On-farm trials in Iowa, 2021: performance of a relative humidity-based warning system for sooty blotch and flyspeck
![]() |
![]() |
![]() |
Summary
Two on-farm trials took place in commercial orchards in central Iowa in 2021. Both demonstration experiments had 12 trees per treatment. Two treatments were tested: “control” and “warning system.” The warning system used relative humidity data to predict the risk of sooty blotch & flyspeck (abbreviated SBFS). The control treatment received the standard spray schedule used by each grower, which was based on calendar-timed spraying. The warning system treatment started once the first cover spray had been applied: the second fungicide cover spray was withheld until a total of 385 hours of relative humidity above 90% after first cover was reached. At both trials, in a relatively dry summer, 3 fungicide sprays were saved when using the warning system, with no decrease in marketable yield. In trial 1, the incidence of marketable fruit in the control treatment was 81% compared to 93% for the warning system treatment. In trial 2, the rate of marketable fruit in the control treatment was 87% and in the warning system treatment was 69%; but the main cause of damage in the warning system trees was not SBFS, but rather insect damage – mainly codling moth and plum curculio. No major SBFS or other disease symptoms were observed that could be related to the use of the warning system. Conclusion: with the SBFS-warning system, 3 fungicide sprays were saved per season, without having an impact on fruit quality.
![]() |
![]() |
TRIAL 1
Site: Jefferson, IA
Trees per treatment: 12
Type of trees: 20-year-old ‘Golden Delicious’ trees on semi-dwarfing rootstock.
Treatments applied
- Control. The trees under the control treatment followed the farm’s standard spray program.
- Sooty blotch and flyspeck warning system. These trees followed a regular spray program through the first cover spray. Additional fungicide sprays were withheld until a threshold of 385 accumulated hours of relative humidity (RH) above 90% was reached. However, the second fungicide cover spray was actually applied after a total of 628 accumulated hours of RH above 90% (August 9th) after first cover spray (May 26th).
- Note: the trees in the warning system treatment received the same insecticide and springtime disease (mainly scab) sprays as the control treatment.
Assessment
One week before harvest, the number of marketable apples per tree was rated. The rating was done by arbitrarily selecting and examining 40 fruit from different parts of the trees. Fruit were considered non-marketable if they had sunken spots or peel damage, imperfections exceeding 5% of the total surface of the apple, or were misshapen. Non-marketable fruit were classified by category: insect damage, rots, russeting, or others. Fruits with mechanical damage, bird pecks, or other sources of damage were grouped in the “others” category.
Results


Average incidence of marketable fruit was significantly higher in the warning system treatment than in the control treatment (figure 2). This suggests that although 3 fewer fungicide sprays were applied in the warning system treatment, the marketable yield washigher. Rots were close to 2% total in the warning system treatment, which was higher than in the control trees (figure 3). We suspected that these rots were caused by insect, physical, or bird damage to the peel that then developed into a rot. On average, there was significant more fruit with russeting (figure 3) in the control trees than in the warning system trees. The fruit with russeting on their skin (figure 4) were located at parts of the trees’ canopy with high exposure to sunlight.
![]() |
![]() |
The application of the second-cover fungicide spray was delayed by 2 months in the warning system trees (August 8) compared to the control trees (June 7). Insect damage was due to feeding and egg-laying by plum curculio, codling moth, and others.
TRIAL 2
Site: Cambridge, IA
Trees per treatment: 12
Type of trees: 20-year-old Golden Delicious on semi-dwarfing rootstock.
Treatments applied
- Control. The trees under the control treatment followed the farm’s standard spray program.
- Sooty blotch and flyspeck warning system. These trees followed a regular spray program through the first cover fungicide spray (May 15). After that, fungicide applications were planned to be withheld until a threshold of 385 accumulated hours of relative humidity (RH) above 90% was reached. The second fungicide cover spray was actually applied at 343 hours of RH above 90% (July 24). The recommended threshold was not reached because it was decided to lower the risk of fruit rots; therefore, a cutoff was called a bit earlier.
- Note: the trees on the warning system treatment received the same insecticide and foliar sprays as the control trees.
Assessment
Same as in experiment 1.


In contrast to trial 1, the average incidence of marketable fruit was significantly higher in the control treatment compared to the warning system treatment (figure 6). However, the main cause of this difference was not SBFS or other diseases, but rather insect damage (figure 7A). A total of 3 fewer fungicide sprays were applied in the warning system treatment than in the control treatment.
![]() |
![]() |
Main take-aways:
- 2021 was a dry year for Iowa, which caused conditions of low relative humidity through most of the spring and summer.
- In each on-farm trial, the SBFS warning system saved 3 fungicide sprays compared to the standard calendar-based spray program, with no loss of control of any disease.
- The on-farm trials will be repeated in 2022.