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The Production of a Noninfectious Lettuce Root Rot Under Controlled Environmental and Soil Conditions. John P. Hartnett, Former Graduate Assistant, Department of Plant Pathology, Cornell University, Ithaca, New York 14850, Present address of senior author: Geigy Agricultural Chemicals, Ardsley, New York 10502; James W. Lorbeer, Associate Professor, Department of Plant Pathology, Cornell University, Ithaca, New York 14850. Phytopathology 61:1153-1158. Accepted for publication 1 April 1971. DOI: 10.1094/Phyto-61-1153.

A noninfectious root rot of lettuce (Lactuca sativa) was produced by placing 10-week-old lettuce plants in soil-crop residue mixtures for 3 weeks (exposure period) which previously had been incubated for 5, 7, or 9 weeks (incubation period). Vascular discoloration and corkiness of the root surface were the two characteristic symptoms produced. The mixtures contained either (i) 200 g organic soil; (ii) 200 g organic soil and 500 ml water; (iii) 200 g organic soil and 300 g chopped fresh lettuce leaves; or (iv) 200 g organic soil, 300 g fresh lettuce leaves, and 500 ml water. Only the last mixture produced vascular discoloration in the roots of the plants during the exposure period. Production of the corky root symptom was not specifically related to any of the four incubated mixtures, but occurred whenever plants were subjected to flooded soil conditions for 1 week, followed by a 2-week drying period. Vascular discoloration occurred when the 300 g of fresh lettuce leaves in the fourth mixture were replaced by either 20 g of chopped dried lettuce leaves or roots, 25 g sucrose, 25 g casein hydrolysate, or 25 g sucrose and 25 g casein hydrolysate. When the length of the incubation period was increased, the amount of vascular discoloration observed after a 3-week exposure period decreased. Increasing the amounts of either lettuce residue or sucrose in the mixtures increased vascular discoloration and root injury. The standard method developed for producing the two symptoms of noninfectious root rot involved transplanting 10-week-old lettuce plants into 2-week-old mixtures of 20 g dried lettuce leaves, 300 g organic soil, and 1 liter water, followed by an exposure period consisting of 1 week under flooded conditions and 2 weeks during which the soil was allowed to dry.