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Keywords: cause and effect relationship: uni- and multi-causal diseases, complex disease aetiology, bio-disposition, components of an interacting system, association or interaction: some relevant questions, aetiological relations between nematode and fungus as partners in disease complexes, plant parasitic nematodes, nematode-nematode interactions: how to determine? Marginal response, threshold parameter, testing of hypothesis, associate effects and joint response, product response models, combined dosage equivalence, modeling for nematode-mycorrhiza interactions, vesicular-arbuscular mycorrhiza (VAM) fungi