Crop compatibility: state of the evidence
The list of pairwise relations between crops and the result of checking each against the literature.
Supported relations
Mixed cropping of carrot and onion reduces carrot damage by Psila rosae and onion infestation by Thrips tabaci (Uvah & Coaker, 1984). Intercropping onion with brassicas reduces Brevicoryne brassicae numbers and flea beetle damage (review by Parker et al., 2013). Potato and tomato should not be placed together because of the shared pathogen Phytophthora infestans. Tomato should not be placed next to maize because of Helicoverpa zea, for which both are host plants.
Unsupported relations
For 20 of the 24 pairs no publication supporting the stated relation could be found. A substantial part of them traces to Riotte (1975), which contains no references to studies. Such pairs are marked as unsupported in the application.
A discrepancy
The cabbage–tomato relation is marked unfavourable with reference to allelopathy. No publication on an allelopathic effect of cabbage on tomato could be found; the one study located (Zhao et al., 2025) examines an aqueous leaf extract under laboratory conditions on courgette, bean and maize. Secondary sources, conversely, list tomato among the crops whose intercropping with brassicas reduces damage (Tukahirwa & Coaker, 1982). The primary source has not been read; the relation is left unchanged pending verification.
Applicability and limitations
- The Parker et al. review also reports negative results: marigolds did not reduce carrot fly numbers, and aromatic herbs did not affect damage to brassicas.
- The literature review is not complete.
Sources
- Uvah, I.I.I. & Coaker, T.H., 1984. Effect of mixed cropping on some insect pests of carrots and onions. Entomologia Experimentalis et Applicata 36(2): 159–167 — abstract open, full text paywalled Open
- Parker, J.E., Snyder, W.E., Hamilton, G.C. & Rodriguez-Saona, C., 2013. Companion planting and insect pest control. Weed and Pest Control — Conventional and New Challenges. IntechOpen — full text open Open
- The American Phytopathological Society. Late blight of potato and tomato. APS Education Center — full text open Open
- Utah State University Extension. Corn Earworm / Tomato Fruitworm (Helicoverpa zea). USU Extension IPM — full text open Open