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Disease incidence in corn was light, with a bit of rust

Watersoaked leaf lesions of Gossrsquos wilt above Below dark green to black spots sometimes called freckles near the edges of expanding lesions  mdash Disease photos by Clayton Hollier
<p>Water-soaked leaf lesions of Goss&rsquo;s wilt, above. Below, dark green to black spots, sometimes called freckles, near the edges of expanding lesions. &mdash; <em>Disease photos by Clayton Hollier</em></p>
&ldquo;Southern corn rust doesn&rsquo;t need a lot of rain to develop. The fungus blows in and it lands where it lands &mdash; and if conditions are right, it develops. That&rsquo;s the situation we had this year.&rdquo;

A generally cooler spring and summer, with fairly frequent rainfall, may have made outside work more pleasant for farmers, but it likely didn’t have a profound effect on disease pressure in corn.

Throughout the Mid-South, plant pathologists reported light disease pressure in corn, and most of that came from southern corn rust.

Corn fields infected with the disease were documented in Mississippi, southern Florida, Georgia and Louisiana.

While 2014 was cooler and wetter than normal, it wasn’t really a lot cooler or a lot wetter, says LSU AgCenter Plant Pathologist Clayton Hollier.

“Southern corn rust is a warm-season pathogen,” he says, “so even though the weather was cooler, it was still hot enough for rust to develop. We also had more rain events than normal, but in the Baton Rouge area, they were not always high rainfall events.

“In many cases, we received one-quarter inch or less in each rainfall event. Southern corn rust doesn’t need a lot of rain to develop. The fungus blows in and it lands where it lands, and if conditions are right, it develops. That’s the situation we had this year.”

Moderate yield losses

In five years of disease plot work, Hollier says, he’s seen moderate yield losses each year, but only two years stand out due to high yield losses caused by southern corn rust — 2010 and 2014.

In naturally infected, non-irrigated plots, he saw as much as a 19-percent yield loss in 2014.

There were also increased reports of southern corn rust this year, especially in the southern half of Louisiana. But it didn’t appear to affect yields as negatively in those areas as in Hollier’s disease plots.

Overall, he estimates that the disease caused a yield loss of less than 5 percent for the state’s commercial corn acres.

Other than southern corn rust, Hollier says there was little to no disease pressure in corn. “We had some areas where northern corn leaf blight popped up more than is normal, but it wasn’t a real concern,”

Another bright spot in 2014 was the absence of Goss’s wilt in Louisiana corn. The bacterial disease was observed for the first time in northeast Louisiana during the 2013 growing season.

Rotation recommended

“Goss’s wilt popped up in a good number of our fields in 2013,” Hollier says. “It scared us, and we were quite anxious to see what would happen in 2014, but we didn’t see any this season.

“After the 2013 season, we recommended that affected growers rotate their fields out of corn and into soybeans. We also buried the residue so it would break down. It worked perfectly, because Goss’s wilt doesn’t affect soybeans.”

To further decrease disease pressure in soybeans, he recommends growers plant on time, and plant as early as is possible in the optimum-planting window.

“When we can get in early, we don’t escape late season problems altogether, but we certainly see less of an impact from late season diseases,” he says.

In the Mississippi Delta, disease pressure in corn was relatively low overall, according to Cleveland, Miss., independent crop consultant Justin George, The cooler, wetter year didn’t appear to negatively impact disease development in corn, he says.

Disease pressure was very low in his area, with the exception of a few late-season spots of common rust and southern rust.

“The rust was found very late in the season and did not require treatment,” George says. “I understand there was some late corn, planted after April 10, that had to be treated in our area.”

No preventive treatment

He noticed some corn earworm feeding in non-Bt corn that produced several different types of toxins and molds.

“There is no preventive treatment for this,” he says. “It may be taxing some of the kernels a bit, but we don’t know for sure. We do know that it’s generally not economical to treat for corn earworms.”

In recent years, northern corn leaf blight has been found at treatable levels in many of the fields George scouts, but that wasn’t the case this year.

“I didn’t see any northern corn leaf blight during the season,” he says. “It was really a calm year. We didn’t treat many corn acres with fungicides.”

George also attributes the decrease in fungicide treatments to lower corn prices. “This year, despite high yield potential, more farmers were not agreeable to preventive treatments because of lower crop prices,” he says.

“A lot of my growers would like to produce corn for rotational purposes, even in a break-even scenario, but $3 corn is not break-even. I don’t know that we’ll have as much corn in 2015 if prices don’t come up substantially.”

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