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Aster yellows or seed maggot?

Friday, December 1, 2000
filed under: Planting Systems

Aster yellows or seed maggot?

Experts plan to monitor symptoms for accurate diagnosis in 2001

During bloom of the 2000 growing season, producers and agronomists alike noticed some odd-looking, misshapen sunflower, some with ray petals growing in the middle of the head, usually in a pie-shaped wedge formation.

So what caused these “petal heads”? Sunflower experts believe either aster yellows or seed maggot activity—or both.

Aster yellows is a minor disease (one of the few diseases caused by a mycoplasma-like organism, not a fungus) that can be transmitted to flax, canola, sunflower, carrots, potatoes, and other plants, primarily spread by six-spotted leafhoppers that migrate from the south. Aster yellows can result in head sterility, misshapen heads, and misplaced ray petal formation on the head.

The sunflower seed maggot causes more localized seed sterility when newly hatched larvae tunnel into the corolla of young blooms.

Mycogen agronomist Bruce Due believes both pests were present in the Northern Plains sunflower growing area in 2000, with aster yellows more prevalent north of I-94, and the seed maggot more prevalent south of I-94.

Due says aster yellows was a problem in canola last year, resulting in some plants that failed to set pods. Sunflower is usually a secondary host, but when infected, it can result in sterile heads, though if infection occurs later, it’s possible only part of the head is affected.

However, he noticed different symptoms in sunflower fields south of I-94, where there is also less canola. Bract damage, a crease of scar tissue on the head, sometimes (but not usually) accompanied by the misplaced ray petals—is more characteristic of seed maggot activity, says Due, adding that he also noticed fly activity on some sunflower heads in southeast North Dakota earlier in the growing season.

He notes two more differences in symptoms south of I-94 compared to north of I-94. More of the damage south was hybrid specific, while damage north was generally more random, regardless of hybrid. “The maggot is pedigree sensitive, with damage that can differ by hybrid. So far that we know, aster yellows is not that specific,” Due says. Further, some heads were absent in affected fields south of I-94. Heavy seed maggot pressure can destroy the developing sunflower bud, but there is nothing Due says he can find in published literature that indicates aster yellows will prevent head development.

In canola, aster yellows is generally localized in field margins, corresponding with activity of the primary carrier, the six-spotted leafhopper, which generally affect field margins. In sunflower, symptoms are spread out more across a field, so that muddies the picture further, on whether aster yellows or the seed maggot is to blame.

“I’m not an entomologist, and I’m not a pathologist. I’m not going to swear on a stack of bibles, but I think we’ve got two things going on here,” says Due.

Agronomists have generally ruled out other causes. It’s not boron deficiency, says Due, because that results in heads breaking from the stalks. “It’s not bud worm activity, because that creates more of a tunnel-shaped scar tissue in the sunflower head. It’s not the seed or self-compatability, that just forms blank seed, when the plant doesn’t accept its own pollen,” says Duane Berglund, North Dakota State University extension agronomist.

Since the problem this year was sporadic and affected only a small area of sunflower heads, there was little damage, says Tom Gulya, USDA plant pathologist, Fargo, N.D. Yield losses from both aster yellows and the seed maggot are usually minor, with treatment generally not recommended. Due agrees that both are minor pests that are not treatable and in fact, he says he probably won’t even refer to either problem in grower meetings this winter, unless someone brings the subject up. Still, the symptoms are enough of a concern that they need to be monitored. “We don’t need another pest problem in sunflower,” says Due.

Crop rotation and breeding more tolerant hybrids would be about the best answers available to address aster yellows or seed maggots in sunflower, if they become more prevalent. Pinpointing which is causing problems where is the first step needed, however.

Symptoms were noticed too late in the 2000 growing season for lab analysis to accurately determine whether damage was aster yellows, says Gulya. Agronomists will be watching for the problem more closely next year. If it reappears, affected heads will be analyzed in the diagnostics lab to determine the true cause of the problem.

Due also plans to monitor the problem more closely next season, and tag heads where there is maggot activity, and watch symptoms develop. “It has everybody’s attention now. I think by the end of the next growing season, we’ll have more answers.” – Tracy Sayler

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