Warning systems are not new. Over the last 50 years, they have been developed by researchers and extension workers. Most warning systems use weather data to predict the risk of outbreaks of plant diseases. The approach works because many disease-causing fungi and bacteria respond sensitively to weather conditions.

Based on 5 years of field trials at Iowa State University’s Horticulture Research Station (link here), we developed a new warning system to control the fungal disease sooty blotch and flyspeck (SBFS). This system counts up the number of hours at or above 90% relative humidity since the first-cover fungicide spray was applied. From there, it delays applying the second-cover spray until this total reaches a threshold of 385 accumulated hours. Once the second-cover fungicide spray goes on, fungicide spray timing changes to a calendar-timed basis (7- to 14-day intervals) for the rest of the season.

At the ISU Hort Farm, this new SBFS warning system saved about 2 ½ fungicide sprays, on average, per season by delaying the second-cover fungicide spray, and controlled SBFS on fruit as well as using the calendar to time the second-cover spray. But how well would this system work in commercial orchards in Iowa?

In 2020 (the first year of our 3-year USDA-CPPM project), on-farm trials were located in two commercial farms in central Iowa: Deal’s Orchard near Jefferson, and Center Grove Orchard near Cambridge. Each trial divided part of a row into two groups of trees: a “control” group that would receive fungicide sprays only on calendar-based timing, and another group where spray timing was determined by the warning system.

The 2020 growing season in Iowa was exceptionally dry. But if you live in Iowa, you may recall something called a derecho. This high-powered (100 mph) windstorm arrived on August 10. An hour later, a large percentage of the apples were on the ground, and even some trees blew over. However, we were able to gather some data from the apples that remained on the trees (Figure 2).

Using the SBFS warning system resulted in savings of 3 fungicide sprays at Deal’s Orchard and 2 sprays at Center Grove Orchard. No SBFS-blemished apples appeared in any of the treatments. This spray savings is consistent with the summer fungicide-spray savings we found in the ISU Hort Farm trials over a 5-year period.  Percent of marketable fruit at harvest (Figure 2) was reduced dramatically by the August 10 superstorm.

One thing to keep in mind when using the SBFS warning system is that it concerns timing of fungicides only. Insecticide sprays may be necessary if scouting thresholds for insect pests are reached during the period between first- and second-cover fungicide sprays.

The 2020 season was challenging for all Iowa growers. Despite the obstacles, our on-farm demonstration trials did show that growers can save several fungicide sprays per season by using the new SBFS warning system while protecting fruit quality. We’ll work with the growers to test the warning system again in 2021 and 2022 - hopefully without derechos!!

We’d like to thank Steve Black (Center Grove Orchard) and Jerald Deal (Deal’s Orchard) for their patience and cooperation with us on these trials.