Xiaoyu Zhanga, Mark L. Gleasona, Gail R. NonneckeB and Natalia A. PeresC
a
Department of Plant Pathology and Microbiology, Iowa State University
B
Department of Horticulture, Iowa State University
C
Department of Plant Pathology, Gulf Coast Research and Education Center, University of Florida
Zhang, X., Gleason, M., Nonnecke, G., and Peres, N. 2015. Strawberry anthracnose: managing a hidden menace.
The Plant Health Instructor. 10.1094/PHI-T-2015-1015-01
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ANSWERS TO PART A
What characteristics of the fungus
Colletotrichum acutatum can cause anthracnose fruit rot (AFR) to appear so suddenly in a field?
ANSWER: Microscopic spores (conidia) of the pathogen can stick to nursery plants and hang around on plant leaves and stems for a long time without showing any symptoms, so the fungus can spread widely in a strawberry field without being noticed. When fruit ripen, the fungus can suddenly cause sunken, brown spots that ruin the fruit.
What are the roles of conidia and appressoria in spreading the disease?
ANSWER: Conidia are the asexual spores of the fungus. They can multiply and spread in splashing water and infect new plants. Appressoria are survival structures that form quickly on plant surfaces from germinated conidia and enable the fungus to withstand stresses from ultraviolet light, unfavorable temperatures, drying out, and attack by other microorganisms.
What are suitable weather conditions for AFR to develop?
ANSWER: Warm and rainy weather between the flowering period and the start of harvest favors outbreaks of AFR.
What strategies is Jack currently using for AFR control (including fungicides and all the cultural practices)? How does each of these strategies reduce the threat of an AFR outbreak?
ANSWER: Jack uses the following methods for AFR control: 1) fungicide spraying, which can kill
C. acutatum; 2) rotating individual fields out of strawberries for 3 years, which will lessen survival of the fungus in that field and thereby help reduce the risk of transmitting AFR from one strawberry planting to a later one; and 3) placing a layer of straw mulch around the strawberry plants to reduce the risk of the pathogen splashing from plant to plant.
a. What is the net income of Sunny Patch Farm when Jack has 70% of his field for U-Pick and 30% for pre-picked strawberry?
ANSWER:
Net income ($ per acre) = (70% U-pick net income + 30% Pre-picked net income)
= 0.7 × (10,000 – 4,000 – 1,000 – 1,000 - 110) + 0.3 × (12,000 – 6,000 – 1,000 – 1,200 - 110)
= 2,723 + 1,107 = 3,830
Net income whole farm ($) = 3,830 × 15 = 57,450
b. What is the net income of Jack’s farm when it has 70% yield loss due to an AFR outbreak?
ANSWER:
Net income ($ per acre) = (100% - 70%) × (70% Gross income of U-pick + 30% Gross income of Pre-picked) - 70% (50% Labor of U-pick + other cost) - 30% (50% Labor of Pre-picked + other cost)
= (1-0.7) × (0.7 × 10,000 + 0.3 × 12,000) - 0.7 × (0.5 × 4,000 + 1,000 + 1,000 + 110) – 0.3 × (0.5 × 6,000 + 1,000 + 1,200 + 110)
= 3,180 – 2,877 – 1,593 = -1,290
Net income whole farm ($) = -1,290 × 15 = -19,350
ANSWERS TO PART B
Describe how the AFR warning system works.
ANSWER: The AFR warning system inputs measurements of wetness duration (total number of hours per day that the plants are wet) and air temperature, collected on the farm, to calculate the risk of an AFR outbreak, using an equation developed from studies of
C. acutatum biology. The weather data can be collected with a commercial weather station that includes sensors to track the weather conditions and a datalogger to record the weather data. A built-in computer chip runs the equation, converting the weather data into an AFR risk rating and issuing a recommendation each day about whether or not to apply a fungicide spray.
Based on the disease triangle concept:
Are there strategies that Jack could use to keep
C. acutatum from getting into his fields?
ANSWER: The AFR warning system inputs measurements of wetness duration (total number of hours per day that the plants are wet) and air temperature, collected on the farm, to calculate the risk of an AFR outbreak, using an equation developed from studies of
C. acutatum biology. The weather data can be collected with a commercial weather station that includes sensors to track the weather conditions and a datalogger to record the weather data. A built-in computer chip runs the equation, converting the weather data into an AFR risk rating and issuing a recommendation each day about whether or not to apply a fungicide spray.
If the fungi were confirmed NOT to be present in the field, would it make sense to spray fungicides against AFR anyway? Why or why not?
ANSWER: If tests do not detect the pathogen in a production field, it is likely that AFR won’t develop there, since one component of the Disease Triangle – the pathogen - is absent. In that case, fungicide sprays would not be needed. But suppose the pathogen WAS present in the field, but at too low a concentration for testing to detect – in other words, below the limit of detection of his test method; should Jack still spray fungicide even though his testing results were negative? Additional factors that may affect Jack’s decision include the degree to which the current weather conditions favor AFR outbreaks, the level of genetic resistance of his strawberry varieties to AFR, and his customers’ reluctance to buy heavily sprayed strawberries.
Based on the information presented in the case:
Would you use the warning system if you were Jack O’Neil?
ANSWER: You could use the warning system to reduce fungicide sprays for the following reasons: meet your customers’ demands for reducing pesticide use; save some money on sprays (about $40 per acre per spray); reduce the risk of fungicide resistance; and protect the environment on your farm by reducing fungicide exposure of pesticide applicators, consumers, and other organisms (for example, pollinators, natural enemies of pest insects, fish, and soil microorganisms).
If yes, how can he make sure that the warning system is working in his field?
ANSWER: Since using the warning system means a significant change in the way Jack manages the AFR threat, it would be a good idea to start small. Trying out the operation of the warning system on a small field for a few years would help him to gain confidence in the new system without risking large losses. If the system works reliably for a few years on the small field, its use can be expanded to more production fields. To meet customers’ demands, Jack could first expand the warning system to pick-your-own strawberry fields (where customers have frequent contact with the plants and fruit) and later to pre-picked fields.
What are possible problems associated with using it?
ANSWER: There are two main risks associated with using the warning system: 1) malfunctions with the weather-monitoring equipment could cause you to miss a critical period for applying a fungicide spray; and 2) prolonged rainy weather can make it difficult to respond to a spray warning in a timely way, even after a warning has been issued.
What additional information would be useful to help you decide?
ANSWER: Before applying the warning system in his farm, he needs to know: 1) how many sprays, on average, he can expect to save with the warning system; 2) whether the weather monitoring system is reliable; 3) where to set up the equipment to collect the weather data on his farm; and 4) how much money he can expect to save (including labor for each management option).
How will using the warning system help Jack to reduce the risk of resistance development?
ANSWER: A disease-warning system can save fungicide sprays, which reduces the risk of resistance development because it lessens the exposure of the fungus to the fungicide. In turn, less exposure means less selective pressure that would favor the predominance of resistant individuals of
C. acutatum on the farm. P>
If Jack decides not to use the warning system, how can he meet his customers’ demands that he cut back on fungicide use?
ANSWER: If he decides not to use the warning system, Jack can try alternative tactics to reduce the risk of AFR. If he uses AFR-resistant varieties, he could potentially need fewer fungicide sprays. But will his customers like the berries from the resistant varieties as well as those from the susceptible varieties he currently plants? Will they grow as well on his farm as the varieties he now grows?
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