Allelopathy and microclimatic modification of intercropping with marigold on tomato early blight disease development

Marigold (Tagetes erecta L.) is a multipurpose crop with ceremonial, ornamental, medical and pharmaceutical uses, and reported antimicrobial properties. In this study we evaluated the effect of marigold intercropped with tomato (Lycopersicon esculentum Mill.) on Alternaria solani (Ellis and Martin) Jones and Grout conidia germination in vitro, on conidial density and tomato leaf damage in vivo, as well as microclimatic changes, compared to tomato intercropped with pigweed (Amaranthus hypochondriacus L.) and monocropped tomato. We found that intercropping with marigold induced a significant (*P < 0.05) reduction in tomato early blight caused by A. solani, by means of three different mechanisms. One was the allelopathic effect of marigold on A. solani conidia germination, as it was shown in vitro conditions; while pigweed did not have any of this inhibitory effect in conidia germination. The second way was by altering the microclimatic conditions around the canopy, particularly by reducing the number of hours per day with relative humidity greater than or equal to92%, thus diminishing conidial development. The third mechanism was to provide a physical barrier against conidia spreading. When intercropped with tomato, pigweed plants worked also as a physical barrier and promoted reductions in the maximum relative humidity surrounding the canopy, but to a lesser extent than marigold. (C) 2003 Elsevier Science B.V. All rights reserved.