Organic and ecological agriculture has increased popularity in recent years mainly due to considering environmental issues, a more sustainable production system and production of healthier food, etc. One of the most important principles for making a farm more sustainable is reducing the use of synthetic agrochemicals and replacing non-renewable petrochemical resources by biologically-based renewable inputs. Biological control strategies are seriously recommended by organic agriculture particularly in today’s integrated pest management (IPM). Biocontrol agents are used in organic agriculture in different ways including conservation, classical and biopesticides. In organic systems, conservation strategies and protection of predators, parasitoids and antagonistic pathogens of pests are recommended, but application of microorganisms is allowed while application of transgenic microorganisms is prohibited. Plant nutrient supply is one of the main issues in organic agriculture, on the other hand, plant susceptibility to pathogens is also undoubtedly affected by nutrition. In greenhouse experiments, replicated groups of common lambsquarters plants raised with different N supplies were sprayed with various isolates and concentrations of Ascochyta caulina, three weeks after planting. Disease development was positively related to increasing plant tissue nitrogen and also to increasing spore concentration. Fungal spore concentration also had a positive effect on plant tissue nitrogen percentage.

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