Pansy Mottle Syndrome: Investigating the role of synthetic auxin herbicides as a possible cause of plant distortion in bedding plant species


Project summary:


There has been an increasing occurrence of pansy Mottle Syndrome affected Pansy and Viola crops in recent years and there is increasing evidence that other bedding plant species may also be affected by similar distortion symptoms. The increase in the problem coincides with a reduction in use of peat-based substrates and a move to dilute peat with other materials including green waste compost.
A recent HDC desk study (McPherson, 2010) hypothesised that one possible explanation to account for the sporadic but economically damaging symptoms may be the presence of trace (1-10ppb) levels of synthetic auxin herbicides. This hypothesis is reinforced by the fact that many green waste samples submitted for PAS 100 testing are known to be contaminated by trace levels of these herbicides and also by the fact that a small number of litigation cases are occurring as a result of synthetic auxin damage, allegedly originating from contaminated substrates (McPherson, pers com). One of the main difficulties is that sensitive plants such as tomato & field bean are known to be sensitive down to concentrations as low as 1-10ppb and this is below the analytical detection limit in substrates. Therefore, routine laboratory analysis cannot determine their presence unfortunately and a host plant bioassay remains the most sensitive method for detection.
As previous investigations to elucidate a primary cause for Pansy Mottle Syndrome have not been conclusive it is considered appropriate to explore the synthetic auxin hypothesis in more detail. The aim of the proposed study is to spike a growing medium with a range of different concentrations of these herbicides e.g. clopyralid, MCPA, 2,4-D and to grow a range of bedding plant subjects including Pansy & Viola in the substrate alongside unspiked controls for comparison.
Germinated seedlings will be assessed in detail for the presence of distortion to the cotyledons, and emerging true leaves to see if early differences become apparent. Sub-samples of the seedlings will then be grown on further to see if they exhibit ‘classic’ pansy mottle symptoms as they mature.
Benefits to industry:
Pansy Mottle Syndrome, whilst sporadic, is economically damaging when it occurs and its unpredictability and uneven distribution makes crop management very difficult.
If a cause for the symptoms can be found it will provide growers with appropriate knowledge by which to help avoid future problems. It will also provide a strong evidence base with which to demonstrate the risk associated with the use of green waste substrates in peat-based growing media.
The result of the work, will be communicated back to the industry via the British Protected Ornamentals Association (BPOA) and via HDC News.
Project code:
PO 015
01 May 2013 - 31 October 2013
AHDB Horticulture
AHDB sector cost:
Project leader:

About this project

Aims and objectives:
Project aims
To investigate the possibility that residual synthetic auxin herbicides, occurring as trace substrate contaminants (possibly following handling of green waste materials) are responsible for the disorder referred to as Pansy Mottle Syndrome
Project objectives
To spike a peat-based growing medium with a range of concentrations of different synthetic auxin herbicides, sow a range of different bedding plant subjects (and field bean as a standard) into the media alongside unspiked controls and grow-on to determine if symptoms consistent with those of Pansy Mottle Syndrome can be reproduced.