Aaron Cato – Extension Specialist – Horticulture IPM, University of Arkansas
Do Old Adages Still Make Sense?
Strawberries grown in the Southeast incur significant losses from fruit rot caused by fungal pathogens. Without proactive cultural practices and an effective spray program, more than 80% of strawberry flowers can be lost to Botrytis fruit rot when environmental conditions are favorable. Botrytis fruit rot primarily damages strawberry fruit when spores infect mature flowers, although symptoms may appear throughout fruit maturation. Due to the impact of Botrytis fruit rot across the region, fungicide spray programs are often designed to protect flowers from fungal spores that cause this disease. Infection of green or red fruit generally only occurs when physical or water damage occurs, or if fruit are touching active mold growth. In fact, recommendations for fungicide protection programs generally inform growers to stop sprays once flowers are not expected to be fruited, generally 30 days prior to the last harvest date.
While Botrytis is considered the most damaging disease of harvested strawberries, in recent years the incidence and impact of anthracnose fruit rot has increased (Figure 1). When warm and wet conditions are present during harvest in the Southeast, anthracnose fruit rot is commonly the most damaging fruit rot disease observed. Spores from the fungus Colleotrichum nymphaeae, the main causal organism of anthracnose fruit rot, can infect many parts of the strawberry plant including flowers, leaves, etc. However, the infection of green and red fruit is largely responsible for the economic losses caused by this fungus. This is in stark contrast to Botrytis fruit rot, which is primarily the result of infected flowers. Recent issues with anthracnose fruit rot are likely due to the high incidence of infected strawberry plants coming from nurseries. When paired with recent excessively warm (77-80°F) and wet springs that favor the spread and infection of spores, excessive losses due to anthracnose fruit rot become an inevitability.
Due to the recent impact of anthracnose fruit rot many growers have questioned the old adages that dictate our protectant fungicide spray programs. One common question is, “When do fungicide sprays need to be terminated when anthracnose is likely to be a major issue?” Although much is known about conditions that drive anthracnose fruit rot problems, the benefit of spraying through the last picking vs. stopping 2-4 weeks prior had not yet been explored, especially outside of more tropical areas of the Southeast. The objective of our trial was to determine the yield impact of terminating foliar fungicide applications zero, two, or four weeks prior to the last week of harvest.
Figure 1.
Anthracnose fruit rot observed on strawberry fruit.
Methods
Strawberry plants of the cultivar ‘Chandler’ were established in a plasticulture production system in Kibler, Arkansas at the University of Arkansas System Division of Agriculture Vegetable Research Station on September 30th in 2020 and September 23rd in 2021. Strawberry plugs were established in two offset rows per bed, plants were arranged in plots of 12 plants and each plot on a row was separated by five ft of empty buffer. Strawberry plots were arranged in randomized complete block design with replications on four different rows separated by 12 ft to prevent drift. A fungicide protection program was administered to each plot and stopped at three separate termination timings: full spray schedule through last week of harvest, two weeks prior to last week of harvest, and one month prior to last week of harvest (when no blooms would be kept to harvest).
Fungicide programs were initiated each season when 5-10% of plants had flowers that would develop into fruit and were made on a weekly basis (Table 1). All applications were made with a CO2 backpack sprayer at 75 GPA. Fungicide programs were terminated for each treatment based on expected harvest dates and included 4-5 weekly applications for treatment 3 (termination one month prior to last week of harvest), 6-7 applications for treatment 2 (termination two weeks prior to last week of harvest), and 8-9 weekly applications for treatment 3 (termination the last week of harvest) (Table 1). Weekly applications were attempted, but rain events often increased or decreased spray intervals. The number of weeks sprayed varied from 2021-2022, as more hot rain events in 2022 warranted earlier and tighter spray intervals. In general, more frequent warm, windy rain events occurred in 2022 compared to 2021, generating higher levels of disease pressure (Table 1).
Table 1. Fungicide spray program applied to strawberry plants of three fungicide termination treatments based on timing from last expected harvest in the spring of 2021 and 2022.
| 2021 Spring – Low Pressure Year | |||
| Date | Fungicide | FRAC Group | Treatments Sprayed* |
| 3/22 | Luna Sensation (7.6oz) | 7 + 11 | 1 2 3 |
| 3/29 | Captan (3.75lb) + Elevate (1.5lb) | M04 + 17 | 1 2 3 |
| 4/6 | Switch (14 oz) | 12 + 9 | 1 2 3 |
| 4/12 | Captan (3.75lb) + Elevate (1.5lb) | M04 + 17 | 1 2 3 |
| 4/21 | Luna Sensation (7.6oz) | 7 + 11 | 1 2 |
| 4/27 | Switch (14oz) + Captan (3.75lb) | 12 + 9 + M04 | 1 2 |
| 5/5 | Captan (3.75lb) | M04 | 1 |
| 5/12 | Luna Sensation (7.6oz) | 7 + 11 | 1 |
| 2023 Spring – High Pressure Year | |||
| Date | Fungicide | FRAC Group | |
| 3/16 | Switch (14oz) + Captan (3.75lb) | 12 + 9 + M04 | 1 2 3 |
| 3/24 | Captan (3.75lb) + Elevate (1.5lb) | M04 + 17 | 1 2 3 |
| 4/1 | Switch (14 oz) | 12 + 9 | 1 2 3 |
| 4/7 | Luna Sensation (7.6oz) | 7 + 11 | 1 2 3 |
| 4/15 | Captan (3.75lb) + Elevate (1.5lb) | M04 + 17 | 1 2 3 |
| 4/21 | Switch (14 oz) | 12 + 9 | 1 2 |
| 4/26 | Luna Sensation (7.6oz) | 7 + 11 | 1 2 |
| 5/3 | Switch (14oz) + Captan (3.75lb) | 12 + 9 + M04 | 1 |
| 5/11 | Luna Sensation (7.6oz) | 7 + 11 | 1 |
Ripe strawberry fruit were harvested twice per week from each plot to assess yield. Harvest began when the first ripe fruit could be picked and continued until fruit quality began to deteriorate due to environmental conditions. In total, 10 harvests occurred in 2021 for the first harvest season, starting April 15th and ending on May 20th. In 2022, plots were harvested 11 times, starting on April 14th and ending on May 19th. Berries were separated into marketable and cull fruit at harvest and then weighed to determine marketable and cull yield in each plot. Cumulative yield per plot across all harvest dates were considered for analysis, and data were corrected for the number of plants that were harvested from each plot. Marketable yield (fruit weight) and percent cull per plant (percentage of cull yield from sum of marketable and cull) were used for analysis. The percent of fruit culled due to anthracnose fruit rot was also assessed to determine the relative impact of anthracnose in 2022 only. Yield data were converted from pounds harvest per plant to yield per acre assuming a plant population of 15000 plants per acre.
Results
The impact of fungicide termination was found to be pronounced in 2022 while minimal impact was observed in 2021 (Table 2; Figure 2). In 2021, no difference was observed in marketable yield or percent of yield that was culled when considering the three fungicide termination timings (Table 2). However, ~3300 less pounds of marketable fruit per acre were observed in 2022 when terminating the fungicide spray program either 2 or 4 weeks prior to the last week of harvest when compared to spraying through the last week. Yields across the board were significantly impacted by hot and wet rains as a large percentage of culls were observed. No significant difference in yield lost to culled fruit was observed in 2022 when considering termination treatment. However, a larger portion of those culled fruit were observed to be due to anthracnose fruit rot in 2022.
Table 2. Estimation of marketable yield, percent of total yield lost to culled fruit, and percent of culled fruit exhibiting anthracnose fruit rot for strawberries with three fungicide termination timings in 2021 and 2022.
| Year | Termination Timing* | Marketable Yield (lbs/acre)* | Loss in Marketable Yield | Yield lost to culls (%) | Culls due to Anthracnose (%) |
| 2021 | Full Schedule | 15347 a | 33.3 a | . | |
| Two Weeks Prior | 15045 a | -302 | 30.5 a | . | |
| One Month Prior | 15008 a | -339 | 40.4 a | . | |
| 2022 | Full Schedule | 8168 a | 53.0 a | 34.2 b | |
| Two Weeks Prior | 4859 b | -3309 | 68.5 a | 51.2 a | |
| One Month Prior | 4754 b | -3414 | 66.5 a | 50.0 a | |
| Combined 2021-2022 | Full Schedule | 11758 a | 41.1 a | . | |
| Two Weeks Prior | 9953 b | -1805 | 45.6 ab | . | |
| One Month Prior | 9882 b | -1876 | 50.9 b | . |
When looking across the two years of this trial a significant impact to marketable yield was observed when terminating fungicide applications early (Table 1). Even though 2021 was not a disease heavy year, the impact of the bad year was significant enough to still realize over 1800 less pounds of strawberry per acre when applications were terminated at least two weeks earlier. Additionally, a 10% increase in total yield lost to culls was observed across the two years when comparing the full spray schedule to terminating a month earlier.
Figure 2. Marketable yield of strawberries subjected to three fungicide termination timings in two separate years.
Conclusions
Although preventative fungicide spray programs have historically been timed based on Botrytis fruit rot infection risk, anthracnose fruit rot must be considered each year. Green and red fruit are susceptible to anthracnose meaning that we can’t just think about protecting flowers when timing fungicide applications. We observed significant yield loss when only eliminating the last two fungicide sprays during fruiting. Fungicide applications at the end of the spring market window offer protection for developing fruit at a time when weekly yields can be considerably high. This is also a time where the weather can be more conducive to anthracnose infection, as highs are often in the 80s and windy rain events are common. Any lapse in protection can quickly lead to rapid infection of fruit and we’ve even seen growers abandon picking as anthracnose quickly took over fields.
Old adages don’t seem to be very fruitful in this case but there are several strategies that you can employ that will keep your fungicide spray program working as good as you remember. First, try to employ cultural controls and augment your plant management strategies in ways that reduce leaf wetness, avoid working or picking when plants are wet, and get all culled/infected fruit out of the field ASAP. Additionally, a few strawberry cultivars acceptable for the Southeast growing region have some resistance to anthracnose. For more information about resistant cultivars see the Southeast Regional Strawberry Integrated Pest Management Guide. Ultimately you need to consider the risk of anthracnose fruit rot with every fungicide you apply during your spring protection program. Although Botrytis is the main focus of programs during flowering, anthracnose will also infect and kill flowers. More importantly, anthracnose within the field will multiply on plant material throughout the early spring and lead to higher risks of fruit infection once conditions become ideal the last month of harvest.
With a limited arsenal of fungicides that effectively manage Botrytis and anthracnose, there are a few factors that should drive your selection of fungicides from week-to-week. First, you need to employ resistance screening on your operation to understand what fungicides work for you. Any weekly spray that doesn’t include an effective fungicide can quickly lead to impactful fruit rot. Be sure to rotate effective modes-of-action with each application (FRAC groups) to prevent resistance and ultimately insure yourself from possible failures. Fungicides with excellent efficacy for Botrytis should take precedence during early and full bloom while fungicides with excellent efficacy for anthracnose should be prioritized during harvest. Although the risk for anthracnose is low early in the season, all fungicide applications made in the Southeast should be rated at least ‘good’ in their effectiveness for both Botrytis and anthracnose fruit rot to prevent excess disease buildup. Products such as Kenja (FRAC 7) and Elevate (FRAC 17) work well for Botrytis but are not effective in preventing anthracnose and should be accompanied with Captan during bloom. Use of Captan during bloom has proven to effectively prevent anthracnose abundance later during fruiting. Luna Sensation (FRAC 7+11) and Miravis Prime (FRAC 12+7) are great products to use as climate conditions shift to favor anthracnose growth. When disease pressure is high consider adding captan alongside other fungicides for enhanced effectiveness. For more information about fungicide efficacy ratings see the Southeast Regional Strawberry Integrated Pest Management Guide.
If you have any questions regarding building an effective fungicide protection program for you operation checkout our new fact sheet Developing a Sustainable Fungicide Spray Program to Prevent Fruit Rot in Strawberry for the Southeast and our YouTube channel which features video guides on building spray programs.
As always give me a call or shoot me a text or picture at 479-249-7352 if you have any questions.