Q: What is the cause of the bottom limbs on my tomato plants turning yellow?
A: Most tomato gardeners have seen the symptoms: lower leaves get dark spots, turn bright yellow, then drop off the plant. As the season progresses most leaves drop off the tomato vine, leading to sun scald if any fruit forms.
The disease is early blight, Alternaria solani. The fungus is present in most soils and can not be eliminated. It attacks tomatoes when humidity and temperatures are high. The best practice to limit disease occurrence is to mulch under the plant IMMEDIATELY after planting. In this way infected soil cannot splash onto lower leaves. Water using a soaker hose, keeping the leaves as dry as possible at all times. Fungicides labeled for garden use can also be used to keep the disease from spreading up the plant.
