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Management of Apple Scab: Simulation with Applescab

Applescab Exercises
Answer Sheet for Instructors

Exercise 1. Calendar Sprays

The students' results will show considerable variability in the timing of sprays and in the total number of applications, and therefore the effectiveness of their spray programs will vary substantially. The key to a successful protectant program is applying the first two sprays on time to prevent early infections. The first must go on at green tip despite the light drizzle that occurs at green tip in the default weather. The second must be applied 7 days later (half-inch green). The subsequent sprays are important, of course, but the timing is not as critical.

The following is an economic report from a simulation with good control during a moderately rainy season, using only 8 applications of Protectan.

The season ended on October 5 with 115.69 scab lesions per tree. Your crop is 799.65 bu./acre. At the current fresh market price of 6.0 this would bring 4797.93/acre. However, 1.08% of your apples are infected. FRESH FRUIT YIELD REPORT Severity Price Yield Revenue /lb. bu. Total None 6.0 791.01 4746.06 Light 4.5 8.64 38.90 Medium 3.0 0.0 0.00 Severe 1.5 0.0 0.00 Total: 799.65 4784.96 Quality losses due to scab: 12.97 Yield losses due to scab: 2.07 Total losses due to scab: 15.04 SPRAY REPORT Fungicide Sprays Cost Protectan 8.0 122.40 Eradican 0.0 0.00 Combocide 0.0 0.00 COST REPORT Fixed Costs: 450.00 Harvest Costs: 879.62 Fungicide Costs: 122.40 Application Costs: 360.00 Total Costs: 1812.02 NET RETURN: 2972.94

The major advantage of a calendar-based protectant fungicide spray program is its simplicity and therefore its ease of management. Calendar programs also tend to be rather conservative and provide effective control even in a rainy season.

The major drawback of calendar-based programs is that they over-control apple scab during dry seasons, making unnecessary fungicide applications, which wastes money and contributes unnecessarily to environmental pollution. It may also be difficult to adhere to a strictly protectant spray program, particularly during a very rainy season, when day after day of rain may prevent the timely application of a protectant spray.


Exercise 2. After-infection Sprays

Again the students' results will show considerable variability, depending on how many infection periods they tried to cover with a single Eradican spray and how many days after an infection period they waited before spraying.

The following is an economic report from a simulation with good control during a season with moderate rainfall, using 6 applications of Eradican.

The season ended on October 5 with 434.69 scab lesions per tree. Your crop is 799.3 bu./acre. At the current fresh market price of 6.0 this would bring 4795.79/acre. However, 3.94% of your apples are infected. FRESH FRUIT YIELD REPORT Severity Price Yield Revenue /bu. bu. Total None 6.0 767.77 4606.61 Light 4.5 31.48 141.66 Medium 3.0 0.05 0.15 Severe 1.5 0.0 0.00 Total: 799.3 4748.42 Quality losses due to scab: 47.37 Yield losses due to scab: 4.21 Total losses due to scab: 51.58 SPRAY REPORT Fungicide Sprays Cost Protectan 0.0 0.00 Eradican 6.0 182.40 Combocide 0.0 0.00 COST REPORT Fixed Costs: 450.00 Harvest Costs: 879.23 Fungicide Costs: 182.40 Application Costs: 270.00 Total Costs: 1781.63 NET RETURN: 2966.79

During the season with moderate temperatures and dry weather, there were fewer infection periods than in the rainy season, but there were several infection periods without rain (as the result of dew) and several eradicant sprays are still required.

The following shows the results of a simulation with just 5 sprays of Eradican.

The season ended on October 5 with 582.42 scab lesions per tree. Your crop is 798.7 bu./acre. At the current fresh market price of 6.0 this would bring 4792.18/acre. However, 5.22% of your apples are infected. FRESH FRUIT YIELD REPORT Severity Price Yield Revenue /bu. bu. Total None 6.0 757.04 4542.24 Light 4.5 41.54 186.94 Medium 3.0 0.11 0.34 Severe 1.5 0.0 0.00 Total: 798.7 4729.52 Quality losses due to scab: 62.65 Yield losses due to scab: 7.82 Total losses due to scab: 70.48 SPRAY REPORT Fungicide Sprays Cost Protectan 0.0 0.00 Eradican 5.0 152.00 Combocide 0.0 0.00 COST REPORT Fixed Costs: 450.00 Harvest Costs: 878.57 Fungicide Costs: 152.00 Application Costs: 225.00 Total Costs: 1705.57 NET RETURN: 3023.95

The major advantage of an after-infection program is that it allows a grower to reduce the total number of sprays compared to a calendar protectant program, particularly during a dry season (6 eradicant sprays during a rainy season and 5 sprays during a dry season compared to 8 protectant sprays on a calendar program). The major drawback of the after-infection program is the difficulty of management. A grower has to monitor the weather and determine the infection periods and then keep track of time after the infection periods. Because it may continue to rain after an infection period, it frequently is not possible to apply the eradicant fungicide within the required time. This adds considerably to the stress of management.


Exercise 3. Multi-season Management

The results in the first year, with 10¹⁰ ascospores per acre as initial inoculum and 8 applications of Protectan at roughly weekly intervals during a moderately rainy season, are given above in Exercise 1. The effective apple scab control during this season reduced the initial inoculum in the second year to 2.0 x 10⁸ ascospores per acre, which allowed us to maintain effective control with only 5 applications of Protectan.

The season ended on October 5 with 33.93 scab lesions per tree. Your crop is 799.61 bu./acre. At the current fresh market price of 6.0 this would bring 4797.64/acre. However, 0.32% of your apples are infected. FRESH FRUIT YIELD REPORT Severity Price Yield Revenue /lb. bu. Total None 6.0 797.05 4782.31 Light 4.5 2.55 11.50 Medium 3.0 0.0 0.00 Severe 1.5 0.0 0.00 Total: 799.61 4793.81 Quality losses due to scab: 3.83 Yield losses due to scab: 0.29 Total losses due to scab: 4.12 SPRAY REPORT Fungicide Sprays Cost Protectan 5.0 76.50 Eradican 0.0 0.00 Combocide 0.0 0.00 COST REPORT Fixed Costs: 450.00 Harvest Costs: 879.57 Fungicide Costs: 76.50 Application Costs: 225.00 Total Costs: 1631.07 NET RETURN: 3162.74

Because of the very effective apple scab control in the second year, the initial inoculum in the third year had been reduced to 5.8 x 10⁷ ascospores per acre (assuming no ascospores blown in from neighboring orchards), and we were able to control apple scab with only 2 applications of Protectan.

The season ended on October 5 with 76.38 scab lesions per tree. Your crop is 799.36 bu./acre. At the current fresh market price of 6.0 this would bring 4796.15/acre. However, 0.72% of your apples are infected. FRESH FRUIT YIELD REPORT Severity Price Yield Revenue /lb. bu. Total None 6.0 793.63 4761.79 Light 4.5 5.73 25.77 Medium 3.0 0.0 0.00 Severe 1.5 0.0 0.00 Total: 799.36 4787.56 Quality losses due to scab: 8.59 Yield losses due to scab: 1.49 Total losses due to scab: 10.08 SPRAY REPORT Fungicide Sprays Cost Protectan 2.0 30.60 Eradican 0.0 0.00 Combocide 0.0 0.00 COST REPORT Fixed Costs: 450.00 Harvest Costs: 879.29 Fungicide Costs: 30.60 Application Costs: 90.00 Total Costs: 1449.89 NET RETURN: 3337.67

Good control of apple scab results in a dramatic reduction in the numbers of ascospores available for dissemination the following season. This reduction in initial inoculum allows the grower to reduce the number of fungicide sprays and still achieve adequate apple scab control.