Both tomatoes and potatoes are susceptible to similar pests and diseases, such as blight, so planting them together can increase the risk of infection for both crops.
Peppers are in the same family as tomatoes (Solanaceae) and can attract similar pests and diseases. Planting them too close together can lead to cross-contamination.
Like peppers, eggplants belong to the Solanaceae family and can share pests and diseases with tomatoes, making them less ideal companions.
Corn can compete with tomatoes for nutrients and space, leading to reduced growth and yield for both crops if planted too close together.
Fennel produces chemicals that can inhibit the growth of nearby plants, including tomatoes. It's best to keep these two plants separated in the garden.
Walnut trees release chemicals called juglones, which can be toxic to many plants, including tomatoes. Avoid planting tomatoes near walnut trees.
Similar to other members of the cabbage family, kale can attract pests that might affect tomatoes, such as aphids and caterpillars.