by Courtney Llewellyn

There are more than 100 different hosts for sooty blotch and flyspeck, two different diseases that commonly occur together on the same fruit. Their spores move by wind and rain, and thus, humid conditions favor sooty blotch and flyspeck (SBFS).

SBFS causes a discoloration or blemish of near-mature fruit. While the discoloration is superficial, and neither disease damages the fruit, the presence of the diseases reduces the grade and market value of the fruit. Jose Gonzalez-Acuna, a research technician at Iowa State University, has been researching a new humidity-based model for SBFS in apples using organic fungicides. The goal is spraying when the relative humidity warrants it.

Gonzalez-Acuna said right now there are two warning systems for SBFS: wetting hours (rain) and a relative humidity model (created in Iowa in 2017). Both focused on that critical second fungicide spray.

Redfree, Liberty and Gold Rush apple varieties were tested in the Iowa trial. They used an organic fungicide spray program, with 1% horticultural oil, Cueva (copper octanoate or copper soap) sprayed through pink and Cueva and Double Nickel 55 (Bacillus amyloliquefaciens) sprayed starting at bloom. The trials featured one control plot with just one cover spray; one plot on a 10- to 14-day calendar spray schedule; and plots using the warning systems at 185, 285 and 385 hours of relative humidity above 90%.

With only year of data so far, the researchers saved four fungicide sprays between the calendar schedule and waiting for the 385 hours of high relative humidity. “There were no significant differences in marketable yield” between the two schedules, according to Gonzalez-Acuna, and “no differences in Redfree and Liberty.” Gold Rush, however, had disease outbreak in the control and 185-hour plots.

He noted that there is no genetic resistance to SBFS, but growers are less likely to see it on earlier ripening varieties versus those that ripen later.

Gonzalez-Acuna and his team will be repeating these trials in 2022. They are looking to determine the best threshold for relative humidity-prompted sprays and to replicate their tests in different regions and with different cultivars.