Walnut trees can cause a problem called walnut wilt.
Walnut and butternut roots exude a toxin called juglone that is capable of killing tomatoes that are growing near these roots. Tomato plants will wilt as if they need water but do not recover. These symptoms resemble those of two fungal wilts, Fusarium and Verticillium. Woody stem tissue of affected tomatoes turns brown, and the plants die. Avoid this problem later in the season by not planting tomatoes in the vicinity of walnut or butternut trees now.