EFFECT OF FIELD SANITATION ON BOTRYTIS FRUIT ROT AND MARKETABLE YIELD OF STRAWBERRY

James Mertely, Craig Chandler, Chang-Lin Xiao, and Dan Legard.  University of Florida, Gulf Coast Research and Education Center – Dover.  

 INTRODUCTION

 

        Botrytis fruit rot, caused by the gray mold fungus Botrytis cinerea, is probably the most important disease of strawberry worldwide.  In Florida, Botrytis fruit rot is a constant threat due to cool temperatures, periodic rainfall, and heavy dews that characterize the winter growing season.  These conditions favor disease development by enhancing spore production and infection efficiency.  In the Dover area, preharvest losses to Botrytis can approach 35% in unsprayed plots, but usually range from 5 to 15% in conventional spray programs.  Postharvest losses are also significant, particularly when the berries are mishandled after harvest.

 

        Due to public and worker concerns associated with the frequent use of fungicides in strawberries, non-chemical measures for Botrytis control are being investigated at the Gulf Coast Research and Education Center in Dover.  These include disease resistance, field sanitation, plant spacing, and plastic tunnels.  Field sanitation, which consists of trimming off old, senescent leaves, or removing cull berries from the production field, is discussed here.

 

METHODS

 

        Replicated field experiments were conducted during the 1995 to 1996, 1996 to 1997, and 1998 to 1999 seasons to investigate the effects of field sanitation on Botrytis fruit rot and yield.  Each plot contained 54 to 72 plants in three beds planted in a traditional raised-bed, plastic-mulched, drip-irrigated production system.  Three non-chemical practices were tested: (i) leaf removal, (ii) fruit removal, and (iii) combined leaf/fruit removal.  Leaf removal consisted of trimming off old senescing leaves after plant establishment (one time), with one or more repetitions later in the season (multiple times).  Fruit removal consisted of removing cull fruit from the alleys between the beds after each harvest.  Leaf/fruit removal combined both practices.  In addition to these “cultural control measures”, conventional and reduced rate fungicide spray programs were also evaluated.  The conventional (full fungicide) program consisted of weekly applications of captan at the full label rate, plus four Rovral bloom sprays.  The reduced-rate (reduced fungicide) program tested Captan at 1/2 to 2/3 full label rate.  Combinations of chemical and cultural control measures were also evaluated, as well as unsprayed controls.

 

        All the experiments were picked twice weekly during the harvest period.  Marketable fruit from the central bed in each plot were counted and weighed.  Cull fruit were categorized and enumerated by category.  Data from the twice-weekly harvests were combined for graphical presentation and statistical analysis.

 

RESULTS

 

        In the 1995 to 1996 experiment, none of the treatments using the cultivar ‘Oso Grande’ developed more than 1% Botrytis fruit rot (Table 1).  ‘Sweet Charlie’ was more susceptible, with average Botrytis incidence of 8.1% in the leaf/fruit removal treatment.  This level was significantly higher than 0.5% recorded in treatments that received full fungicide sprays.  As expected, the treatment with the highest incidence of Botrytis also had the lowest yield of marketable fruit.  However, none of the yields were significantly different from each other (P=0.16), due to a high amount of variability in the test.

 

        In the 1996 to 1997 experiment, Botrytis fruit rot incidence ranged from 2.8 to 3.8% in the chemical control treatments and from 8.2 to 11.0% in the cultural control treatments (Table 1).  Correspondingly, marketable yields in the cultural treatments and unsprayed control were significantly lower than in the chemical control treatments.  Leaf removal and leaf/fruit removal reduced Botrytis levels significantly, but neither treatment resulted in a significant increase in marketable yield.  Two opposing factors may account for these results.  Botrytis colonizes strawberry leaves when they are young, but remains inactive.  As the leaves age and begin to die, the fungus produces spores, which infect flowers and eventually invade the fruit.  Removing older leaves after plant establishment suppresses Botrytis, but deprives the plant of some green foliage, and a possible source of stored nutrients.  This may explain why the full fungicide program yielded 2120 flats/acre (10.25 lbs per flat) while the combined full fungicide/leaf removal treatment produced only 1900 flats, a significant decrease.  A similar yield reduction did not occur when only cull fruit were removed in a treatment receiving full fungicide sprays (2180 flats).

 

        In the 1998 to1999 experiment, fruit rot losses were very high.  Botrytis incidence ranged from 11.8 to 16.2% in the fungicide treatments to 17.9 to 21.4% in the cultural control treatments.  Changes in experimental design may account for the increased levels of disease, since the growing season was unusually dry.  In 1998 to 1999, plant spacing was reduced from 16 to 12 inches, plot size was increased from 54 to 72 plants, and beds of ‘Camarosa’ were substituted for open areas between experimental blocks.  Like the previous season, disease levels in the fungicide treatments were lower than in the cultural treatments and the unsprayed control.  However, the results were not as clear statistically.  For example, Botrytis fruit rot in the leaf removal and leaf/fruit removal treatments was statistically similar to the unsprayed control and the reduced fungicide treatments.  Yields in the leaf removal treatments were comparable to the unsprayed control, but significantly lower than treatments receiving fungicide sprays.  However, fruit removal, provided yields similar to several fungicide treatments.  Apparently, the leaf removal and reduced fungicide treatments provided moderate to good disease control in 1996 to 1997, but were less effective in 1998 to 1999 when disease pressure was high.

 

CONCLUSIONS

 

        Cultural practices had relatively minor effects on Botrytis fruit rot incidence.    Even when leaf removal reduced Botrytis significantly, a corresponding yield increase was not obtained, probably because some leaves were still photosynthetically active when they were cut.  Considering the high cost of trimming foliage (i.e., $300 to 400 per acre) and the limited reduction in Botrytis fruit rot that was obtained, we suggest that growers direct their resources to more effective control measures like fungicide applications.  Removing cull fruit from the experimental area did not reduce disease levels or increase yields significantly.  While this research did not address the issue of removing cull fruit from within the plant canopy during each harvest, it did not find any benefit from picking up discarded fruit in the alleys and carrying them out of the field.

Table 1.  Harvest data from cultural control field experiments carried out during the 1995 to 1996, 1996 to 1997, and 1998 to 1999 seasons.

Season & treatment no.	Treatment description	Cultivar	Average fruit weight (grams)		Marketable Yield (flats/acre)		Botryis fruit rot (%)
1995-1996
1	Full fungicide (FF)	Oso Grande	18.8	ab1	1543	ns2	0.2	a
5	FF+ leaf and fruit removal	Oso Grande	18.2	ab	1384	ns	0.1	a
10	Leaf and fruit removal only	Oso Grande	18.1	ab	1540	ns	0.7	a
1	Full fungicide	Sweet Charlie	17.1	b	1484	ns	0.5	a
5	FF + leaf and fruit removal	Sweet Charlie	17.8	ab	1532	ns	0.5	a
10	Leaf and fruit removal only	Sweet Charlie	15.6	c	1145	ns	8.1	b
1996-1997
1	Full fungicide	Sweet Charlie	17.7	a	2117	ab	2.8	a
2	FF + leaf removal (one time)	Sweet Charlie	17.5	a	2089	a-c	4.6	a
3	FF + leaf removal (multiple times)	Sweet Charlie	17.4	a	1898	c	2.9	a
4	FF + fruit removal	Sweet Charlie	17.7	a	2178	a	3.8	a
5	FF + leaf and fruit removal	Sweet Charlie	17.6	a	1958	bc	3.4	a
6	Reduced fungicide (RF)	Sweet Charlie	17.4	a	2090	a-c	3.5	a
7	RF + leaf and fruit removal	Sweet Charlie	17.2	a-c	2029	a-c	3.3	a
8	Leaf removal only (multiple times)	Sweet Charlie	16.8	bc	1643	d	8.2	b
9	Fruit removal only	Sweet Charlie	17.3	ab	1683	d	11.0	bc
10	Leaf and fruit removal only	Sweet Charlie	16.8	c	1616	d	8.3	b
11	Control	Sweet Charlie	17.2	a-c	1610	d	12.6	c
1998-1999
1	Full fungicide	Sweet Charlie	17.0	ns	1854	a	13.4	ab
5	FF + leaf and fruit removal	Sweet Charlie	17.2	ns	1710	a-c	11.8	a
6	Reduced fungicide	Sweet Charlie	16.3	ns	1740	a-c	15.6	bc
7	RF + leaf and fruit removal	Sweet Charlie	16.5	ns	1667	bc	16.2	bc
8	Leaf removal only (multiple times)	Sweet Charlie	16.7	ns	1504	d	17.9	cd
9	Fruit removal only	Sweet Charlie	16.6	ns	1554	cd	21.4	d
10	Leaf and fruit removal only	Sweet Charlie	16.8	ns	1401	d	18.3	cd
11	Control	Sweet Charlie	16.5	ns	1468	d	20.7	d

 

¹  Within a column and each growing season, values followed by the same letter are not significantly different                    

according to a protected Fisher’s least significant difference (LSD) test (P < 0.05).

²   Not significantly different (P < 0.05).