Effect of crop rotation on pink root rot disease of onion in Isfahan

The pink root rot disease، Phoma terrestris (syn. Pyrenochaeta terrestris) causes considerable losses in onion production of Isfahan، Iran. In this study، the effect of a suitable crop rotation system on 441 onion fields in the two main onion growing areas، Baraan and Ghahderijan for the two dominant cultivars، Texas-Early-Grano and Yellow-Sweet-Spanish was evaluated during 2009-1010. The results indicated that، in the fields where onions were grown in rotation with cereals، the disease severity and infection percentage were the least in comparison with other rotation types. Also، the orchards that were recently converted into field crops showed only 2. 83 percent disease severity. Infection rate of onion in rotation with vegetables، alfalfa، and when onion was planted more than one time in the last four years was 17. 66، 27. 00 and 36. 16%، respectively. Between the cultivated varieties، Texas-Early-Grano showed less infection and disease severity (20. 5%) compared with Yellow-Sweet-Spanish (24. 78%).