The unknown malady plaguing a small portion of Arkansas' rice crop has been identified as hydrogen sulfide toxicity.
“When we began working on this mystery it had a lot of similarities to hydrogen sulfide toxicity (where soil, under flood, becomes toxic through the production of hydrogen sulfide gas),” says Rick Cartwright, Extension plant pathologist. “There were some new aspects, however, that threw us off the chase a little.”
First, many of the fields didn't have a history of the problem. However, the main spanner in the deductive works was internal crown rot.
“We'd never seen crown rot associated with hydrogen sulfide — nor has anyone else I've talked to. However, as we worked on it this summer, we began receiving reports on high sulfates around wells in some of the problem fields. That got our attention.”
When Cartwright and colleagues first saw the problem, they speculated that Grandstand herbicide was involved. “That was the only common denominator in the fields we studied early,” he said. “As research progressed, though, it became apparent that Grandstand was not involved — nor was any other herbicide. It was just a coincidence.”
A few weeks ago, searching for answers, Cartwright and Chuck Wilson, Arkansas Extension rice specialist, traveled to a northeast Arkansas field they'd worked a decade ago. The field had suffered true hydrogen sulfide toxicity, says Cartwright — the water even stank of sulfur.
“When we got involved 10 years ago, the roots in that field would rot off early in the season. We suggested that the water be pulled off so air could get to the roots. That's an old-time remedy. Anyway, the crop grew back and made it to harvest.”
Even though growers in the area hadn't been draining and drying to prevent straighthead, “we suggested they start doing so. Before reproductive stages, just dry the soil a little, let new roots grow out and flood again. That doesn't cure the problem forever, but it interrupts the toxicity factor.”
Revisiting the old problem field, the men dug up rice plants and thoroughly checked them. Sure enough, the field's crop had gotten sick again, and the insides of the crowns were beginning to rot.
“From that, and based on the symptoms and sulfate levels being reported in some of the fields, we were very suspicious that it was hydrogen sulfide toxicity. We went in a big circle and were misled by what we'd seen earlier.”
The crown rot was a new symptom, says Cartwright, probably because “we weren't looking at the rice late enough in the season. Also, the fields from a decade ago were obviously suffering from an abundance of sulfur. That isn't the case in many of the newly affected fields.”
Studies on this will continue. There's still “an organism or two” that may be aggravating the problem.
“We're searching for every solution — a production system twist, a soil remedy or something else,” says Cartwright. “Some of the wells on the Grand Prairie are at such low capacity, draining and drying isn't an option. If they drain those fields, flooding them back up isn't an option. We need to find an answer for those growers, too.”
At some level, Cartwright has seen the problem in 15 or 16 east Arkansas rice counties.
“I've found it by just stopping at a field that looks suspicious from the road. It mostly occurs closer to wells, but that isn't always the case — I've found it far from wells, too. Whatever the case, this seems to be an increasing problem for us.”