Tomatoes are 95 percent water, with most varieties soaking up summer rains to become pump and juicy on the vine. Water – along with sunshine and nutrient-rich soil – is needed at every stage of the growing process. Otherwise, tomatoes won't grow, blossom and produce fruit. The manner in which water is delivered, either via rain or via a controlled system, can affect tomatoes in a positive way, or can also have devastating results.
Cracking and Splitting
A tomato's fragile skin can easily start splitting and cracking due to inconsistencies in soil moisture. Once a tomato starts to ripen, it forms a protective skin that helps it during harvest, but if rain is heavy during ripening time and the tomato receives too much water, that skin will crack and split. Laying mulch and setting up a regular watering schedule can help prevent splitting and cracking by keeping the soil moisture consistent despite irregular rain.
Blossom End Rot
A tomato plant can start to rot at its blossoms, causing the blossom end of the fruit to turn brown. If rain has been inconsistent, your tomato plants are not getting enough nutrition, especially calcium, because the soil is dry and not able to deliver the proper nutrients to the plant. To remedy this, be sure to lay mulch on your garden to retain moisture and don't rely only on rain to water your garden. Water your garden daily to keep the soil consistently moist. If splitting or cracking has occurred, remove the bad tomatoes and apply a calcium chloride spray to the new growth.
A strong, heavy rain can help keep your tomato plants free from spider mites, which are barely visible to the human eye, and aphids, which are tiny pear-shaped insects that like to hang out on leaves. The pressure from a heavy rain can keep these insects at bay by washing them away, but if there hasn't been a hard rain in a long time, soapy water or a chemical spray may be needed to control the insects.
Fungal blight can devastate a tomato plant, with the spots first appearing on lower leaves. Fungal blight rarely occurs during dry times, but once the rainy, humid season hits, fungal blight can quickly destroy a tomato plant. To control blight, fungicide sprays should be applied prior to a rain event.