A simple model was constructed, tested and used to determine the potential yield of cumin (Cuminum cyminum). Using model outputs and data obtained from 228 fields, yield gap was determined. Yield gap varied considerably among regions (from 2.42 to 0.68 ton ha
1). Stepwise regression on data collected from fields showed that 73% of yield gap variation in 228 fields could be explained by fungal diseases (Fusarium oxysporum and Alternaria burnsii), inappropriate sowing dates and successive planting. Therefore, these were considered to be the main reducing factors in the studied regions, with 38% contributed by fungal diseases, 30% by sowing date and 5% by successive planting. When 67% of surveyed fields (averaged for all fields) were infected with these diseases and when there was a 3-mm increment in precipitation, infection increased about 1%. Our results indicated that 1% of the fungal infection increase equals to a yield loss of 150 kg per hectare. Sowing date of 63% of fields were also not within the appropriate range. Therefore, appropriate sowing date and all possible approaches to alleviate the effects of fungal diseases are the most fundamental advices to fill the gap between potential and actual yield of cumin in Khorasan provinces, Iran. Detailed descriptions of the model, important physiological parameters and other important state variables are also presented.

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