Plant health care - An overview
From Allotments4All
Diagnosis, treatment, prevention and control
Plant healthcare - principles, general prevention and specific methods.
Plants that are healthy are less susceptible to attack by pests and diseases, and the health of the plant can be maintained in a number of ways.
•Quality of plants. Where a plant certification system is applicable, purchasing certified stock will give the best chance that the plant is healthy and free from problems such as viral diseases. This means that not only is the purchased plant of good quality, but there is less likelihood that diseases will be introduced into any existing stock that is being grown. On a general basis, plants should be examined when being purchased. They should have a good fibrous root system, and be free from obvious physical damage, and signs of pests and diseases. The plant should be well established and appear healthy.
•Selection of resistant plants. Where certain pests or diseases are known to be a problem, the selection of resistant cultivars can reduce the risk. The carrot, Daucus carota is prone to attack by carrot root fly Psila rosea, and there are some resistant varieties available, like Sytan, available from Marshalls or Flyaway from Thompson & Morgan.
•Selection of suitable plants for the available site. It is necessary to assess the needs of the plant in conjunction with the growing conditions, for example shade loving plants would not grow well in hot, sunny sites. Plants that need high light levels and sharp drainage, like the alpine Cobweb Houseleek, Sempervivum arachnoideum would not grow well in shady, boggy, conditions.
•Timing. Plants should be purchased at the optimum time for planting so that they can establish themselves easily. For example, planting shrubs in the autumn means that the soil is still warm and there is a better chance that a good root system will ensue. Bare root trees should only be purchased during their dormant period during winter. Certain pests and diseases have particularly active phases within their lifecycle, and an example would be to grow early varieties of potato to avoid the effects of potato blight, the fungus Phytopthera infestans, as the crop would be harvested before the disease has a chance to affect it.
•Soil texture and structure. The characteristics of the soil affect plant health. In an ideal garden soil the constituent proportions would be: water 25%, air 25%, organic matter 5%, and mineral particles 45%. Some plants prefer a very free-draining soil, whilst others such as bog plants prefer much moister conditions. A clay soil will tend to hold water and nutrients, although can be prone to water-logging in winter, whilst a sandy soil holds less water and nutrients are likely to leach out in winter rains. The addition of organic matter can improve all types of soil.
•Soil pH and nutrients. Most plants do well at a pH level of around 6.5, however some have different requirements. Shrubs such as Rhododendron sp. need acidic soils, with a pH of 5.5 – 6.0. The pH level of the soil will affect the uptake of nutrients in the plant. Where it is intended to grow crops on a commercial scale, it is necessary to identify their exact pH and nutritional needs, and get a soil analysis test done. DEFRA booklet RB209 gives useful advice.
•Temperature. Temperature affects enzymes within the plant and most plants will only photosynthesise efficiently in the range 0 – 36o C. Growth processes are affected as respiration increases with temperature, as does transpiration. Excessively high temperatures during the day can mean that the plant loses more water than is available to it, and so it will suffer. Temperature will affect the germination process.
•Light levels. The duration and intensity must be sufficient for photosynthesis. Levels of light between 1000 and 30,000 lux in the wavelength range 400 nm to 700nm are required, and this can be particularly critical for plants under indoor cultivation. They can be affected adversely by dirty glass reducing light levels, or a positive effect induced by the use of lamps that emit the correct wavelength and intensity of light. Plants in the outdoor environment can be affected by their situation. If the aspect is open, or south-facing, the light levels will generally be high, but if the plant is sited under trees or shrubs, the light levels will be reduced.
•Air movement. Hot drying winds should be avoided as leaves may become desiccated, and a sheltered site will assist this. Rose powdery mildew, Spaerotheca pannosa, thrives in hot dry conditions, so these should be avoided for Rosa spp. Good air circulation is important however, as humid conditions can promote the growth of other fungal diseases like downy mildew Perenospora destructor on Allium spp. and bacterial diseases.
•Pollution. Where air quality is poor due to pollution, or salt laden winds in coastal areas, and soil becomes contaminated, plants with good tolerance to these conditions can be selected, such as thrift, Armeria maritime which is tolerant to salt, and trees like Alnus and Betula spp. which are tolerant of roadside pollution.
•Plant management and cultivation. Pests and diseases can be deterred in a number of ways, and good hygiene practices and cultivation methods will help to increase plant health.
•Remove plant debris, as this can provide shelter and breeding grounds for pests and diseases.
•Prune away dead, damaged and diseased wood to remove sources of disease and help to prevent the spread of disease.
•Clean tools thoroughly after use to help to prevent the spread of viral disease from plant to plant.
•Weed regularly to remove host plants.
•Add organic matter to the soil to improve structure and fertility and ensure that plants nutritional needs are met.
•Irrigate when necessary. Watering once a week thoroughly to moisten the soil to a depth of at least 15cm is more effective than light sprinklings on the surface, which will evaporate more quickly and also encourage shallow rooting.
•Apply the principles of crop rotation to lessen the risk of building up pests and diseases. For example, when planting a new rose, don’t plant where an old rose was sited. Don’t grow potatoes on the same part of your allotment year after year; choose a different part, following a three or four year cycle.
•Walk around the growing area on a regular basis and look carefully for signs of pests and diseases. Regular inspections can detect problems at an early stage, which makes them easier to tackle when dealt with promptly.
•Encourage natural predators by providing habitats for them to supply their needs for shelter and food.
How the knowledge of the biology of pests and diseases enables effective control techniques.
Possessing knowledge of the biology of a pest or disease makes it possible for the horticulturalist to choose the most effective ways to combat them.
Many pests and diseases become more active as temperature increases. If action is taken at the start of this period, it can be more successful than after the pest or disease has become more established.
Knowing the times when pests start to become a problem is important for growers. Where chemical control is used in agriculture, commercial growers make use of forecasting techniques so that they get to know when Carrot root fly, Psila rosea is prevalent and can time their spraying operations to maximise effect.
The way that a pest or disease feeds can determine the type of chemical method that will have the best results.
Sap-sucking insects such as Myzus persicae can be controlled with a systemic insecticide. This type of insecticide is absorbed into the sap of the plant, so the insect will ingest it when feeding.
Insects that bite and chew such as the larvae of Pieris brassica do not digest so much sap in proportion to their body size, and so are better controlled with a stomach poison.
The predators of pests are commonly used as biological control, and making sure that these are applied in optimum conditions – at the best temperature to ensure activity including feeding and reproduction – means they will be more effective. Sometimes these conditions are similar to the needs of the pest, sometimes they differ, as in the case of predatory mite Phytoseiulus persimilis, used as a biological control for Tetranychus urticae, the glasshouse red spider mite. The pest prefers low humidity, and the predator prefers high humidity, so this is a case in which introducing the predator and increasing the level of humidity to suit the predator will have the best effect of controlling the pest.
Knowing the lifecycle of pests and diseases can determine cultivational methods. If a pest or disease over-winters under debris, then by clearing away debris the habitat is removed and the pest or disease controlled.
Similarly, it is important to know the lifecycle of any predators that are used for control. Aphidoletes aphidimyza, used as a biological control for Myzus persicae, pupates only when day lengths exceed 15 hours, so needs to be used between late spring and early autumn if the population is to be sufficient to deal with the pest.
Virus infected plants cannot be cured. This means that there is little point applying products in an effort to remedy the symptoms of the virus, the only way to destroy that batch of virus is to destroy the plant.
However, vectors like insects and mites transmit viruses, and pruning knives, wind and water also transfer them. It is possible to use means to control these, for example sterilising pruning knives, controlling the insects and mites by chemical, biological or barrier methods.
Many biological controls need to be used immediately they are purchased, so consideration needs to be given about making available resources to administer them at the correct time.
Chemical controls may need to be used, for example in the case of the larvae of vine weevil, Otiorhynchus armadillo, which may be active in container grown plants over the winter period, when biological controls may not be effective at low temperatures.
The main points of health and safety and other legislation that applies to pest and disease control in the UK.
Many countries all over the world adhere to the International Plant Protection Convention, which sets standards for plant health quality assurance, called International Standards for Phytosanitary Measures. These countries have inspection bodies, National Plant Protection Organisations, who can issue a phytosanitary certificate to accompany the plants all the way through their transportation to the retailer, and which verifies that the plants have been produced under conditions and inspected to a level that ensures that the risk of infestation with pests and diseases is very low. The certificates ensure traceability of the plants back to the third party originators, and are used for countries outside the EU. For trade wholly within the EU, plant passports are used instead for certain types of plant, including plants like seed potatoes, fruit and ornamental trees of Malus spp. and other ornamentals such as Viburnum spp.
The Plant Health Act 1967 gave powers for authorities to impose restrictions to protect plants from attack by pests and diseases which may be prevalent in foreign countries, and to prevent the spread of pests and diseases.
The Plant Health (Great Britain) Order 1993 as amended, deals with current requirements under UK law, covering the powers of government officials to enter and inspect production and storage facilities and to prohibit the movement of plants that are suspected to be contaminated with pests and diseases. It lists specific banned insect, mites and nematode pests together with fungal, viral and bacterial diseases. It mentions import restrictions on many plants and plant materials, which include seed potatoes, certain citrus fruits and timber products, strawberry plants, plum and apple trees, and also covers contaminants which could be found on used agricultural machinery for re-sale. A list of plants that require phytosanitary certification is shown, and includes cultivars and species of plants such as Narcissus and Begonia.
The Health and Safety at Work provided powers for governments to issue regulations and orders to ensure that employers meet their responsibilities for ensuring the health and safety of those involved in the business, whether they are staff, subcontractors, customers or the general public.
The Control of Substances Hazardous to Health Regulations 2002 requires that employers ensure that employees are protected from exposure to hazardous substances such as chemicals that may be found in pesticides, or microorganisms. They must undertake risk assessments to help them decide what steps to take to prevent harm by eliminating or reducing the risk, control the risk, monitor the control measures and any exposure to employees, and carry out health checks on employees where appropriate. Employers are responsible for training employees properly, and for preparing plans to deal with any accidents that may happen involving hazardous substances.
The Control of Pesticides Regulations (COPR) 1986 as amended shows types of pesticides which are approved for use, the application methods, and it aims to protect the health of people, creatures, plants and the general environment, whilst controlling pests efficiently and humanely. It demands that users follow codes of practice to administer pesticides safely, that suppliers and employers store pesticides safely. The Control of Pesticides [Amendment] Regulations, 1997 requires that users such as employees and contractors undergo formal training through bodies like Lantra and achieve a certificate of competence, and levels of competence that can be achieved include PA1, PA2 and PA6 certification. The Plant Protection Products Regulations 2003 covers the approval procedures for manufacture and regulation of plant protection products including pesticides and also lays down rules on the labelling and other health and safety information which must be provided by the manufacturer to the consumer.
The Contaminants in Food (England) Regulations 2003 sets out requirements for the control in levels of contaminants such as pesticides including incorporating EC Directive No. 194/97 which sets maximum levels for nitrates in Lactuca sativa (lettuce) and Spinacia oleracea (spinach). Different levels apply to indoor crops, and to processed spinach, and also vary by season. There is a review underway, and the UK is monitoring the levels. Data is collected for this review (EC Regulation No. 864/99). This legislation also covers disposal of foodstuffs which do not comply with the legislation but which is not deemed to be harmful to countries outside the EC, and makes it an offence to mix crops that do comply with crops that don’t.
The UK Pesticides Guide Blue Book shows a list of all approved products at date of publishing; it is now possible to access updated information via the internet www.pesticides.gov.uk. It is illegal to use unapproved products. The Green book shows information on pesticides direct from the pesticide manufacturers.
