Optimising disease biocontrol in propagation

The nursery stock propagator’s productivity plan is based on three propagation productivity principles: ‘maintaining cleanliness’, ‘optimising rooting potential’ and ‘delivering disease protection’.
Hardy ornamental nursery stock species develop roots from cuttings at different speeds. The rooting speed, propagation conditions, and quality of cutting material, can influence rooting efficiencies (often referred to as the ‘cutting strike rate’). Slow-rooting cuttings are especially vulnerable to infection by pathogens that infect the stem base. If infected, cuttings may, depending on the pathogen, develop a dark brown or black lesion that extends upwards from the basal end. Infected cuttings generally lead to poor root development, or in severe cases, to a complete loss of rooting potential. Trays of cuttings that fail to produce healthy rooted plugs represent an unwelcome economic loss to the business, that can often be avoided.

A hygiene programme is a significant part of the disease control strategy. The use of peroxyacetic acid, e.g. Jet 5, at a 1% v/v dilution, is the nurseryman’s standard disinfectant treatment. Most pathogen spores will succumb to the treatment, if the recommended concentration, and pathogen exposure, is maintained during the disinfection process. Since organic matter will reduce the disinfectant’s efficacy, this should be removed from benches, or used capillary matting, by washing, drying and brushing, before the disinfectant treatment is applied.

Rooting rates can be improved by applying a rooting hormone dip treatment prior to sticking. Root initiation and development can be encouraged by watering-in the cuttings with the plant biostimulant Vidi Parva. At the same time, Vidi Parva should be tank mixed with Trianum-P to prevent the entry of diseases such as Pythium, Fusarium and Rhizoctonia. Trianum-P contains the unique disease antagonist Trichoderma harzianum (strain T22). A second application of Vidi Parva and Trianum-P should be applied 10-14 days later.

In 2020, a development study demonstrated that Vidi Parva has a useful secondary benefit of increasing biomass production in T. harzianum (T22). The increase was significant at the commercial application rate, compared to dilute solutions and the water only control. In practice, Vidi Parva may contribute to T. harzianum’s speed of establishment and an elevated population density, enabling Trianum to give better protection against disease infections.

The fungicide propamocarb hydrochloride and Trianum-P can be safely integrated in the same treatment programme (https://www.koppert.co.uk/side-effects-database/). Propamocarb hydrochloride has no effect against T. harzianum (T22) conidiospores or hyphae. This safety is due to propamocarb hydrochloride’s selective activity against fungi belonging to the Oomycota (e.g. Pythium and Phytophthora), but it has no effect against Ascomycota (i.e. Trichoderma). This dual treatment constitutes a ‘weaken and kill’ strategy, where the fungicide weakens the target, and T. harzianum completes the killing process. For further details on the propamocarb hydrochloride approval for use in ornamental propagation, visit https://secure.pesticides.gov.uk/offlabels/search.asp and search for Extension of Authorisation document 2016-0796.