Exercise 1. Disease resistance
The uncontrolled epidemic on the potato variety with low resistance results in complete defoliation that terminates the season a few days earlier than normal and yields a net loss of profit. The following is the economic report found by clicking on Show Report in the Economics menu:
Season ended on September 23 with 100.0% blighted foliage. Your crop is 27016.49 kg./ha.
At the current market price of 0.11/kg this would bring 2971.81/hectare
31.89% of your tubers are blighted. YIELD REPORT
Yield
(kg) Revenue
Total Potential Yield: 33639 3700.29
Yield Losses due to Plant Blight: -6623 -728.48
Yield Losses due to Tuber Blight: -8616 -947.81
Net Yield: 18400 2024.01
SPRAY REPORT
Sprays Fungicide (kg)
Protectant: 0 0.0
Systemic: 0 0.0
COST REPORT
Fixed Costs: 2716.78
Spray Costs: 0.00
Application Costs: 0.00 Net Profit: -692.77/hectare
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A moderate level of resistance slows the epidemic somewhat but by itself does not control late blight well enough to produce a profitable yield:
Season ended on September 28 with 98.53% blighted foliage.
Your crop is 28391.29 kg./ha.
At the current market price of 0.11/kg this would bring 3123.04/hectare
27.45% of your tubers are blighted.
YIELD REPORT
Yield
(kg) Revenue
Total Potential Yield: 33639 3700.29
Yield Losses due to Plant Blight: -5248 -577.25
Yield Losses due to Tuber Blight: -7795 -857.41
Net Yield: 20597 2265.64
SPRAY REPORT
Sprays Fungicide (kg)
Protectant: 0 0.0
Systemic: 0 0.0
COST REPORT
Fixed Costs: 2716.78
Spray Costs: 0.00
Application Costs: 0.00
Net Profit: -451.15/hectare
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The potato variety with high resistance to late blight produces a profitable yield without any additional disease management measures, but, of course, it runs the risk of selecting a virulent population of Phytophthora infestans:
Season ended on September 28 with 37.18% blighted foliage.
Your crop is 32460.98 kg./ha.
At the current market price of 0.11/kg this would bring 3570.71/hectare
0.0% of your tubers are blighted.
YIELD REPORT
Yield
(kg) Revenue
Total Potential Yield: 33639 3700.29
Yield Losses due to Plant Blight: -1178 -129.58
Yield Losses due to Tuber Blight: -0 -0.00
Net Yield: 32461 3570.71
SPRAY REPORT
Sprays Fungicide (kg)
Protectant: 0 0.0
Systemic: 0 0.0
COST REPORT
Fixed Costs: 2716.78
Spray Costs: 0.00
Application Costs: 0.00
Net Profit: 853.93/hectare
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Exercise 2. Protectant Fungicides
Applying a protectant fungicide on a weekly schedule very effectively controls the late blight epidemic. Instead of a net loss, there is a substantial net profit, even higher than that with the highly resistant variety:
Season ended on September 28 with 3.77% blighted foliage.
Your crop is 33518.82 kg./ha.
At the current market price of 0.11/kg this would bring 3687.07/hectare
0.0% of your tubers are blighted.
YIELD REPORT
Yield
(kg) Revenue
Total Potential Yield: 33639 3700.29
Yield Losses due to Plant Blight: -120 -13.22
Yield Losses due to Tuber Blight: -0 -0.00
Net Yield: 33519 3687.07
SPRAY REPORT
Sprays Fungicide (kg)
Protectant: 8 10.0
Systemic: 0 0.0
COST REPORT
Fixed Costs: 2716.78
Spray Costs: 35.20
Application Costs: 79.04
Net Profit: 856.05/hectare
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Exercise 3. Systemic Fungicides
The cost per kg of the systemic was more than 8 times that of protectant, but because the dose of the systemic was very much lower and only two sprays were applied (versus eight for the protectant) the total cost for the systemic sprays was 32.85 as compared to 114.24 for the protectant spray program. Two sprays of the systemic fungicide produced nearly as effective control of the epidemic as eight sprays of the protectant:
Season ended on September 28 with 15.69% blighted foliage.
Your crop is 33245.78 kg./ha.
At the current market price of 0.11/kg this would bring 3657.04/hectare
0.0% of your tubers are blighted.
YIELD REPORT
Yield
(kg) Revenue
Total Potential Yield: 33639 3700.29
Yield Losses due to Plant Blight: -393 -43.25
Yield Losses due to Tuber Blight: -0 -0.00
Net Yield: 33246 3657.04
SPRAY REPORT
Sprays Fungicide (kg)
Protectant: 0 0.0
Systemic: 2 0.44
COST REPORT
Fixed Costs: 2716.78
Spray Costs: 13.09
Application Costs: 19.76
Net Profit: 907.41/hectare
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The use of the systemic was more profitable than the use of the protectant fungicide, largely because of the reduced application cost. However, there is a risk associated with the systemic. It is far more likely than the protectant fungicide to select a fungicide resistant population of Phytophthora infestans.
Exercise 4. Effects of weather
Under the hot, dry conditions, there was still a severe late blight epidemic with no fungicide applied, but it was substantially less severe than the epidemic under cool, wet conditions, and it even yielded a modest profit:
Season ended on September 28 with 70.7% blighted foliage.
Your crop is 31126.01 kg./ha.
At the current market price of 0.11/kg this would bring 3423.86/hectare
0.0% of your tubers are blighted.
YIELD REPORT
Yield
(kg) Revenue
Total Potential Yield: 33639 3700.29
Yield Losses due to Plant Blight: -2513 -276.43
Yield Losses due to Tuber Blight: -0 -0.00
Net Yield: 31126 3423.86
SPRAY REPORT
Sprays Fungicide (kg)
Protectant: 0 0.0
Systemic: 0 0.0
COST REPORT
Fixed Costs: 2716.78
Spray Costs: 0.00
Application Costs: 0.00
Net Profit: 707.08/hectare
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Applying 8 protectant sprays at 7-day intervals during hot, dry weather provides very effective control:
Season ended on September 28 with 1.21% blighted foliage.
Your crop is 33602.31 kg./ha.
At the current market price of 0.11/kg this would bring 3696.25/hectare
0.0% of your tubers are blighted.
YIELD REPORT
Yield
(kg) Revenue
Total Potential Yield: 33639 3700.29
Yield Losses due to Plant Blight: -37 -4.04
Yield Losses due to Tuber Blight: -0 -0.00
Net Yield: 33602 3696.25
SPRAY REPORT
Sprays Fungicide (kg)
Protectant: 8 10.0
Systemic: 0 0.0
COST REPORT
Fixed Costs: 2716.78
Spray Costs: 35.20
Application Costs: 79.04
Net Profit: 865.23
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Reducing the number of protectant sprays to a total of 6 at 10-day intervals reduces the spray cost and improves the profitability during hot, dry weather:
Season ended on September 28 with 5.26% blighted foliage.
Your crop is 33491.51 kg./ha.
At the current market price of 0.11/kg this would bring 3684.07/hectare
0.0% of your tubers are blighted.
YIELD REPORT
Yield
(kg) Revenue
Total Potential Yield: 33639 3700.29
Yield Losses due to Plant Blight: -147 -16.22
Yield Losses due to Tuber Blight: -0 -0.00
Net Yield: 33492 3684.07
SPRAY REPORT
Sprays Fungicide (kg)
Protectant: 6 7.5
Systemic: 0 0.0
COST REPORT
Fixed Costs: 2716.78
Spray Costs: 26.40
Application Costs: 59.28
Net Profit: 881.61
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Two systemic sprays during the hot, dry season also gives very effective control of late blight:
Season ended on September 28 with 1.86% blighted foliage.
Your crop is 33580.38 kg./ha.
At the current market price of 0.11/kg this would bring 3693.84/hectare
0.0% of your tubers are blighted.
YIELD REPORT
Yield
(kg) Revenue
Total Potential Yield: 33639 3700.29
Yield Losses due to Plant Blight: -59 -6.45
Yield Losses due to Tuber Blight: -0 -0.00
Net Yield: 33580 3693.84
SPRAY REPORT
Sprays Fungicide (kg)
Protectant: 0 0.0
Systemic: 2 0.44
COST REPORT
Fixed Costs: 2716.78
Spray Costs: 13.09
Application Costs: 19.76
Net Profit: 944.21
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During the hot, dry season, a single spray of the systemic fungicide also provided good late blight control, but its profitability was slightly less than that with two sprays:
Season ended on September 28 with 16.6% blighted foliage.
Your crop is 33236.86 kg./ha.
At the current market price of 0.11/kg this would bring 3656.05/hectare
0.0% of your tubers are blighted.
YIELD REPORT
Yield
(kg) Revenue
Total Potential Yield: 33639 3700.29
Yield Losses due to Plant Blight: -402 -44.24
Yield Losses due to Tuber Blight: -0 -0.00
Net Yield: 33237 3656.05
SPRAY REPORT
Sprays Fungicide (kg)
Protectant: 0 0.0
Systemic: 1 0.22
COST REPORT
Fixed Costs: 2716.78
Spray Costs: 6.54
Application Costs: 9.88
Net Profit: 922.85/hectare
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The two systemic sprays during hot, dry weather yielded a higher profit than they did during cool, wet weather (944.21/hectare versus 907.41/hectare).
Exercise 5. Disease thresholds
Under these conditions, late blight is first detectable on August 8. Starting the protectant sprays on August 9 and repeating on August 14, 19, 24, and 29 and September 3, 8, and 13, for a total of 8 sprays, we get the following results:
Season ended on September 28 with 83.23% blighted foliage.
Your crop is 27253.02 kg./ha.
At the current market price of 0.11/kg this would bring 2997.83/hectare
26.62% of your tubers are blighted.
YIELD REPORT
Yield
(kg) Revenue
Total Potential Yield: 33639 3700.29
Yield Losses due to Plant Blight: -6386 -702.46
Yield Losses due to Tuber Blight: -7254 -797.93
Net Yield: 19999 2199.90
SPRAY REPORT
Sprays Fungicide (kg)
Protectant: 8 10.0
Systemic: 0 0.0
COST REPORT
Fixed Costs: 2716.78
Spray Costs: 35.20
Application Costs: 79.04
Net Profit: -631.12/hectare
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Repeating this with the systemic fungicide, applied on August 9, 17, and 24, we get:
Season ended on September 28 with 71.3% blighted foliage.
Your crop is 30248.5 kg./ha.
At the current market price of 0.11/kg this would bring 3327.34/hectare
16.28% of your tubers are blighted.
YIELD REPORT
Yield
(kg) Revenue
Total Potential Yield: 33639 3700.29
Yield Losses due to Plant Blight: -3390 -372.95
Yield Losses due to Tuber Blight: -4925 -541.79
Net Yield: 25323 2785.54
SPRAY REPORT
Sprays Fungicide (kg)
Protectant: 0 0.0
Systemic: 3 0.66
COST REPORT
Fixed Costs: 2716.78
Spray Costs: 19.63
Application Costs: 29.64
Net Profit: 19.49/hectare
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It is clear that although it appears to start late, late blight is a rapidly developing epidemic, and the use of a threshold to start the spray program is not practical.
Exercise 6. Sanitation
Moving the cull pile to 100 meters from the field resulted in a significant delay in the development of the epidemic and reduced losses nearly as much as applying a protectant fungicide on a weekly schedule at a cost of over $100 per hectare:
Season ended on September 28 with 51.88% blighted foliage.
Your crop is 32124.2 kg./ha.
At the current market price of 0.11/kg this would bring 3533.66/hectare
0.0% of your tubers are blighted.
YIELD REPORT
Yield
(kg) Revenue
Total Potential Yield: 33639 3700.29
Yield Losses due to Plant Blight: -1515 -166.63
Yield Losses due to Tuber Blight: -0 -0.00
Net Yield: 32124 3533.66
SPRAY REPORT
Sprays Fungicide (kg)
Protectant: 0 0.0
Systemic: 0 0.0
COST REPORT
Fixed Costs: 2716.78
Spray Costs: 0.00
Application Costs: 0.00
Net Profit: 816.88/hectare
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Adding only 50 volunteer potato plants per hectare increases the level of initial inoculum and substantially advances the onset of the epidemic, resulting in a higher level of disease at the end of the season:
Season ended on September 28 with 99.75% blighted foliage.
Your crop is 26896.93 kg./ha.
At the current market price of 0.11/kg this would bring 2958.66/hectare
31.03% of your tubers are blighted.
YIELD REPORT
Yield
(kg) Revenue
Total Potential Yield: 33639 3700.29
Yield Losses due to Plant Blight: -6742 -741.63
Yield Losses due to Tuber Blight: -8346 -918.01
Net Yield: 18551 2040.65
SPRAY REPORT
Sprays Fungicide (kg)
Protectant: 0 0.0
Systemic: 0 0.0
COST REPORT
Fixed Costs: 2716.78
Spray Costs: 0.00
Application Costs: 0.00
Net Profit: -676.13/hectare
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Removing these volunteers would give you the outcome that we saw in the previous run, which we said was roughly equivalent to applying 8 sprays of protectant fungicide at at a cost of about $100/hectare.
Exercise 7. Certified Seed
With uncertified seed, the epidemic develops early in the absence of fungicides:
Season ended on September 25 with 100.0% blighted foliage.
Your crop is 27394.25 kg./ha.
At the current market price of 0.11/kg this would bring 3013.37/hectare
31.39% of your tubers are blighted.
YIELD REPORT
Yield
(kg) Revenue
Total Potential Yield: 33639 3700.29
Yield Losses due to Plant Blight: -6245 -686.92
Yield Losses due to Tuber Blight: -8600 -945.98
Net Yield: 18794 2067.38
SPRAY REPORT
Sprays Fungicide (kg)
Protectant: 0 0.0
Systemic: 0 0.0
COST REPORT
Fixed Costs: 2716.78
Spray Costs: 0.00
Application Costs: 0.00
Net Profit: -649.40/hectare
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With certified seed, the epidemic is sufficiently delayed that you get a respectable yield, even without the use of fungicides:
Season ended on September 28 with 37.05% blighted foliage.
Your crop is 32674.81 kg./ha.
At the current market price of 0.11/kg this would bring 3594.23/hectare
0.0% of your tubers are blighted.
YIELD REPORT
Yield
(kg) Revenue
Total Potential Yield: 33639 3700.29
Yield Losses due to Plant Blight: -964 -106.06
Yield Losses due to Tuber Blight: -0 -0.00
Net Yield: 32675 3594.23
SPRAY REPORT
Sprays Fungicide (kg)
Protectant: 0 0.0
Systemic: 0 0.0
COST REPORT
Fixed Costs: 2716.78
Spray Costs: 0.00
Application Costs: 0.00
Net Profit: 877.45/hectare
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If certified seed added $313/hectare to the production costs, the net profit without the use of fungicides would be $564.45/hectare. Of course, we would also want to apply fungicides, and using certified seed would allow a reduction in the fungicide cost, which we will see in the next exercise.
Exercise 8. Integrated Tactics
1. Uncontrolled epidemic
Season ended on September 21 with 100.0% blighted foliage.
Your crop is 25656.87 kg./ha.
At the current market price of 0.11/kg this would bring 2822.26/hectare
34.51% of your tubers are blighted.
YIELD REPORT
Yield
(kg) Revenue
Total Potential Yield: 33639 3700.29
Yield Losses due to Plant Blight: -7982 -878.03
Yield Losses due to Tuber Blight: -8854 -973.92
Net Yield: 16803 1848.34
SPRAY REPORT
Sprays Fungicide (kg)
Protectant: 0 0.0
Systemic: 0 0.0
COST REPORT
Fixed Costs: 2716.78
Spray Costs: 0.00
Application Costs: 0.00
Net Profit: -868.44
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This clearly is a "worst case" scenario, and no grower would let this happen.
2. Fungicide spray (a single systemic fungicide application on July 13)
Season ended on September 28 with 86.47% blighted foliage.
Your crop is 30161.72 kg./ha.
At the current market price of 0.11/kg this would bring 3317.79/hectare
14.08% of your tubers are blighted.
YIELD REPORT
Yield
(kg) Revenue
Total Potential Yield: 33639 3700.29
Yield Losses due to Plant Blight: -3477 -382.50
Yield Losses due to Tuber Blight: -4245 -467.00
Net Yield: 25916 2850.79
SPRAY REPORT
Sprays Fungicide (kg)
Protectant: 0 0.0
Systemic: 1 0.22
COST REPORT
Fixed Costs: 2716.78
Spray Costs: 6.54
Application Costs: 9.88
Net Profit: 117.59/hectare
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3. Partial resistance (a moderate level of late blight resistance plus a single systemic spray)
Season ended on September 28 with 55.32% blighted foliage.
Your crop is 31876.21 kg./ha.
At the current market price of 0.11/kg this would bring 3506.38/hectare
0.0% of your tubers are blighted.
YIELD REPORT
Yield
(kg) Revenue
Total Potential Yield: 33639 3700.29
Yield Losses due to Plant Blight: -1763 -193.91
Yield Losses due to Tuber Blight: -0 -0.00
Net Yield: 31876 3506.38
SPRAY REPORT
Sprays Fungicide (kg)
Protectant: 0 0.0
Systemic: 1 0.22
COST REPORT
Fixed Costs: 2716.78
Spray Costs: 6.54
Application Costs: 9.88
Net Profit: 773.18/hectare
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4. Moving cull pile (added to the above tactics, moving the cull pile from 10 meters away from the field to 100 meters away from the field)
Season ended on September 28 with 34.7% blighted foliage.
Your crop is 32680.84 kg./ha.
At the current market price of 0.11/kg this would bring 3594.89/hectare
0.0% of your tubers are blighted.
YIELD REPORT
Yield
(kg) Revenue
Total Potential Yield: 33639 3700.29
Yield Losses due to Plant Blight: -958 -105.40
Yield Losses due to Tuber Blight: -0 -0.00
Net Yield: 32681 3594.89
SPRAY REPORT
Sprays Fungicide (kg)
Protectant: 0 0.0
Systemic: 1 0.22
COST REPORT
Fixed Costs: 2716.78
Spray Costs: 6.54
Application Costs: 9.88
Net Profit: 861.69/hectare
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5. Removal of volunteers (added to all of the above tactics, removal of the volunteer potatoes prior to planting)
Season ended on September 28 with 26.16% blighted foliage.
Your crop is 32953.81 kg./ha.
At the current market price of 0.11/kg this would bring 3624.92/hectare
0.0% of your tubers are blighted.
YIELD REPORT
Yield
(kg) Revenue
Total Potential Yield: 33639 3700.29
Yield Losses due to Plant Blight: -685 -75.37
Yield Losses due to Tuber Blight: -0 -0.00
Net Yield: 32954 3624.92
SPRAY REPORT
Sprays Fungicide (kg)
Protectant: 0 0.0
Systemic: 1 0.22
COST REPORT
Fixed Costs: 2716.78
Spray Costs: 6.54
Application Costs: 9.88
Net Profit: 891.72
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Comparing the net profit to that of the previous run, where the volunteers were not removed, the marginal return is about $30/hectare, so we could invest up to that amount in the removal of volunteers.
6. Certified seed (added to the above tactics, using certified seed)
The complete program of integrated tactics clearly gives the best disease control, with only about 1% of the foliage blighted at the end of the season.
Season ended on September 28 with 0.91% blighted foliage.
Your crop is 33618.34 kg./ha.
At the current market price of 0.11/kg this would bring 3698.02/hectare
0.0% of your tubers are blighted.
YIELD REPORT
Yield
(kg) Revenue
Total Potential Yield: 33639 3700.29
Yield Losses due to Plant Blight: -21 -2.27
Yield Losses due to Tuber Blight: -0 -0.00
Net Yield: 33618 3698.02
SPRAY REPORT
Sprays Fungicide (kg)
Protectant: 0 0.0
Systemic: 1 0.22
COST REPORT
Fixed Costs: 2716.78
Spray Costs: 6.54
Application Costs: 9.88
Net Profit: 964.82
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However, if you subtract the $313/hectare cost of the certified seed, the net profit comes out to be $651.82/hectare. Under these circumstances, the marginal return on the investment in certified seed is less than the marginal cost and is thus not economically justified for the control of late blight alone. But late blight control is only a small side-benefit of certified seed; its primary purpose is the control of potato viruses. The certified seed cost should really be added to the "fixed costs" with respect to late blight control and should have been included in the profit calculations of all of the exercises.
Applying 4 systemic sprays at 14-day intervals beginning on June 29 will give you nearly the same level of disease control as the entire integrated program, but the profits are lower because of the increased fungicide and application costs. The main reason, however, for not using 4 systemic sprays is the risk of selecting a fungicide-resistant Phytophthora infestans.
Season ended on September 28 with 1.1% blighted foliage.
Your crop is 33596.75 kg./ha.
At the current market price of 0.11/kg this would bring 3695.64/hectare
0.0% of your tubers are blighted.
YIELD REPORT
Yield
(kg) Revenue
Total Potential Yield: 33639 3700.29
Yield Losses due to Plant Blight: -42 -4.65
Yield Losses due to Tuber Blight: -0 -0.00
Net Yield: 33597 3695.64
SPRAY REPORT
Sprays Fungicide (kg)
Protectant: 0 0.0
Systemic: 4 0.88
COST REPORT
Fixed Costs: 2716.78
Spray Costs: 26.17
Application Costs: 39.52
Net Profit: 913.17/hectare
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