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item Weller, David

Submitted to: International Crop Science Congress Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 9/26/2004
Publication Date: 9/26/2004
Citation: Cook, R.J., Weller, D.M. 2004. In defense of crop monoculture. in: new directions for a diverse planet. International Crop Science Congress Proceedings, Brisbane, Australia, Crop Science Society.

Interpretive Summary: Traditionally, crop monoculture has not been considered a sustainable practice because of the enrichment of soilborne pathogens, which infect roots and reduce yields. However, worldwide there are many examples of crop monoculture leading to the development of suppressiveness in the soil to soilborne pathogens. The best example is take-all decline, which develops during wheat or barley monoculture following an outbreak of the root disease, take-all. Disease suppressiveness associated with monoculture results from the buildup in the soil of antagonistic microorganism, which colonize and protect the roots against pathogen attack. By managing antagonistic microorganisms in the, farmers can reap the benefits of natural disease protection without off-farm inputs.

Technical Abstract: Because yields typically decline starting with the 3rd or 4th consecutive crop, crop monoculture is commonly considered as not sustainable. This yield decline is due largely to soilborne plant pathogens adapted to the roots of that crop. For high-value fruit and vegetable crops, yields are maintained with monoculture using soil fumigation or soil solarization. Soils can also be sanitized by flooding, which may account for the success of paddy rice monocultures. Our work in the U.S. Pacific Northwest has focused on diagnosis and control of four root and crown diseases of wheat and barley, namely take-all caused by Gaeumannomyces graminis var. tritici, Fusarium crown rot caused by Fusarium culmorum and Fusarium pseudograminearum, Rhizoctonia root rot caused by Rhizoctonia solani AG8 and R. oryzae, and Pythium root rot caused by several Pythium species. Herein, we describe a remarkable and apparently wide-spread microbiological control (disease suppression) in the rhizosphere that is responsible for the well-documented decline of take-all and coordinate increase in crop yield following one or more outbreaks of the disease and continued monoculture of wheat or barley. Since this disease suppression is specific for take-all, other strategies are under development for control of the other three root and crown diseases, including in direct seed systems. The strategies include the development of transgenic resistance in barley to Rhizoctonia root rot using the ThEn42 gene from Trichoderma harzianum for production of a 42-Kda endochitinase, selection of wheat cultivars for tolerance to Fusarium crown rot, and use of a systemic seed-treatment chemicals and current-year seed for seedling protection against Pythium root rot.