Performance of plant protection products against Thielaviopsis on Viola©
Thielaviopsis basicola infects members of at least 15 plant families to cause black root rot resulting in uneven growth of seedlings and failure of establishment in newly potted nursery stock. There is usually a slow decline in plant vigour, until the plants are put under stress, for example in warm weather or drought. The roots develop dark brown speckled areas where long-lived resting spores (chlamydospores) are formed in the pale-coloured host cells. The fungus also produces abundant colourless endospores which are released outside the root and can be spread in run-off water.
In 2013, UK growers of pot and bedding plants and nursery stock recently became concerned about black root rot and the limited number of plant protection products that are available to them. In particular, Cercobin WG (thiophanate-methyl), can be used as a drench over ornamental plants, but only once per crop, and only if they are in containers in a permanent greenhouse. The UK levy-funded research body, AHDB Horticulture, agreed to fund a series of studies to find alternative treatments. An initial scoping study (Wedgwood, 2013) determined that other chemical active ingredients and biological products might be effective against the pathogen. This led to efficacy experiments (Wedgwood, 2014, 2015).
Wedgwood, E. 2016. Performance of plant protection products against Thielaviopsis on Viola©. Acta Hort. (ISHS) 1140:355-358