Tomatoes are at the end of their productive period for the spring. If they are left in place you will be more likely to produce spider mites and fungal diseases than more tomatoes. Pull the plants early in the month and consider planting a fall crop late in the month. Recommended varieties for the fall include Red Snapper, HM 8849, Tycoon, Celebrity, Red Deuce, Phoenix, BHN 968, Ruby Crush and Valley Cat. Tycoon, Celebrity, BHN 968 and Valley Cat are nematode resistant. The Roma canning tomato is also often planted in the autumn because it produces fruit very quickly and always beats the cold weather.
If spider mites have been a problem in the past apply a preventive spray of 2 tablespoons per gallon of water to the new transplants every week. Spray under the leaves. If the old tomato plants show evidence of nematodes be sure that you use a nematode resistant variety for the fall and do not put the pulled plants into the compost pile. Discard them in the garbage. The same treatment is also recommended for pulled plants that are infested with spider mites.