Strawberries 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, 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 for our region. 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 12 plant plots
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. 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 (7.6 oz)7 + 11
1 2 3
3/29
Captan (3.75 lb) + Elevate (1.5 lb)M04 + 17
1 2 3
4/6
Switch (14 oz)12 + 9
1 2 3
4/12
Captan (3.75 lb) + Elevate (1.5 lb)M04 + 17
1 2 3
4/21
Luna Sensation (7.6 oz)7 + 11
1 2
4/27
Switch (14 oz) + Captan (3.75 lb)12 + 9 + M04
1 2
5/5
Captan (3.75 lb)M04
1
5/12
Luna Sensation (7.6 oz)7 + 11
1
2022 Spring- High Pressure Year
Date Fungicide FRAC Group Treatments Sprayed*
3/16
Switch (14 oz) + Captan (3.75 lb)12 + 9 + M04
1 2 3
3/24
Captan (3.75 lb) + Elevate (1.5 lb)M04 + 17
1 2 3
4/1
Switch (14 oz)12 + 9
1 2 3
4/7
Luna Sensation (7.6 oz)7 + 11
1 2 3
4/15
Captan (3.75 lb) + Elevate (1.5 lb)M04 + 17
1 2 3
4/21
Switch (14 oz)12 + 9
1 2
4/26
Luna Sensation (7.6 oz)7 + 11
1 2
5/3
Switch (14 oz) + Captan (3.75 lb)12 + 9 + M04
1
5/11
Luna Sensation (7.6 oz)7 + 11
1
*Number indicate which fungicide termination treatment was sprayed each week: 1 –
last week of harvest, 2 - two weeks prior to last week of harvest, 3 - one month prior
to last week of harvest
Ripe strawberry fruit were harvested twice per week from each plot to assess
yield. 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. 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
.
*Yield or percent followed by a different letter within each year and column are statistically
different according to a Tukey’s HSD at α=0.05.
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.
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 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 your 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.
Aaron Cato
