Septoria leaf spot can cause significant defoliation in blueberries.
Septoria leaf spot can cause significant defoliation in blueberries.

Fish extracts have long been touted as nutrient enhancers in organic production.

But now researchers at the University of Georgia have shown the products also can suppress leaf diseases and pests in organic blueberries, according to a news release.

Harald Scherm, a university plant pathologist, led a research team that compared four fish extracts and one or two biological fungicides at three on-farm trials on rabbiteye and southern highbush blueberries.

Four of the six products—Omega Grow, Organic Gem, Organocide, and SeaCide—suppressed Septoria leaf spot and leaf rust, two common diseases in organic blueberries.

But the effects on leaf beetle and foliar fertility were less clear.

"The impact of the fish products on the leaf beetle was inconsistent, and the products did show some benefits in boosting foliar nutrition, but more so for plants growing in low-fertility sites than for plants growing in soils with higher fertility rates," Scherm said in the release.

Organocide and SeaCide improved plant vigor and leaf retention in a low-vigor site in 2009 but not in a high-vigor site in 2010, according to the research report.

None of the treatments translated to higher flower bud set for the next growing season.

Across all three trials, Organic Gem resulted in higher sodium concentrations in leaf tissue.

Although the levels did not appear to be detrimental, Scherm says it would be advisable to apply the product only in rotations to minimize any potential negative effects.

In two of the three trials, some products increased phosphorus concentrations, providing a potential nutritional benefit, according to the research report.

Scherm's work reinforces what research literature had already reported—fish extracts don't directly control disease pathogens. Instead, they boost defense mechanisms in the plant.

Septoria leaf spot strikes late in the season, typically after harvest.

Many growers may brush it off.

But Scherm says they need to pay attention because it can cause significant defoliation. Without leaves, photosynthetis can't occur.

And photosynthetis is responsible for converting light to energy, which will affect flower buds that are formed for next season's crop.

Scherm's work was funded by a two-year $119,000 grand from Southern Sustainable Agriculture Research & Education.