On-farm performance of a relative humidity-based warning system for sooty blotch and flyspeck in Golden Delicious and Granny Smith

November 30, 2022
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Content Author:
Jose Gonzalez Mark Gleason

Summary

Two on-farm trials took place in commercial orchards in central Iowa in 2022. Both demonstration experiments had 10 trees per treatment. Two treatments were tested: “grower standard” 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, mostly based on calendar-timed sprayings. The warning system treatment started once the 1st cover spray had been applied: the 2nd fungicide cover spray was withheld until a total of 385 hours of relative humidity above 90% after 1st cover or an 8-week after 1st cover spray was reached. Just like in 2021, at both trials, in a relatively dry summer, 3 fungicide sprays were saved when using the warning system, with no evidence of a decrease in marketable yield. In trial 1, the incidence of marketable fruit in the control treatment was 77% compared to 79% for the warning system treatment. In trial 2, the rate of marketable fruit in the control treatment was 97% and in the warning system treatment was 94%. ‘Site 1’ farm had high incidence of woolly aphid. No major SBFS or other disease symptoms were observed that could be related to the reduction of fungicide spray. We conclude that with the SBFS-warning system, 3 fungicide sprays were saved, without having an impact on fruit quality.

Figure 1. Relative humidity sensor/datalogger installed at site 2 in spring 2022 (left); ‘Granny Smith’ apple trees one week before harvest (right) at trial site 2.
sensor farm

 

map1

Trial 1

Site: Jefferson, IA

Trees per treatment: 10

Type of trees: mature ‘Golden Delicious’ trees on semi-dwarfing rootstock.

Treatments applied

  • Grower standard (control). The trees followed the farm’s standard full-spray program.
  • Sooty blotch and flyspeck warning system. The trees followed a regular spray program through first cover spray. Additional fungicide sprays were withheld until a cut-off date of 8-weeks after 1st cover spray was reached, as the spray threshold of 385 accumulated hours of relative humidity (RH) above 90% was not reached during this time period.
    • Note: the trees in the warning system treatment received the same insecticide and springtime disease (mainly scab) sprays just 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

fig 2
Figure 2. Average percentage of marketable fruit in on-farm demonstration trial site 1, 2022.
Fig 3
Figure 3. Average percentage of non-marketable fruits per tree at trial 1, 2022.

Average incidence of marketable fruit was very similar in both treatments: 77% for control treatment and 79% for the warning system treatment (figure 2). Fruit damage was mainly due to woolly aphids (figure 3 and 4). This suggests that although 3 fewer fungicide sprays were applied in the warning system treatment, the marketable yield was equivalent.

Figure 4. Woolly aphid damage on shoot and fruit at trial site 1.
woolly aphid woolly aphid

Application of the second-cover fungicide spray was delayed by 6 weeks in the warning system trees (July 29th) compared to the control trees (June 16th).

 

Trial 2map

Site: Cambridge, IA

Trees per treatment: 10

Type of trees: mature Granny Smith on dwarfing rootstock and high-density cropping system.

Treatments applied

  • Grower standard (control). The trees followed the farm’s standard full-spray program.Sooty blotch and flyspeck warning system. The trees followed a regular spray program through first cover spray. Additional fungicide sprays were withheld until a cut-off date of 8-weeks after 1st cover spray was reached, as the spray threshold of 385 accumulated hours of relative humidity (RH) above 90% was not reached during this time period.
    • Note: the trees in the warning system treatment received the same insecticide and springtime disease (mainly scab) sprays just as the control treatment.

Assessment

Same as in experiment 1.

fig 6
Figure 6. Average marketable yield (%) per tree with standard error bars (with 95% confidence) one week before harvest in trial site 2, 2022.
Fig 7
Figure 7. Average non-marketable yield (%) per tree with standard error bars (with 95% confidence) one week before harvest in trial site 2, 2022.

Average incidence of marketable fruit was higher than in trial site 1, with 97% for control treatment and 94% for the warning system treatment (figure 6). The main cause for non-marketability was insect damage – mostly due to plum curculio but at a very low incidence ~3% (figure 7). Just like in trial site 1, this suggests that although 3 fewer fungicide sprays were applied in the warning system treatment, the marketable yield was equivalent.

Main takeaways            

  • Two cultivars assessed: Granny Smith and Golden Delicious, with very similar results.
  • 2022 was the 3rd dry year in a row for Iowa, which caused conditions of low relative humidity through most of the spring and summer – thus, low disease pressure was observed.
  • 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.