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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 Disclaimer Whilst reports issued under the auspices of the HDC are prepared from the best available information, neither the authors nor the HDC can accept any responsibility for inaccuracy or liability for loss, damage or injury from the application of any concept or procedure discussed. The results and conclusions in this report may be based on an investigation conducted over one year. Therefore, care must be taken with the interpretation of the results. Use of pesticides Only officially approved pesticides may be used in the UK. Approvals are normally granted only in relation to individual products and for specified uses. It is an offence to use nonapproved products or to use approved products in a manner that does not comply with the statutory conditions of use, except where the crop or situation is the subject of an off-label extension of use. Before using all pesticides check the approval status and conditions of use. Read the label before use: use pesticides safely. Further information If you would like a copy of the full report, please email the HDC office ([email protected]), quoting your HDC number, alternatively contact the HDC at the address below. HDC Stoneleigh Park Kenilworth Warwickshire CV8 2TL Tel – 0247 669 2051 The contents of this publication are strictly private to HDC members. No part of this publication may be copied or reproduced in any form or by any means without prior written permission of the Horticultural Development Company. © 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