungicide. That's true in both commercial fields and my own plots.”
Cartwright summed up his fungicide test work this way: in a long-time rice production field in a semi-dwarf long grain (including Cocodrie, CL131 and Cybonnet), “we're getting a benefits from a fungicide.
“We see a benefit 40 percent of the time taller varieties like Francis and Wells are sprayed. That means such varieties need intense scouting and good decision making.”
Medium grains (Medark, Jupiter, Bengal) and hybrids in the tests almost never get a response from fungicides.
“Sometimes the medium grains will break even. That happens 10 percent of the time. On the hybrids, that percentage is much less — we may have broken even one time. On those varieties, just because you've got a lot of sheath blight low means nothing to yield. The disease has to get up higher in the plant to really do any damage.”
What about Wells and smut?
“Wells isn't one we've had particular trouble with in kernel smut. It's one that is moderately susceptible.”
There are preventive compounds, though.
“Because we can't scout for or predict the smuts, if you're growing a susceptible variety in a field that normally turns your combines black, we recommend a fungicide treatment. That's at least during the boot stages. So there are two things to consider: a susceptible variety along with a history of intense smut.”
If the main tillers in your field are in the swollen leaf stage but aren't heading yet, Cartwright said 4 ounces of Tilt — or the equivalent of another product — per acre is a consistent treatment for smut.
“If you get any earlier than that — say, when the panicles are just about to start into boot — the 6-ounce rate seems to provide more consistency.
“Spray earlier than that and inconsistency sets in. We've used 8 ounces and 10 ounces up to a week after mid-season. But even those high rates will fail if applied really early. If I was going in before any boot formation, I'd use the highest rate and hope for the best. Most of the time, you'll get activity — it just isn't a definite.”