Features
A Cream of a Crop
Irish organic strawberry production involves the drafting of a rotational plan to reduce the incidence of soil borne diseases and to achieve a nutrient balance. The main source of nutrition is farmyard manure. Pests and diseases are controlled by biological means and by good crop hygiene. Weed control is achieved by mechanical methods or by the use of crop mulches. Organic strawberry production requires a high level of management skills and success is achieved by the use of well-balanced rotations, nutrient budgeting and good crop husbandry techniques.
Good fencing, shelter, and a reliable source of irrigation are essential for organic strawberry production. Well-maintained hedgerows and field margins are an important addition to the infrastructure on an organic farm. Dry banks and ponds are also an advantage. These provide protection and a good habitat for birds, small animals and insects, many of which act as predators and parasites on crop pests
Ground Preparation and Planting
Runners propagated from certified stock should be used to ensure that the plants are free of disease. The ground used should not have been planted with strawberries for a period of ten years at least to avoid the risk of root disease and ellworm infection. Strawberries grow best in slightly acid (pH 6.5) deep, well-drained soil with a loam texture. The ground is ploughed to a depth of 20-25 cm using a digger plough. A strawberry plantation will remain productive for two to three years and a relatively heavy dressing of farmyard manure (FYM) is required to supply adequate plant nutrients over this period. The FYM is applied to the soil surface and ploughed in. Raised beds 1.0m wide mulched with black polythene are constructed at 1.5m centres. The runners are planted in double rows spaced at 45cm at a spacing of 40cm in the row. The use of the polythene mulch allows for a more rapid increase in soil temperature and earlier fruit. Fruit yields are also increased.
Soil borne diseases
Red core root rot is a serious soil borne disease of strawberries, which persists in the soil for a number of years. A minimum seven-year rotation is required to control this disease. Experienced growers prefer a 10-year rotation for good control of this and other soil borne strawberry diseases.
Nutrition
Strawberries remove less nutrients from soil than most other crops. With the standard plant population of 27,000 plants per hectare only 38 kg of nitrogen (N), 5 kg of phosphorus (P) and 44 kg of K is removed annually from the soil. Although the nutrient offtakes are low the strawberry crop benefits from a high level of soil fertility. A soil analysis should be carried out so that the appropriate dressing of FYM can be calculated. Due to the small size of the root system the root zone is relatively restricted. The strawberry plant is a poor forager and grows best when the nutrient content in the root zone is high. In the sixty days between flowering and fruit production the fresh weight of the strawberry plant is increased by 125 per cent and sufficient nutrients must be available during this peak demand period to support the plant growth. The strawberry crop will occupy the ground for a period of two to three years and sufficient FYM must be applied during that period to support the crop. The strawberry plant extracts over 90 per cent of its nutrient requirements from the soil within a 25 cm radius of the central crown of the plant. One tonne of FYM contain 5 kg N, 1 kg P and 10 kg K. When all the above factors are taken into consideration, a dressing of 35 t/ha FYM is required to support the strawberry crop for a period of three years.
Weed control
Weeds are controlled by the use of the black polythene mulch under the crop and the use of straw in the interrows. When straw is not available mechanical means of weed control are used in the interrow. The "tearaway" spring tined harrow, the Kelmink rotary harrow and the brush weeder are adjustable cultivators which are suitable for this purpose.
Pests and diseases
Soil borne diseases are controlled by rotation. When the use of soil sterilants and fungicides is discontinued, organisms which parasitise or compete with disease organisms build up in the soil over time. These reduce the infection levels of most diseases to a tolerable level. Examples of these organisms are trichoderma, protozoa, fungii, bacteriophage and bacteria. Trichoderma is active against root diseases, some protozoa species feed on bacterial disease organisms, fungii attack nematodes, bacteriaphage infect and kill bacteria and some bacteria species control fungal diseases, e.g fusarium. Some of these organisms are at present being developed as a biological method of disease control. The cultivars Florence, Eros, Emily and Everest have shown good tolerance to red core root rot and crown rot. Symphony is very resistant to soil borne diseases.
Botrytis: Botrytis is one of the most virulent diseases of organic strawberries. Due to the low concentrations of soil nitrogen organic strawberries are less susceptible than conventional crops. This disease can cause considerable crop losses in a wet year. The use of raised beds with black polythene encourages moisture runoff and will reduce the level of infection. Irrigation by T-tape rather than overhead irrigation also reduces infection. Removal of all ripe and infected fruit also reduces the source of infection. This disease is less severe in well-ventilated crops grown under cover than in field crops. The losses commonly recorded due to botrytis infection are 10-15%.
Mildew: Powdery mildew does not infect field crops and occurs in protected crops only in the autumn. The most susceptible cultivars are Elsanta and Tango. The newer cultivars Florence, Everest and Symphony are resistant to this disease.
Redspider: This pest occurs regularly in protected strawberries in dry, warm summers but is not a serious pest in field crops. Good control is achieved by the application of Phytoselius persimilis in protected crops.
Vine weevil: This pest can cause severe damage to strawberry crops in the second and third year of cropping. The application of Heterorhabditis megidis (parasitic eelworm) gives good control of this pest. Several species of carabid beetles feed on vine weevil. These carabids build up to a considerable population under organic production due to non-use of pesticides. As a result vine weevil is a less serious pest in organic crops. Other common pests of strawberries are less prevalent in organic crops. This is due to the exclusion of pesticides and the build up of natural predators and competitors and a more gradual supply of nutrients such as N-excessive nitrogen promotes soft, lush growth, which is more attractive to pests.
Crop management
The main crop management issues which arise, are runner control and crown thinning. Runners are produced from mid-summer onwards. These are produced at the expense of fruit yield and quality and must be removed. In the case of mulched beds these runners must be removed by hand. In the second and third year of production individual plants begin to produce additional crowns. These crowns produce large numbers of flowers and fruit. The fruit size and quality is reduced when too many crowns are produced. In the second and third year the number of crowns should be reduced to two in the case of normal plants and to three in the case of large plants.
Economics
Investigations in the EU have shown that yields of organic strawberries can be as high as 12-15 tonnes/ha. The premium paid on organic strawberries is 50 to 100%. On this basis profit margins for organic strawberries compare favourably with those of conventional crops.
Gillian Westbrook has been appointed to the new position of
General Manager with IOFGA. Coming from a background in the regulatory area...
