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​Strawberry Anthracnose: Managing a Hidden Menace

THE CASE

Part A

Jack's farm​

Jack O’Neil owns Sunny Patch Farm, located near Des Moines, Iowa, with 15 acres in strawberry production. Most of his income is from customers and their families who come out to Sunny Patch Farm to pick their own berries (Figure 1) and buy Jack’s homemade jams and pies. In addition, Jack sells pre-picked strawberries twice a week at the Des Moines Farmers Market.

Figure 1. Pick-your-own customers at Sunny Patch Farm.

Strawberry is one of the most productive and high-value crops in the world, but it must be managed carefully to suppress diseases and insect pests that can damage the crop. Jack’s main defense against strawberry diseases is spraying numerous applications of synthetic chemical fungicides on the crop.

The strawberry harvest comes in a 4-week-long period during late May and early June. He harvests the planting in years 2 to 5, and then rotates to another crop on that site for 3 years while harvesting strawberries from additional fields. This “matted row” system maintains berry production while reducing the risk of spreading diseases – including anthracnose fruit rot - from one strawberry planting to the next. Because Jack farms in a cold climate, he covers his strawberries with 4 inches of oat straw each year in late November to avoid freeze damage to the plants during the winter months. In April, he moves some of the straw off the plants and places it between the rows, but also packs straw around the strawberry plants in order to reduce the risk of soil splashing onto the berries. The income and costs for Jack’s farm are estimated and listed in Table 1.

Table 1. Sunny Patch Farm’s annual income and costs (per acre).  

Anthracnose fruit rot: an insidious enemy.

The disease that scares Jack the most is anthracnose fruit rot (AFR). The fungus that causes AFR, Colletotrichum acutatum, is notorious for its sneaky behavior. Unlike most fungi that cause crop diseases, this one can hang around on leaves and stems for a long time without showing any symptoms (symptoms = visible effects of a disease on plants). But when the weather becomes warm and rainy, C. acutatum suddenly can attack ripening fruit, causing sunken, brown spots that make the fruit unfit for sale (Figure 2) (Louws et al. n.d.). An AFR epidemic can spell disaster for a commercial farm like Jack’s: more than half of the fruit can be ruined.

Figure 2. Anthracnose fruit rot on ripe strawberry.
(Reprinted, by permission, from Louws et al., 2014)

Twenty years ago, shortly after Jack had started his strawberry business, he was hit by an AFR epidemic. Walking his fields one day after a thunderstorm, just as the harvest period was beginning, he noticed a few fruit with sunken, brown spots (Figure 2). The very next day, the damaged fruit were five times more numerous. Despite immediately starting to fight back with fungicide sprays, he ended up losing more than 70% of his crop to AFR. He was even forced to suspend pick-your-own activities and farmers-market sales that year, disappointing scores of his customers. He worried they might not return to his farm next year or just buy non-local strawberries in the grocery store.

In addition to taking a serious financial hit, Jack was bewildered by the suddenness of the AFR outbreak and how badly he had failed to stop it. So he sought advice from Dr. Nancy Muller, a plant pathologist at Iowa State University and an expert on strawberry diseases. Nancy told Jack about AFR’s ability to hide in plain sight: its microscopic spores, called conidia, can spread rapidly across a field during warm, rainy weather, establishing invisible infections on plants. Once fruit began to ripen, further rainy weather could cause innumerable fruit infections and a crop failure like the one Jack had experienced.

Figure 3. Conidium (C₁) and appressorium(A) attached to the leaf surface produce a new conidium (C₂).
(Reprinted, by permission, from Leandro et al., 2001)

The fungus that causes AFR, Colletotrichum acutatum, is a hitchhiker. It can stick to nursery plants with its own natural glue and a special survival structure called an appressorium (Figure 3, A), which is too small to see with the naked eye, and then move hundreds of miles in a shipment of young plants to Jack’s farm. Once the plants start growing, warmth and rainfall wake up the fungus and it uses its asexual spores, called conidia (Figure 3, C), which move in raindrops, to spread to other plants. Twenty years ago, when Jack noticed the fruit rot symptoms it was already game over— too late for him to control the disease. In the aftermath of that disaster, Nancy suggested that Jack spray fungicides on his fields every 10 days during and after the flowering period to keep AFR under control.

Since then, Jack has followed Nancy’s spray advice to the letter. He has never had another AFR epidemic, but sometimes wonders whether it’s necessary to apply so many fungicide sprays.

Current disease management strategies at Sunny Patch Farm

Every year, Sunny Patch Farm purchases strawberry crowns (small plants with roots and leaves attached) from nurseries in other states. Jack picks strawberry varieties that grow well in Iowa and whose fruit have excellent flavor, uniformly red color, and high yields, such as Honeoye, Jewel and Kent. However, all of these varieties are susceptible to AFR, so fungicide sprays must be part of his AFR management program.

When rainfall and dew during the flowering and fruit ripening period keep plants wet for more than 12 hours at a time and average temperature ranges from 77o F (25 C) to 86o F (30 C), C. acutatum will spread through a strawberry field, infect leaves and fruit invisibly, and eventually cause AFR symptoms. In Iowa, these disease-favorable conditions occur in some years but not others; in other words, they are sporadic. But when disease-favorable conditions do occur, and especially if they are prolonged, AFR can become a raging epidemic unless Jack uses fungicide sprays. Because he cannot predict weather, he views each year as a potential AFR outbreak. He sees fungicide as “cheap insurance,” since fungicide spraying is much less expensive for him than enduring an AFR epidemic. In a typical growing season, he applies five sprays against AFR.

PART A QUESTIONS:

1. What characteristics of the fungus Colletotrichum acutatum can cause anthracnose fruit rot (AFR) to appear so suddenly in a field?
   

2. What are the roles of conidia and appressoria in spreading the disease?
   

3. What are suitable weather conditions for AFR to develop?
   

4. 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?
   

5. 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?
   
   

   b. What is the net income of Jack’s farm when it has 70% yield loss due to an AFR outbreak?

   *Net income = Gross income - total cost
   * During an AFR epidemic, assume that Jack’s labor cost is reduced by 50% due to loss of harvestable yield.