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The Production of Phytotoxins from Surface Crop Residues¹

ABSTRACT

Experiments were conducted to determine if crop residues, common to eastern Washington, were likely to cause phytotoxicity problems with no-till planting of winter wheat (Triticum aestivum L.) The possible role of microorganisms was assessed also. Mats (5 to 8 cm thick) of lentil (Lens culinaris Medic.), pea (Pisum sativum L.), wheat, barley (Hordeum vulgare L.), and bluegrass (Poa pratensis L.) residues were spread over bare soil in Sept. 1975. Water extracts of the residues and soil beneath them were bioassayed weekly for wheat-seedling phytotoxins, and the residues were plated biweekly to determine the numbers of fungi, bacteria, and pseudomonads from Sept. 1975 through May 1976. All residues produced wheat-seedling root inhibitors, but only after conditions became favorable for microbial growth. Toxin production from these straws was irregular and was generally preceded by wet weather with air temperatures above freezing but below 15°C. Lentil- and pea-straw extracts were most toxic to wheat seedlings in the fall and early winter, causing up to 90% root growth inhibition but little or no inhibition later. Wheat- and barley-straw extracts were less inhibitory. However, these residues intermittently produced seedling inhibitors in the fall, late winter, and until late April when the straw dried. Bluegrass straw extracts showed toxic substances primarily in late winter and early spring. The delayed toxin production could explain why fall-killed bluegrass sod was detrimental to no-till seeded spring wheat, while spring-killed no-till seeded sod produced excellent wheat yields. The toxin(s) extracted were apparently different from those described by other researchers, since they (i) inhibited root growth only, (ii) were produced at lower temperatures, (iii) were produced cyclically over longtime periods, (iv) were stable when autoclaved, and (v) were not soluble in the organic solvents tested.

¹ Contribution from the ARS, USDA, Western Region, in cooperation with Washington State Univ., Pullman, WA 99164. Sci. Pap. no. 4764.

² Soil Scientist, Microbiologist, and Soil Scientist, respectively.

Received for publication March 9, 1977. Accepted for publication June 15, 1977.

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