Yellowing leaves can be a sign of different issues that may be effecting your tomato plants. There are disease and fungi that create leaf patterns that may turn leaves yellow. There are nutritional issues that may turn leaves yellow. There are insects and pests that, you guessed it, will turn your tomato plant leaves yellow. Those reasons are reason of concern and often merit a specific treatment or response from the gardener.
There are however, other reasons for leaves to turn yellow that aren't red or should I say yellow flags of alarm. Determinate variety tomatoes die out once they reach a set height and set fruit. That is a normal yellowing and life cycle of the tomato plant. The video highlights my indeterminate varieties of tomatoes and follows 4 planting areas of the 'Brandywine' variety tomato. With 7-10 days of 95-105 degree heat and nearly 100% humidity... a healthy tomato plant responds. That response is often yellowing leaves. And that is nothing to worry about. The tomato sheds leaves knowing it can fall over, set roots to the ground from its stem and grow strong again. It doesn't know we have it staked up.
You do have to make sure you keep the basics going. Make sure they are being fed, watered and inspected and managed for diseases and pests. When all those thing are in check... sometimes yellow leaves are normal. The 'Brandywine' always gets yellow leaves and 'leaf die out' come the high heat in my area. My new variety 'Matt's Wild Cherry' is yellowing heavily. Around this yellowing tomato are healthy green tomato plants that have been cared for the same way. The only difference is the heat, humidity and how different tomato varieties respond to the climate in your Zone. There is something you can do and that is to try out different tomato varieties each year and remove varieties that do not do well in your Zone.
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