Soil health management and biodiversity: the central pillars of plant disease management in organic agriculture

Summary

There are some important differences between organic and conventional farming systems that have the potential to greatly affect the importance of various pathogens within farming systems. The most important difference between organic and conventional systems is the organic approach to soil fertility management and the unavailability of highly effective pesticides. As a consequence, organic plant disease management is based almost entirely on prevention through the use of resistances, rotations, hygiene and seed health, organic matter management, and diversification strategies.
Soil fertility in organic systems is achieved through the careful management of organic matter inputs, rotations, and the use of leguminous crops wherever possible for nitrogen fixation. While tillage operations are mostly related to mechanical weed control as no herbicides are available, they also enhance mineralization processes and thus short term availability of nutrients. Compared to conventionally managed soils that frequently receive soluble mineral nitrogen fertilizers and that are managed with very simple rotations or with more or less continuous monocultures, organically managed soils usually have higher contents of organic matter and higher soil microbial biomass and activity.
The effects of organic soil management on the occurrence and importance of soil borne diseases are manifold. On the one hand, the increased use of legumes in the rotations can lead to problems different from conventional soils due to the great susceptibility of many legumes to broad host range pathogens and nematodes such as Phoma medicaginis, Fusarium avenaceum and other Fusaria, Pratylenchus, Paratylenchus, and Meloidogyne species, e.g. On the other hand, it has been shown that many soil borne diseases are suppressed in well managed organic soils. Disease suppression may be due to direct suppression of the pathogens but also to improved resistance of the plants triggered by improved soil structure in soils high in organic matter, the diversified soil flora and fauna, and often improved mycorrhization. One of the most critical factors with respect to plant health is the often strong reliance of organic farmers on the plough as ploughing destroys soil structure, strongly affects soil organisms and may lead to decreased organic matter contents in the soil overall. This counteracts many positive effects of organic soil management and there is much to learn within the farming systems to improve weed management without compromising soil quality. Approaches combining minimum and no-till with living and dead mulches will be discussed and their potential effects on soil borne diseases.
The usually reduced nutrient availability in organically managed systems affects plant size and density and the composition of the plants especially with respect to dry matter and proteins. In addition, resistance induction through soil organisms can significantly change overall susceptibility above ground. This changes the importance of several airborne diseases. Obligate pathogens such as rusts and mildews are therefore often less important in organic field crops. With respect to the downy mildews including Phytophthora infestans such effects are less evident, however, making the downy mildews very important pathogens in organic systems as there are only few organic pesticides and biocontrol agents available for the control of airborne diseases and many of those are not as efficient as conventional products. The often higher incidence of weeds can also affect the microclimate. In addition to these differences seed borne pathogens are generally of greater importance in organically managed systems as only few if any possibilities exist for the control of seed borne pathogens except for hot water treatments which are at times difficult to apply.
To determine the importance of pathogens on agricultural crops, yield loss assessments are usually done with healthy controls as reference. This is very difficult in organic systems due to the unavailability of effective fungicides. Comparisons in conventional soils are not very informative. Approaches based on assessments on a fine spatial scale and regression analysis including nutrient availability have resulted in important insights on the interactions between varietal characteristics, nutrition, and yield loss in potatoes due to P. infestans under organic conditions.
One of the most powerful tools available to farmers for managing diseases and other stress related problems in plants is the use of biodiversity at all levels starting from biodiversity in the soil microbiota to diversity of endo- and epiphytes to plant diversity and structural diversity at the landscape level. With respect to soil-borne pathogens a powerful management tool is the use of suppressive composts. Examples of disease reductions for Rhizoctonia solani in potatoes and for several soil borne pathogens on leguminous crops will be presented. For the management of air-borne pathogens the use of plant diversity within and among species is well known to reduce many disease problems. In addition, the management of the surrounding vegetation can be of importance. Here questions of scale at which diversity is applied may be important with respect to practicality. Geostatistical models on the one hand and adaptation of diversification strategies to the machinery or vice versa can be very helpful in finding solutions especially at the inter-specific level. At the intra-specific level the availability of germplasm with appropriate resistance diversity is an important issue.
Pathogens constantly adapt and evolve in response to their host genetic composition. In contrast, current crop varieties are actively hindered in their adaptation to the pathogens as no genetic change over time in the form of co-evolution is possible in genetically uniform varieties. To deal with this problem breeding for diversity in annual and perennial crops has been promoted with more or less success since about 100 years. Spectacular successes have been achieved in coffee in Colombia resulting in yearly savings in fungicide use of up to 100 Mio $ US (Alvarado et al., 2008a,b). For cereals and several other crops there are many initiatives working on diversification strategies including breeding for diversity. An important aspect is the involvement of farmers in the breeding process. These approaches are generally leading to highly adaptable and resilient crops that will make an important contribution in future agriculture under conditions of increasing unpredictability and climate change. Examples will be presented and the potentials and difficulties including some legal consequences will be discussed.
References
Alvarado, G., Huver, E. Posadas S., Hernando, A., and Cortina G. 2008a. I. La variedad Castillo: Una variedad de café (Coffea arabica L.) con elevada productividad y amplia resistencia a enfermedades. Fitotecnia Colombiana 8:1-21.
Alvarado, G., Huver, E., Posadas S., Hernando, A., and Cortina G. 2008b. II. Las variedades Castillo regionales: Variedades de café (Coffea arabica L.) con alta productividad, elevada resistencia a enfermades y adaptacion especifica. Fitotecnia Colombiana 8:22-38.