For the past few years my tomato leaves keep turning yellow on the bottom of the plant, which then works its way to the top of the plant. It’s very frustrating; I still get tomatoes, but the plants don’t produce like they used to. Any suggestions?
—Larry in Ohio
The most important thing to know about growing tomatoes is that they love warm soil and air temperatures. In an attempt to harvest as early as possible, many northern gardeners plant their entire crop the same day in May (usually around the middle of the month through Memorial Day). That’s fine when it’s a warm dry spring, but a rainy, cool season is the perfect storm for fungal issues like early blight or septoria leaf spot.
In my zone 5/6 garden I’m actually planting earlier, only with protection and certainly not my whole crop all at once. I’ll put in four or five plants at the end of April under double floating row covers over 11 gauge wire. The spun bound translucent fabric acts as a greenhouse for the plants and (when the stars align) provides early tomatoes to brag about to garden friends. On the other hand, when Mother Nature doesn’t cooperate, the tomatoes struggle in the cold, wet soil.
One key to fighting the fungal diseases on tomatoes is succession planting. After that May planting, continue planting into midsummer for most of the country. My last planting is on July 4th; I’ll put a big cherry or another early maturing variety in the ground. The plants love the warm soil and are rarely affected by fungal diseases that late in the season. It’s important to rotate planting, so tomatoes aren’t growing in the same part of the garden every year. Mulch will help keep soil borne fungal spores from splashing up onto the foliage of the plant.
Another trick is to remove the lower leaves at planting time to make it harder for those spores to reach the foliage. Be sure to give plants plenty of air circulation by planting them at least three feet apart. Grow lots of different varieties, as each one will react differently to disease.
Tomatoes enjoy a hot dry summer, but in case they don’t get one, be sure to use the techniques above. There’s also a great organic fungicide called Serenade which will keep the plants healthy if a cool, rainy season is what we are dealt.