In Defense of Crop Monoculture

Because yields typically decline, crop monoculture is commonly considered as not sustainable. This yield decline is due largely to soilborne plant pathogens adapted to/specialized for the roots of the 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 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 with wheat-intensive cropping systems, 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. Introduction Crop rotation, which we define as the practice of growing different crops in sequence in the same field, is nearly as old as agriculture itself and remains the centerpiece of most cropping systems worldwide. Crop monoculture, which we define as the practice of replanting the same crop species in the same field, with no break to a different crop, has an equally long but less successful history, except for some high-value fruit and vegetable crops grown with the aid of soil fumigation or soil solarization, and also possibly for paddy rice. “Crop monoculture” is also used to describe large areas planted to the same crop species, e.g. the vast area planted to wheat each year in the North American Great Plains. For purposes of our defense of crop monoculture, like crop rotation, we mean crop monoculture in the temporal and not the spatial sense. Growers that practice crop monoculture generally do so for economic reasons. The selected crop is the most profitable and any profitability loss from yield declines are less than that which occurs from any rotational options available. In these situations, the ability to minimize the losses associated with monoculture can provide the best option to increase productivity and profitability. Many names have been given to the phenomenon of yield decline with monoculture, including replant problem, autotoxicty, and monoculture injury. Even perennial crops such as alfalfa and grasses are prone to show yield (or stand) decline over time, typically starting in the third or fourth year following their establishment. Similarly with annual crops, yields typically decline starting in the third or fourth year of the monoculture, although some yield decline may occur already in the second year of monoculture (Cook and Baker, 1983). Because of these yield declines, crop monoculture is commonly considered as not sustainable. Crop rotation, like tillage, is an invention of agriculture. Indeed, annual plants in the wild reseed themselves in more or less the same places year after year—and without tillage. The occurrence of wild plants in polycultures could provide a kind of rotational benefit in cases where the seeds of one species happen to fall on the site occupied the previous year by a different species. In the case of wheat, the