Richard Blacharski1, Dan Legard2, and Jerry Bartz1.
1University
of Florida, Department of Plant Pathology, Gainesville.
2University
of Florida, Gulf Coast Research and Education Center-Dover.
INTRODUCTION
Botrytis fruit rot is a major disease affecting strawberries in Florida. The disease begins with infection of the flowers during periods of cool and wet weather and remains quiescent until the fruit ripens. Although the disease is an important preharvest fruit rot, it is also important when it develops during either shipping or storage. The current USDA grade A standard for strawberries allows no more than five percent decay in a shipment. Botrytis fruit rot is the major limiting factor to shipping Florida strawberries long distances and limiting overall storage life. Since there are no fungicides labeled for postharvest application on strawberry, the current methods for control of this disease relies on preharvest fungicide applications and rapid cooling after harvest. Currently, protectant fungicides like Captan and Thiram are applied throughout the season. Additional applications of fungicides are applied during wet weather or peak bloom periods. Little information is available about the benefits of these fungicide programs on postharvest disease control.
In order to answer these questions, a two year replicated study was conducted during the 1997 to 1998 and 1998 to 1999 seasons. This study investigated the effect of preharvest fungicide applications on the control of postharvest Botrytis fruit rot. Strawberries were grown using commercial practices in raised beds that had been fumigated with 98:2 methyl bromide: chloropicrin and covered with black plastic mulch. Three different fungicide treatments were tested. Captan was applied at full (3 lbs active ingredient (a.i.) / acre) and 2/3 rates (2 lbs a.i./ acre), with and without Rovral bloom sprays (1 lbs a.i. / acre). Thiram was applied at 1.4 lbs a.i./acre the first season and 2.6 lbs a.i./acre the second, with and without Rovral bloom sprays . The third treatment featured Rovral applied two times as two sprays each during peak flowering periods as per label. Fruit were evaluated for postharvest observation four times each season by hand harvesting fruit directly into plastic clamshell containers. The fruit were then forced-air cooled to 3 °C and stored at 4 °C and 95% RH for 14 days. Beginning at day-3, the fruit were observed each day for incidence of Botrytis rot. The container contents were left undisturbed to avoid bruising or mechanical dispersal of decay.
During the 1997 to 1998 season, rainfall exceeded area averages. Conditions were extremely favorable for disease development. As a consequence, the postharvest disease control provided by the fungicide treatments varied with harvest. Overall, fruit treated with captan at full and 2/3 rate plus bloom applications of Rovral, had the lowest incidence of Botrytis rot in storage. Captan applied at two-thirds rate without bloom sprays and Thiram without bloom sprays did not provide sufficient control. On average, both treatments developed more than 5% decay on or before the fifth day of storage. Fruit harvested from the Rovral bloom-spray treatment and those from the unsprayed plots both had greater than 20% decay by day-8.
For the 1998 to 1999 season, rainfall was below normal, which decreased the potential for gray mold development. Captan and Thiram applied at full rate provided the best control over the storage interval. Overall, Thiram provided better control than the average of the different Captan rates, and bloom applications of Rovral did not influence the incidence of decay. Fruit from the unsprayed control had an incidence of 5.7% decay within 5 days of storage.
The control of postharvest disease begins with preharvest fungicide applications in the field. Fungicides should be applied at full label rate throughout the season to provide the best control of both preharvest and postharvest Botrytis fruit rot. Additional applications of fungicide during peak bloom periods are important during prolonged periods of wet weather that coincide with temperatures of 68 to 72 °F. However, when conditions are unfavorable for disease development (as during the 1998 to 1999 season), bloom spray applications of Rovral do not appear to reduce disease incidence during storage. With the application of Captan or Thiram at full label rate, use of proper cold storage can keep decay incidence below 5% for up to 10 days. A 10-day minimum storage life affords a longer shipping interval and access to more distant markets for Florida strawberries. The addition of modified atmosphere storage and more exact temperature maintenance during shipping could provide additional disease control and maintenance of quality. However, if fruit are exposed to prolonged periods of warming in an unrefrigerated display, decay in excess of 15 to 20% could develop within 48 hours.
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1997 to 1998
season |
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1998 to 1999
season |
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|
Fungicide treatment (rate / acre) |
Day 5 |
Day 8 |
Day 11 |
Fungicide treatment (rate / acre) |
Day 5 |
Day 7 |
Day 10 |
||||||
|
Captan full rate (3 lbs ai) |
6.5 |
abc1 |
10.0 |
a |
15.9 |
ab |
Captan full rate (3 lbs ai) |
2.0 a2 |
3.1 |
a |
5.3 |
a |
|
|
Captan full rate + Rovral
(1 lb ai) |
5.5 |
abc |
8.1 |
a |
9.6 |
a |
Captan full rate
+ Rovral (1 lb ai) |
3.9 b |
5.3 |
a-c |
8.2 |
ab |
|
|
Captan 2/3 rate (2 lbs ai) |
1.1 |
a |
8.1 |
a |
18.3 |
ab |
Captan 2/3 rate (2 lbs ai) |
3.1 b |
5.7 |
a-c |
8.2 |
ab |
|
|
Captan 2/3 rate + Rovral |
2.6 |
a |
10.0 |
a |
14.4 |
a |
Captan 2/3 rate + Rovral |
4.4 b |
7.7 |
bc |
12.2 |
bc |
|
|
Thiram 1/2 rate (1.4 lbs ai) |
11.2 |
bcd |
18.2 |
b |
22.7 |
b |
Thiram full rate (2.6 lbs ai) |
2.5 a |
3.3 |
a |
5.5 |
a |
|
|
Thiram 1/2 rate + Rovral |
3.2 |
ab |
8.9 |
a |
10.8 |
a |
Thiram full rate + Rovral |
2.9 ab |
3.9 |
ab |
6.4 |
a |
|
|
Rovral bloom only |
15.3 |
cd |
26.0 |
b |
26.0 |
b |
… |
… |
… |
|
… |
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|
|
Unsprayed control |
17.5 |
d |
42.9 |
c |
51.2 |
c |
Unsprayed control |
5.7 c |
9.8 |
c |
32.2 |
c |
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1
Numbers followed by the same letters
are not significantly different according to Duncan’s multiple range test (P
d 0.05). Values are from harvest 3 and
represent the percentage of berries with decay.
2 Numbers followed by the same letters are not significantly different according to Duncan’s multiple range test (P d 0.05). Values are the mean of 4 harvests and represent the percentage of fruit with decay.