30 Review Received: 21 June 2015 Revised: 7 August 2015 Accepted article published: 1 September 2015 Published online in Wiley Online Library: 6 October 2015 (wileyonlinelibrary.com) DOI 10.1002/ps.4145 Plant disease management in organic farming systems Ariena HC van Bruggen, a* Abraham Gamliel b and Maria R Finckh c Abstract Organic farming (OF) has significantly increased in importance in recent decades. Disease management in OF is largely based on the maintenance of biological diversity and soil health by balanced crop rotations, including nitrogen-fixing and cover crops, intercrops, additions of manure and compost and reductions in soil tillage. Most soil-borne diseases are naturally suppressed, while foliar diseases can sometimes be problematic. Only when a severe disease outbreak is expected are pesticides used that are approved for OF. A detailed overview is given of cultural and biological control measures. Attention is also given to regulated pesticides. We conclude that a systems approach to disease management is required, and that interdisciplinary research is needed to solve lingering disease problems, especially for OF in the tropics. Some of the organic regulations are in need of revision in close collaboration with various stakeholders. © 2015 Society of Chemical Industry Keywords: organic agriculture; plant diseases; cultural control; biological control; soil health; systems approach 1 INTRODUCTION Organic farming (OF) can be defined as ‘an ecologically, economi- cally and socially responsible way of farming, providing an endur- ing supply of safe and healthy food and fibers, with the least possi- ble losses of nutrients and energy, and the least negative impacts on the environment, as regulated by certification agencies’. 1,2 Worldwide, OF has increased tremendously in importance over the past 20 years, including in developing countries, and the global market for organic products reached a value of almost $US 72 billion in 2013. 3 OF is governed by the idea that all natural pro- cesses within an agroecosystem are mutually dependent on each other, and that management should aim at achieving and sup- porting self-regulation through natural processes. 4 This has been laid down in detail in the OF standards as formulated by the Inter- national Federation of Organic Agricultural Movements (IFOAM) (http://www.ifoam.org/about_ifoam/standards/index.html). Thus, solutions to problems are primarily sought within the ecological possibilities of the farming system. Cropping systems based on monocropping of genetically uni- form varieties and high external inputs, typical of conventional farming (CF), have favored epidemic development of many plant diseases caused by fungi, bacteria, nematodes and viruses (van Bruggen AHC et al., accepted for publication). 5 – 8 The degradation of soil structure and soil quality by loss of soil organic matter favors root diseases. Soil fumigation creates a biologically impoverished, substrate-rich environment favoring the explosive development of plant pathogens that happen to (re)enter the fumigated soil. In addition, high concentrations of nitrogen and imbalances in plant nutrition lead to greater susceptibility to a variety of root and foliar pathogens. 9 The relation between root disease suppression and a large number of soil health characteristics has been reviewed in detail. 8 Compared with CF, OF systems generally have: (i) higher plant diversity in time and space; (ii) rotations and cover cropping, which lead to a higher soil organic matter content; (iii) a higher biomass, diversity and activity of soil microorganisms and fauna; (iv) an increase in water holding capacity, reduced run-off and increased rooting depth, leading to enhanced water use efficiency; (v) improved cation exchange capacity, increased internal cycling and reduced loss of nutrients. 10 – 12 These intrinsic differences between OF and CF systems have resulted in differences in the occurrence and intensity of plant diseases and pests, but these are reviewed elsewhere and will not be discussed in this review. 8,13 In this review, we describe the various disease management options in OF that can also be useful for other farmers and researchers who strive for greater ecological sustainability. We discuss the practices used to manage disease development and conclude with some suggestions for research on plant disease management in OF. 2 PLANT DISEASE MANAGEMENT IN ORGANIC AGRICULTURE Pest and disease control in OF is to a large extent based on the maintenance of soil fertility by balanced crop rotations, includ- ing nitrogen-fixing crops, winter cover crops, intercrops, addi- tions of manure and compost and reductions in soil tillage. 14 – 16 ∗ Correspondence to: Ariena HC van Bruggen, Department of Plant Pathology and Emerging Pathogens Institute, University of Florida, Gainesville, FL 32611, USA. E-mail: ahcvanbruggen@ufl.edu a Department of Plant Pathology and Emerging Pathogens Institute, University of Florida, Gainesville, FL, USA b Agriculture Research Organization, ARO Volcani Center , Bet Dagan, Israel c Faculty of Organic Agricultural Sciences, Ecological Plant Protection, University of Kassel, Witzenhausen, Germany Pest Manag Sci 2016; 72: 30–44 www.soci.org © 2015 Society of Chemical Industry