New thinking on pest and disease control puts plant feeding first.

Horticulture has lost many of its conventional synthetic pesticides previously widely used for pest and disease control. The reasons for this -- such as public demands for much reduced residues in food and the environment or the limited return on research and development investment offered by horticulture -- are well known and need no elaboration.

Emerging consequences mean that the long-accepted mantra, in which pest and pathogen organisms were the primary focus for destruction, is outmoded. New thinking reverses these objectives. Concentration centres now on feeding the plants first and gaining pest control second. Skilled husbandry and natural biological control link in achieving plant health as the primary objective. Research and development are identifying nutritionally active chemicals that block pest and pathogen activity by secondary biological or physical means.

This new approach benefits all sectors of horticulture. It further enhances our environmentally benign image for consumers of our products, whether these are fruit and vegetables, flowers, gardens or landscapes. And, oddly, it also means the return of some materials that were once discarded.

Potassium bicarbonate

Potassium and calcium have longstanding records as nutrients stimulating plant health. Potassium bicarbonate and carbonate are simple chemicals used daily in the food-processing industry. When used on crops they inhibit and eradicate powdery mildew fungi.

Solid potassium bicarbonate, once dissolved in water, should be used quickly with a suitable wetter to increase adherence to the foliage. This chemical only affects those powdery mildew colonies that are directly coated. The control possibly results from the alkaline nature of bicarbonate solutions.

Users should find the best rates and frequencies of application to fit their own needs. Ranges of between 1.0 to 3.0 per cent solutions (1.0kg to 3.0kg of potassium bicarbonate dissolved in 100 litres of water) are normally effective. Higher concentrations may cause scorch. It is wise to test the response on a part of a crop or planting first before treating larger areas.

Potassium bicarbonate has Commodity Status from the Pesticides Safety Directorate (PSD). This recognises that it is not a pesticide and requires only that users apply a Food Grade (99.5 per cent purity) chemical. Use covers ornamental and edible crops. Adjuvants such as Break-Thru S240 increase the spreading qualities of potassium bicarbonate solutions by up to 25-fold and extend their rain-fastness.


Orthophosphate is the form of phosphorus usually used in fertilisers and taken up by roots. Other forms like phosphites and phosphonates are absorbed through leaves. These have been added to pesticides, increasing the control of downy mildews, Bremia and Phytophthora-induced diseases for many years.

Recently, combinations of both potassium and phosphorus (K and P nutrients) became available. These combine fertiliser qualities that increase plant health with diminishing disease incidence. In the UK they are sold as the liquid foliar fertiliser

Farm-Fos 44, with a nutrient value of 0:32:29 per cent by volume. Foliar sprays are applied at a minimum of 200 litres per hectare and a maximum concentration of one per cent solution of Farm-Fos 44.


Slugs cause enormous losses and impair quality right across most horticultural areas. They graze the foliage and roots of young seedling, reducing stand densities and delaying establishment; they ruin the quality of maturing plants and allow entry by pathogens such as grey mould (Botrytis cinerea). Amesbury-based Certis has developed Ferramol for slug and snail control. This contains one per cent ferric phosphate and 99 per cent inert bait from durum wheat and an attractant.

Only four slug species are of major significance as causes of crop damage in Britain. Ferramol adversely affects them all.

Recommended rates are between 12.5kg and 50kg per hectare, with the lower ones being applied for preventative rather than eradicant action. There is a maximum dose limitation of 200kg per hectare. This allows up to 16 times the lowest rate application in any single use.

Under normal circumstances only one or two applications are needed. Usually applications are made at emergence or at transplanting — this will provide protection throughout the plant’s life and avoid pellets from being lodged in the harvestable portions of crops such as cabbage or cauliflower heads.


Sulphur is well established as a fertiliser but it is also capable of reducing the impact of leaf-invading fungi and especially powdery mildews. Sulphur is available from a range of suppliers and classed as a broad-spectrum foliar fungicide, nutrient and acaricide. Only two applications per planting are permitted.

There are derivatives, such as calcium polysulphide, that are very active against powdery mildews but these are not registered for use in the UK.

Brassicas, in particular, contain high levels of organic sulphur compounds (isothiocyanates) in their roots. The traditional practice of green manuring uses these compounds as natural soil sterilants. Chopped brassicas are incorporated in the soil and these indirectly reduce pest and pathogen populations.

Plant breeders at Plant Solutions in Cobham, Surrey, have produced new Brassica types such as Caliente Mustard, which contain enhanced concentrations of isothiocyanates specifically for use as green manuring, break and biofumigant crops.


Nemaslug from Becker Underwood, contains nematodes pathogenic to slugs and snails. This product is applied up to five days before seedling emergence or transplanting, with more applications at growth stages that are especially susceptible to slug damage, such as tuber formation in potatoes or the delicate shoots of dahlias and delphiniums.

For potatoes, in particular, applications are needed six to seven weeks prior to harvesting the young susceptible tubers.

This product is effective at temperatures between 5°C to 25°C. It contains infective juvenile nematodes in an inert carrier and is mixed with water, forming a suspension and drenched onto crops.

For Brussels sprouts, asparagus and salads or herbaceous plantings that are exposed to slugs over prolonged times and hence especially susceptible, split applications at 50 per cent of the recommended rate may be used.

Supplied by Plant Solutions, DiPel DF uses a similar approach. This contains the bacterium Bacillus thuringiensis and is used to control insect caterpillars on cabbage, cauliflower, Brussels sprouts and broccoli. Dose rates for brassicas are 0.75kg to 1.0kg of the product per hectare, applied as soon as the first feeding larvae are seen, and repeated at seven- to 10-day intervals until the end of the hatching period. Rates of application are increased in line with the intensity of caterpillar infestations. If the cabbage moth (Mamestra brassicae) is present, higher doses are needed.

Tracer or Spinosad approaches resemble the ones mentioned above. This is a selective insecticide derived from natural soil fungi. Products of this type are collectively named Naturalyte by their manufacturer, Dow AgroSciences, and sold in Great Britain by Chelmsford-based Landseer.

It is active against cabbage root fly, diamond back moth, small white butterfly and thrips. Use Tracer for both module and planting drenches at 200ml of the product per hectare in 200-600 litres of water per hectare. Only four applications are permitted per crop, and the last one must be at least three days before harvest. This product must be kept well clear of water courses as it is highly toxic to aquatic life.

Canada-based Premier Horticulture markets biological controls for soft rot pathogens related to Pythium by using peat-based composts with the bacterium Bacillus subtilis MBI 600 added (PRO-MIX) added. This also controls damping off (Rhizoctonia) in vegetable, flower and bedding seedlings, and use is associated with increased germination, plant height and dry weight.

Natural plant extracts

Another recent development is Majestik, which is a natural plant product from Certis. This is a contact insecticide, excluded by the PSD from the Control of Pesticides Regulations and Plant Protection Products Regulations because it acts by physical means only.

Certis also sells spraying oil, which is an insecticidal and acaricidal hydrocarbon oil with similar action.

Garlic oil is marketed for a wide range of purposes from slug to powdery mildew reduction by Garlic Farms (UK). Its product, Garlic Barrier Garshield, is a mixture of garlic and citrus extracts with protective and systemic properties against fungi and bacteria. The garlic oil contains sulphur compounds and to these are added ascorbic acid and flavonoids from citrus.

Garlic Farms recommends its products for swedes and lettuce in the field. The mixes are applied at one per cent dilutions (one litre of concentrate to 100 litres of water) to run-off. Rain-fastness is improved by adding Codacil vegetable oil at 1:1 mixture.

Integrated pest management

All these products are ideal components for integrated pest management (IPM) strategies. They tend not to have restriction periods associated with them and so are best used as the final component in an IPM strategy and applied just before harvest.
Where harvesting is delayed, further applications may be made without risk of building up residue levels in the crop.

Professor Geoff Dixon is managing director of GreenGene International

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