The problem of root-knot nematodes is getting a harder look in corn production, says Austin Hagan, plant pathologist with the Alabama Cooperative Extension System.
“I've been looking for a few years now at root-knot nematodes in cotton. Traditionally, root-knot was not thought to be a yield-limiting factor in corn — but it is now. We've shown that if you do a summer/fall nematode soil assay and you come up with 100 larvae per 100 cc's of soil, that is the economic damage threshold where you start losing yield in corn. It's a fairly low count, but at that level we're seeing a 3 percent to 11 percent yield reduction from cotton root nematodes in corn,” he says.
A variety trial was conducted this past year where plots were treated with Counter soil insecticide, says Hagan. Over the 13 varieties, there was a 12.5-bushel yield increase where there was a lot of root-knot nematode activity.
“Also in these trials, we monitored the rate of reproduction of the nematodes on the corn varieties. Basically, all of the varieties were good hosts for that nematode. We're not only seeing a problem on corn with root-knot nematodes reproducing and suppressing yields, but we're also setting ourselves up if you come back with cotton the following year. The nematodes will be there, and they will be knocking down cotton yields. And the yield losses we see in cotton are far more dramatic than the ones we see in corn.
“In cotton, we have fewer control options for this nematode. Actually, the only non-host crop for sure is peanuts. Some soybeans also have root-knot resistance. So if you have a problem, that might be another way to go,” he says.
To insure disease control in corn, Hagan says it's best to plant early. “If you do that, you won't have rust, and you won't have northern corn leaf blight and Southern corn leaf blight. Also, grow a good variety.”
If you plant corn after wheat, “all bets are off” as far as disease pressure goes, he says.
In some parts of the United States, fungicides are required in corn production, says Hagan. “Thankfully, they don't do that here. In some cases, lenders are requiring it. The only time we've really seen a sizeable yield response on corn is in Baldwin County near the Gulf Coast when rust gets on the crop. We did see about a seven to eight-bushel gain in a test this past spring where we used Headline and had no disease activity. But at current prices, that won't pay for the fungicide.
“Again, if you plant late corn, rust becomes an issue. In those situations, fungicides have not been working well. It is recommended that they be applied after silking, and they don't do a good job when they're put out that late,” says Hagan.
If you have sandy soils, it's important to go ahead and sample for nematodes, says Leonard Kuykendall, regional Extension agent for central Alabama. “On your soil tests, it has never been as important. In conservation-tillage programs, take the top 3 inches, and do that annually. In conventional tillage, go down as far as you plow — 5 to 6 inches. If you have fields that you're not sure about or new fields, take the top 6 inches. Or, if you have fields that haven't been fertilized in a while, take a sample from the top 2 to 3 inches, and then go down in the same hole and take one from 3 to 6 inches,” he says.
Now, more than ever, growers need to know what they have so they won't spend more money than is necessary.