A key feature of the UK protected herb industry is its diversity, in terms of both the production systems used and the plant species produced. The major culinary herb species produced under protection are basil, chives, coriander, dill, mint, parsley, rosemary, sage, tarragon and thyme. Production systems include all year round (AYR) glasshouse production of pot herbs for supermarkets, seasonal production of pot or cut herbs in unheated structures for garden centre sales and culinary use, and glasshouse propagation of both culinary and pharmaceutical herbs for field planting. Due to this diversity and the fact that many herb species are poorly adapted to UK growing conditions (low light, cooler temperatures), individual growers expect to encounter a wide range of pests and diseases. Another problem is that pests and diseases on perennial stock plants may be carried over from one season to the next so that planting materials may already be affected at the time of propagation. Damage due to pests and diseases is difficult to quantify but in the pot herb sector, one grower estimated a 5-10% loss in turnover. This was due to plants which were unmarketable when required for sale (resulting in disruption to production and the supply chain), plants with less vigorous growth and extra labour costs for trimming of damaged leaves. Another grower estimated that losses of up to 80% for individual pot herb crops can occur, for example, due to unacceptable numbers of aphids on parsley.
Use of pesticides on herbs is more restricted than on other protected crops as there are very few pesticides approved for use on protected herbs. However, for many pests and diseases, pesticides can be used under Extension of Approvals for Minor Use (EAMUs), formerly known as Specific Off-Label Approvals (SOLAs).
In addition to the loss of active ingredients following pesticide reviews and changes in approval regulations, there are certain pest and disease problems for which effective pesticides have never been approved for herbs. For example, there are no approved pesticides for the control of shore flies on herbs. Similarly, there are no fully effective fungicides for control of vascular wilts of basil (due to Fusarium oxysporum f. sp. basilici), parsley and mint (Verticillium dahliae).
Even when appropriate active ingredients are available, other factors may limit the use of pesticides on protected herbs as follows:
- The harvest intervals required for some pesticides on protected herbs may make them impractical for use, since many herb crops have a very short production period, and pest and disease problems often occur in the 2-3 weeks prior to sale.
- There are existing and potential problems of resistance to some pesticides.
- There are growing demands from customers and retailers to reduce or eliminate the use of pesticides.
- Non-chemical management strategies are required for production of culinary herbs to organic standards.
- Protected propagation of herbs produced for the medicinal and cosmetic markets also requires minimal pesticide usage.
Given these pressures, combining cultural and biological control methods with minimal use of compatible pesticides within an Integrated Pest Management (IPM) programme is the only sustainable strategy for pest and disease control on protected herbs. Protected cultivation in general provides the opportunity for a wider range of non-chemical crop protection techniques to be adopted, particularly with respect to the use of biological control. Growers of AYR pot herbs are routinely using biological control methods for pests and non-chemical methods for control of pathogens (e.g. UV treatment within re-circulating water systems). An increasing number of growers of other protected herbs are using IPM strategies wherever possible. Retailers continue to demand high quality produce, with ‘zero tolerance’ of pests and diseases. Thus IPM methods need to be highly effective, and development of suitable IPM control strategies is needed for some pests and diseases.