Solanaceae — the nightshade family
The nightshade family. It supplies potato, tomato, pepper, eggplant and tobacco — crops with almost nothing in common agronomically, which share a chemistry and a pathogen list that make them a genuine rotational unit.
A botanical family. Membership is a fact about the plants and is derived from the verified identities, never listed by hand.
Glycoalkaloids
The family characteristically produces steroidal glycoalkaloids, and their distribution within the plant is what makes these crops edible: they concentrate in leaves, stems and green tissue rather than in the ripe fruit or the mature tuber. Potato tubers greened by light exposure accumulate them, which is why storage in the dark is a food-safety practice and not a cosmetic one.
One pathogen list, five crops
Late blight, bacterial wilt, Verticillium, root-knot nematode and several viruses move freely between the cultivated Solanaceae. That makes rotation within the family no rotation at all — following potato with tomato carries the inoculum forward — and it is the most practically useful thing the family tells a grower.
Volunteers and weedy relatives
The family’s disease cycles persist outside the planted crop. Potato volunteers grow from tubers missed at harvest and carry late blight and viruses into the following season, and several weedy nightshades host the same pathogens on field margins and in the rotation crop itself. Controlling volunteers and related weeds is therefore part of the rotation rather than a tidiness measure, and a rotation that is nominally out of the family but full of volunteers is not a rotation at all.
Warm-season crops in temperate systems
The cultivated members are frost-sensitive crops of tropical American origin grown far outside that range, which is why transplanting, protected cultivation and short-season cultivars feature so heavily in their agronomy. The family did not evolve for the seasons most of it is now grown in.
What this is not
- The vegetables group holds most of these crops, but potato sits in roots and tubers because its post-harvest behaviour is that of a storage organ.
- Peppers and eggplants are botanically berries and are handled as vegetables, which is a culinary convention rather than a botanical one.