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New information shows how crop diseases get head start

Tom J. Bechman rows of corn in the field
DISEASE AT WORK: Recent reports by BASF indicate that fungi which invade corn leaves can be at work two weeks before symptoms appear. If so, this corn could already be infected.
Study using an electron microscope indicates disease starts affecting plants several days before visual symptoms show up.

A debate continues over whether the best method for deciding on fungicide applications on crops is to scout and monitor disease — the wait-and-see approach — or to plan on applying fungicides in advance. Proponents of Integrated Pest Management support the wait-and-see method. Farmers with larger acreages and some industry folks contend planning ahead and lining up product and applicators is the practical solution today.

Recently, BASF, makers of Veltyma fungicide for corn and Revytek fungicide for soybeans, injected a different point of view into the debate. Spokespersons contend that even if you’re waiting to see symptoms, disease already has a head start in the field.

“We want growers to understand what the latency period is and why it’s important,” says Kimberley Tutor, technical marketing manager for plant health, working with corn and wheat fungicides.

The latency period is the gap between when the disease initiates infection in the plant and when the first visual symptoms appear, she explains. It’s about 15 days for gray leaf spot in corn, 14 to 20 days for tar spot in corn and seven to 10 days for frogeye leaf spot in soybeans.

“Disease organisms are at work during the latency period, already disrupting normal activities within leaves,” Tutor says. “The result can be more stress on plants, and it can affect yield.”

How disease works

BASF grew corn plants inside a greenhouse and inoculated leaves with gray leaf spot. At different intervals after inoculation, they examined leaf tissue under an electron microscope. Tutor explains that the electron microscope images illustrate clearly how infection begins. By day 5 after infection, the fungus begins moving through tiny stomata on leaves, which are openings that allow materials in and out. By day 9, infection inside the leaf disrupts normal leaf activity.

The fungus also produces a toxin that causes portions of leaf cell tissue to begin collapsing. While you may notice tiny lesions by day 12 if you look closely, typical small lesions that you might observe during casual scouting don’t appear until day 15.

“That’s over two weeks from when the infection actually began,” Tutor says. “By day 19 in the greenhouse study, the fungus grew back out through stomata onto the leaf, ready to begin reproduction. It produces spores which can spread the disease to other plants.”

What it means

BASF recommendations note that if you’re only going to apply fungicide once in corn, tasseling to R1 is a good target. “It’s still the sweet spot,” Tutor says.

However, it you’re in an area where tar spot has a foothold, you might consider applying earlier, she says. Some growers may want to apply as early as V10 for tar spot. One strategy is to apply around the V12 to V14 stage, then come back around R3.

Tutor says when applied properly, BASF corn fungicides do not result in arrested ear syndrome, regardless of time of application. However, if you’re adding adjuvants, she emphasizes checking with the manufacturer to make sure the product you intend to apply is safe for corn at the stage you want to apply.

TAGS: Fungicide
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