Sudex cover crops can kill and stunt subsequent tomato, lettuce and broccoli transplants through allelopathy

Grass cover crops can be harvested for biomass or used as a surface mulch to reduce erosion, improve soil structure, suppress weeds and conserve moisture. There is concern, however, that such plantings may affect subsequent crops. We studied the effects of sudex, a sorghum hybrid used as a cover crop, on subsequent crops of tomato, broccoli and lettuce started from transplants. Within 3 to 5 days of being transplanted into recently killed sudex, all three crops showed symptoms of phytotoxicity including leaf necrosis, stunting and color changes. There was 50% to 75% transplant mortality in all three species. Plant growth and development, as determined by biomass measurements, were also significantly affected. Yields of mature green tomato fruit and marketable broccoli and lettuce heads were reduced significantly. Tomato, broccoli and lettuce should not be transplanted into sudex residue for at least 6 to 8 weeks, or until the residue has been thoroughly leached.