Tomato plant yellowing timing question

This is my second year successfully growing cherry tomatoes. And actually getting some larger varieties to yield.

My question is, is it normal for a tomatoe plant to go yellow in February or am I doing something wrong? I assume they need more potassium.

Web used good quality potting mix and our own compost. Also some pumice for drainage. And this year a timer watering system. Good growth, late fruit, and now large harvest. But the plants are yellowing in February and we get white mildew mold every year too.

Any recommendations or questions?