Can soil management support crop disease control? : The case of Fusarium wilt in banana

Crop diseases imply a high cost to control them and losses in the yield. Besides, effectiveness of conventional control methods such as pesticides can decrease in their effectiveness. There is a greater awareness of the fact that the optimal crop disease management system requires a combination of different approaches. Soil management may be a component that would modulate the expression of crop diseases. Despite of reports on the influence of soil conditions in the control of diseases, results are inconsistent. Based on the literature, a conceptual framework is proposed, where three types of soil influence in disease incidence are identified: a direct influence, where the pathogen is affected; an indirect influence, where the plant’s response to the disease is modulated, and a third, where both the first and the second occur simultaneously (Chapter 1). The model was examined for the case of Fusarium wilt (Fusarium oxysporum f. sp. cubense or Foc) in banana (Musa sp.). Tropical Race 4 (Foc TR4), one strain of this soil borne fungus could sweep away 80% of the whole bananas. It represents a high risk for the large-scale banana production and also for small-system. Foc Race 1 devastated the Gros Michel cultivar in Latin America and the Caribbean (LAC) during the 20th century. Social and economic impacts were truly significant. The was the shift of the Gros Michel to cultivars from de largely resistant to Foc Race 1 sub-group Cavendish (Musa AAA). With the reports of the new strain Foc TR4 and its wide range of susceptible cultivars, the research in the disease was reactivated. Foc can survive in the soil for decades. To better understand the Foc-soil-banana relationship, the research was divided in five phases in where different research tool with advantages and disadvantages were combined: literature, greenhouse experiments, field experiments and farm survey. The research questions were: i) Which soil properties are known to have an important role in disease expression, ii) Does the relation between Foc and soil differ for TR4 and R1?, iii) How does the infection build up over time?, and iv) Does the relationship differ under different soil types or agro-ecological conditions? Chapter 2 presents a retrospective analysis of commercial plantations of Cavendish in the banana-systems of Costa Rica. The chemical and biological soil properties linked to crop production were determined. Three farms with high productivity and three with low productivity and their areas with the highest and lowest relative productivity were identified. Soil abiotic and biotic properties and production level were analyzed. Soils with high pH, low acidity, high Ca and Mg, low Fe and Cu, and larger populations of microorganisms were linked to a higher production. The Chapter 3  re-takes the research line from the last century where reports were obtained with the model Foc Race 1 and banana Gros Michel. The disease according to soil management, focused on pH (lower than 5.2 and higher thar 6.0) and N (low, medium, and high) was studied. A first experiment in Costa Rica (Gros Michel and Foc Race 1) and a second experiment in the Netherlands (Cavendish and Foc TR4 and Foc Race 1) showed that a pH lower than 5.2 accelerated disease symptoms, expressed as a lower plant biomass. In addition, low pH was found to produce disease symptoms caused by Foc Race 1 in Cavendish banana, even though Cavendish is resistant to this Foc strain (Experiment 2). The tested soil properties have a considerable potential in modulating Fusarium wilt in banana not only in Gros Michel but also in Cavendish bananas. These results were confirmed in another greenhouse experiment (Chapter 4), using 8 representative soil types where banana is cultivated with the same treatments. Even though low pH was linked with higher wilting and lower biomass, each soil had a different behavior. Each soil should be considered as a specific package of soil conditions that will naturally modulate crop behavior towards diseases. The main results found in the greenhouse phase were translated into experimental plots at the field (Chapter 5). In two agroecological banana regions in Costa Rica, during four crop cycles, the studied soil properties, all significantly important in crop production (pH, N, Ca + Mg y Mn) influenced Gros Michel banana response to Foc Race 1. High pH (higher than 6.0) decreased disease incidence and its interactions with the rest of the soil properties modulated disease. Additionally, time was found to play a crucial role in the disease where the highest wilt incidence became present after the second evaluated crop cycle. To validate preliminary results, a greenhouse experiment was carried out (Chapter 6), along with retrospective surveys at different plantations in a predominantly Gros Michel banana-producing region in Costa Rica with presence of Foc Race 1. The greenhouse experiment included two representative soils from the area and the same treatments that were applied in the previously. For the survey, 49 locations were classified according to the aggressiveness of the disease. The greenhouse experiment verified the effect of pH and its interaction with N in disease. Soil sample analyses showed that the most infected areas had a higher soil acidity, Ca, Mg. and a lower content of soil organic matter (SOM). Furthermore, environment may also play a role in the expression of the disease. Soil properties can influence the expression of Fusarium wilt in banana, but it is complex. Aspects such as soil type and environmental conditions play a role in disease expression, crop predisposition, or both. The systematic study approach (literature review, greenhouse experiments, field experiments) let us unravel the complexity of interaction between the soil, Foc and banana. Although there is no “silver bullet” to solve the problem of crop diseases, especially Foc in banana, soil management emerges as a complex, but real option to be included in the disease control package (Chapter 7). This would be feasible as long as the particular soil conditions and the possible ecological implications are considered before implementing soil management practices, as has been demonstrated for the case of Fusarium wilt in banana.