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Horticultural
Development
Company
Grower summary
SF 111
Latent botrytis on cold stored
strawberry runners
Final Report 2009
© 2009 Agriculture and Horticulture Development Board
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© 2009 Agriculture and Horticulture Development Board
Headline
Latent Botrytis cinerea was detected in cold-stored A+ and waiting bed strawberry plants, cv.
Elsanta, from each of six propagators, in the crowns, trimmed petioles, roots and leaves.
Background and expected deliverables
Background
Work in 2008 (Project SF 84) showed that latent Botrytis cinerea was present in the crowns
of cold-stored A+ cv. Elsanta strawberry runners from five different propagators at levels
ranging from 6% to 38%. This was the first report of latent B. cinerea in strawberry crowns.
At the same time, infection was confirmed in petiole stubs from pre-storage leaf trimming (6%
to 55% of plants), and in old expanded leaves (12% to 82% of plants) of some samples.
Only A+ plants were sampled in 2008. These plants are taken directly from mother plants
with runners growing at high densities. Waiting bed plants, which are grown at about half the
density of A+ runners, are also used in the UK. These are transplanted and allowed to
become multi-crowned before sale. Some propagators buy-in their runners for waiting bed
production.
Waiting bed plants are now principally grown in mainland Europe as the
Continental climate allows a more predictable production of good flower initials.
Both A+ and waiting bed plants are lifted in winter, usually between November and
December, size-graded and then cold-stored at -1.7°C. Plants are removed from store for
planting between February and July, depending on the fruiting period required. Fungicides
are usually applied during flowering to prevent B. cinerea infection of the fruit.
It is possible that the denser growing conditions of A+ runners, compared with waiting bed
runners, might provide conditions more favourable to botrytis pre-lifting through higher
humidity and/or increased volumes of senescent tissue from shading. Lower plant infection
might occur if fungicide programmes applied pre-lifting include products active against
Botrytis cinerea.
The importance of latent botrytis inside the crowns and leaves of plants at planting on the
subsequent incidence of botrytis in fruit is unknown. There is potential to cause damage in a
new plantation through growth within the plant and/or sporulation on leaves resulting in
flower and fruit infection.
© 2009 Agriculture and Horticulture Development Board
Expected deliverables
The expected deliverable from this project is increased knowledge on the occurrence of
latent botrytis in strawberry plants at planting and on infection pathways leading to flower and
fruit botrytis. Ultimately, earlier treatment to control botrytis infection, during propagation
and/or soon after planting, could allow reduced application of fungicides at flowering and a
consequent reduced risk of pesticide residues in harvested fruit.
The objectives of this project were:
1. To determine the occurrence of latent B. cinerea in commercial strawberry plants at
planting, by testing samples of both A+ and medium density waiting bed plants of the
same variety (Elsanta);
2. To examine whether growing conditions and treatments at runner production-sites affect
botrytis incidence at planting by collating information on crop husbandry with occurrence
of latent B. cinerea.
Summary of project and main conclusions
Occurrence of latent Botrytis cinerea and Colletotrichum acutatum
Ten batches of ex-cold store strawberry plants, cv. Elsanta, were examined for possible
latent infection by B. cinerea and C. acutatum using a range of tests. Petiole stubs and
internal crown tissue were examined for latent infection by plating onto agar after surface
disinfection; roots, crown tissue, old leaf debris and emerging leaves were examined by
humid incubation at room temperature followed by microscope examination. Plant samples
were supplied by propagators in the Netherlands (7 samples), Germany (2) and the UK (1),
and consisted of six batches of A+ runners (crown diameter 15-23 mm) and four batches of
medium waiting bed plants. Fifty plants were tested from each sample.
B. cinerea was detected within crown tissue and petiole stubs by growth from these tissues
onto agar. It was also detected in or on roots, leaves, debris and crown tissue by mycelial
growth following humid incubation.
Combining the results of the three test methods, B. cinerea was detected in all 10 samples at
levels ranging from 6% to 84% (mean incidence 21%) (Table 1). Internal infection in crowns
was found in nine of the samples, at levels ranging from 2% to 18%. Generally B. cinerea
was recovered from only one of six internal crown sections examined, indicating localised
© 2009 Agriculture and Horticulture Development Board
infection. Petiole stubs were present on around 25-50% of plants in a sample, usually with
just one or two per plant. B. cinerea was recovered from petiole stubs in eight of the 10
samples. Levels of infection ranged from 2% to 16%, expressed as a proportion of all plants,
or 5% to 50% expressed as a proportion of plants with one or more petiole stub present.
Growth of B. cinerea following humid incubation of tissues occurred in nine of the samples, at
levels from 2% to 78% of plants (Table 1).
No C. acutatum was found in any test.
Table 1: Recovery of B. cinerea from strawberry plants cv. Elsanta ex-cold store from three
tissue incubation tests (crown and petiole stub isolations and plant damp incubation), 2009
Sample
code and
type
A+ runners
BX09/99
BX09/42
BX09/47
BX09/57
BX09/65
BX09/68
Source
(propagator
code)
Any
test
PG5
PG4
PG6
PG7
PG8
PG9
30
84
14
16
12
6
Mean
Waiting bed
BX09/58
BX09/66
BX09/69
BX09/74
27.0
PG7
PG8
PG9
PG4
6
14
8
16
Mean
11.0
% of total plants with B. cinerea
Within
Within
On incubated
crown
petiole stub
plants
6
18
6
8
6
2
7.7
0
4
4
10
4.5
14
16
8
6
0
0
7.3
18
78
0
2
6
4
18.0
4
4
2
0
2
6
2
6
2.5
4.0
Comparison of A+ and waiting bed plants
In all three incubation tests, the mean level of latent infection by B. cinerea was greater in A+
runners than in waiting bed plants (Table 1). However, there was a large variation between
samples of A+ runners, with one (BX09/42) showing very high levels, one showing
moderately high levels (BX09/99) and four showing lower levels, similar to those found in the
four waiting bed samples.
Four propagators (coded PG7, PG4, PG8 and PG9) supplied both A+ runners and waiting
bed plants. For two of the suppliers, there was little difference in the levels of infection
between A+ runners and waiting bed plants.
For the other two suppliers, one had
considerably greater and the other had slightly greater levels of B. cinerea infection in the A+
runners than the waiting bed plants.
© 2009 Agriculture and Horticulture Development Board
Levels of infection in relation to propagator crop husbandry and fungicide use
Infection levels of B. cinerea in young plants in relation to propagator crop husbandry and
fungicide use was examined, and limited conclusions could be drawn from the data supplied
by propagators. All samples post cold-storage contained plants with botrytis, but no records
were supplied of botrytis incidence in the propagation fields. Some propagators provided
records of repeated applications of powdery mildew fungicides during the summer, and some
products would also have controlled botrytis. Two propagators used Rovral (iprodione) and
thiram (both active against botrytis) in December not long before lifting the runners.
A comparison of the results on latent B. cinerea infection levels in 2009 obtained in this
project was made with those obtained in 2008 in project SF 74. Levels found in 2009 were
generally lower than those found in 2008. Combining the results of the three test methods,
four out of five A+ samples tested in 2008 had infection levels greater than 50%, compared
with only one out of 10 samples tested in 2009. Similarly with internal crown infection, four
out of five samples had over 20% infection in 2008 and none of the samples exceeded this
level of infection in 2009. Further work is required to determine the relative importance of
season, source, fungicide use, and possibly other factors, on the differing levels of latent B.
cinerea in strawberry plants at planting.
Financial benefits
Ultimately, if the incidence of botrytis in propagation material can be reduced, and if this is
shown to reduce risk of fruit rot, then fungicide application during flowering to control fruit
botrytis could be reduced. A secondary benefit for growers would be an end to the picking
disruption entailed by delaying harvesting (or picking and destroying) fruit developing within
fungicide sprayed flowering crops.
Action points for growers
None at present. Further work is required to determine the importance of latent B. cinerea in
young plants at planting on crop production and fruit botrytis.
© 2009 Agriculture and Horticulture Development Board