There are an increasing number of products under research and commercially available, that are marketed as plant biostimulants.
They are declared to contain a mix of biologically active substances such as amino acids and nutrients in a complex known and designed to elicit a ‘strengthening’ response in the plant that makes them more tolerant to so called biotic (pest and disease) and abiotic (environmental) stress factors. They are generally registered for sale as nutritional supplements outside the scope of the pesticide regulations (COPR) though they may have clear crop protection benefits. Regulation is changing. After receiving a positive vote by the Commission in October 2017, new EU ‘Fertilising Product’ regulations will give a stronger legal framework for use, and are expected to be fully applied by 2021.
The benefits of using biopesticides include: improved plant health with potentially lower environmental impact because of reduced pesticide inputs; increased operator and consumer safety; and a sustainable plant induced ‘immunology’ not prone to resistance. With fewer pesticides getting approval it is important that those we have remain effective, and incorporating biostimulants into the spray programme could provide one way of doing this.
Conventional crop protection products (and for that matter, pharmaceuticals) have a reassuringly definable content of active substances. Conversely biostimulants contain sometimes seemingly trace contents of less definable substances to which their efficacy in the plant is attributed and we may doubt as to whether they work at all! Plant physiology suggests otherwise. Micro nutrients, e.g. Zn, Mn, Si, and peptides e.g. Harpin and flavonoids (the ‘bitter’ compounds in plants known to confer plant resistance) e.g. glutathione, are all known to regulate essential plant processes at very low levels and are only required in low doses – for example, Harpin @1.5g active substance per hectare. The science is less familiar to us but it is science nevertheless.
Probably the best known biostimulant is phosphite. Conventional phosphate fertilisers typically have the phosphate component (PO4 ) bound to additional elements such as iron or hydrogen. Phosphonate compounds also have the phosphite component (PO3) bound to additional elements such as potassium, aluminium, copper, zinc. The different effects of these two compounds on crops are remarkable given that they are chemically very similar (see diagram). Phosphite is not directly fungicidal but highly systemic in the plant via both the xylem and phloem and works rapidly by inducing an immune response in the plant. It is particularly effective against Oomycete diseases such as downy mildew. Products include NT Phyte P Plus, Fortify Cu and Phusion.
Compounds containing silicon applied in a soluble silicate form enable it to be assimilated into structural and cuticular cells. Treated plants typically exhibit thickened cuticles and longer leaf hairs that physically resist attack by surface pathogens – in particular, powdery mildew. When a pathogen attempts to penetrate the leaf there is a subsequent accumulation of Si and Mn at the point of entry and induced production of phenolics that are toxic to the disease. Applied to pests, silicon also acts as a physical pesticide blocking spiracles and disrupting the exoskeletal cuticle. Commercially available products include Sion and Sentinel. The latter also contains organic Salicylic acid which aids silicon transport and elicits pesticidal effects in its own right. The combination results in contact activity against thrips, aphids, mites, caterpillars and molluscs. Trials data demonstrates that when applied with pesticides a similar level of control is achieved from lower doses or improved control at normal rates of use.
The Harpin protein (peptide chain of amino acids) is well documented for the unparalleled number of positive effects it has on defence and survival responses in plants. It does this by locating with receptors in the plant that switch on genes responsible for regulating biochemical pathways including tolerance to stress, nutrient transport and calcium deposition, photosynthesis and the production of flavonoids. Raised yield, improved cell integrity against splitting and disease e.g. botrytis, fruit coloration and improved harvest parameters are some of the benefits claimed for the active substance. Commercially available products are ProAct and Frostect.