Development of disease control strategies for organically grown field vegetables (DOVE) (OF0168)

Summary

This is the final report of Defra project OF0168 (DOVE). The attached main report starts with a more detailed and comprehensive Executive Summary, from which this text has been extracted.
This project aimed to improve understanding and awareness of diseases in organic field vegetables. Diseases pose a threat to both quality and yield, and both must be managed if organic production is to expand and remain sustainable.
Objective 1.
To review the significance of diseases in organically grown field vegetables and the potential effectiveness of current control strategies when the scale of production is increased.
Objective 2.
To evaluate novel techniques and strategies for control of diseases in organically grown field vegetables
Objective 3.
To monitor disease development in organic crops in relation to rotation and size of enterprise.
Objective 4.
To update, produce and publicise advisory literature for disease control in organic field vegetables.
Literature review
The philosophy of disease control in organic systems was reviewed together with recent literature on diseases and disease control in organic field vegetables. A wider literature on disease control in conventional agriculture and horticulture was also considered in order to provide novel approaches for organic producers.
Novel approaches
The benefits of using wider plant spacing and mulches of paper or black, red or blue polythene for control of lettuce diseases was investigated in two replicated field experiments on cv. Saladin in 1999. Lettuce variety mixtures were used to investigate their value for control of downy mildew. Mixtures often have positive effects in addition against pests, weeds and abiotic stress. Glasshouse experiments with foliar spray treatments were carried out to determine the spectrum of activity of a range of organic treatments against lettuce downy mildew, grey mould of lettuce and dark leaf spot of brassicas. All treatments gave significant control of lettuce downy mildew and dark leaf spot, but none (except the conventional standard) showed activity against grey mould. Field evaluation of foliar treatments was carried out in 2000 and 2001. It would appear that some organic treatments can provide effective control of foliar diseases, but they need to be used as protectant sprays with a short interval between treatments. Soil amendments with green crop residues, composts, manures and other treatments were evaluated in glasshouse pot experiments against various soil-borne diseases with mixed results.
Disease monitoring
Disease observations were made on 5 organic farms selected to provide different scales of production and contrasting regional differences. The results provide new information to suggest that larger scale production is more prone to disease problems. This observation is consistent with other epidemiological studies, including the mixture effect, which suggest that increasing the diversity of potential disease hosts per unit area tends to reduce the probability of infection. The presence of the weed parasite spores and of semiochemicals released from the weeds could have some influence in restricting disease development in crops through induction of resistance. Growers reported that potato blight was the most frequent problem each year during 1999-2001, followed by downy mildews on onions, brassica seedlings and lettuce, celery leaf spot and leek rust.
Conclusions
Diseases are common in organic vegetable crops, but severe infection leading to significant losses of yield or quality affected few crops (11%). Seed-borne diseases are important and availability of healthy seed would reduce losses in vegetables. Soil-borne diseases continue to trouble organic growers and can be managed through rotation or avoiding badly infested areas. Organic growers can exploit genetic diversity through cultivar and species mixtures, but development and guidance is required to develop practical systems. Disease management using organic conditioners, biological control agents and soil amendments merit investigation in farm-scale experiments.
Key components of disease management strategies for organic vegetables have been made available to growers in a booklet produced as part of this project.