While cucumbers and tomatoes can technically be grown together, there are several good reasons why it's not recommended:
Both cucumbers and tomatoes are heavy feeders that require lots of nutrients, water and sunlight. When planted close together, they will compete for these resources, leading to stunted growth and reduced yields for one or both crops.
Cucumbers and tomatoes are susceptible to many of the same pests and diseases, like aphids, mildew and wilt. When planted in close proximity, these issues can easily spread from one crop to the other, compounding the problem.
Cucumbers thrive in high humidity environments, while tomatoes prefer lower humidity. The high humidity that benefits cucumbers can promote fungal diseases in tomatoes.
Cucumbers have a sprawling, vining growth habit, while tomatoes grow upright. This difference in structure makes it difficult to provide ideal support and spacing for both crops when planted together.
Some research suggests cucumbers may produce allelochemicals that inhibit the growth of tomato plants. The exact mechanisms are still being studied, but this potential allelopathic effect is another reason to avoid planting them together.
While cucumbers and tomatoes can be grown in the same garden, they are best planted in separate beds or with ample spacing between them to avoid the issues mentioned above.