Now that the hot days of summer have finally arrived, gardeners are beginning to notice the effects of heat on plants. Many gardeners recognize that wilting and leaves with brown or "burned" margins are plant symptoms associated with hot weather and poor watering practices. But in some plants, the fruit, not the leaves, tell you that either you're not watering enough or that the plant species or variety you are growing just does not tolerate hot weather.
A good example is blossom-end rot of tomatoes, a problem that many gardeners encounter, but one that few associate with hot weather. Blossom-end rot appears as a leathery, water-soaked spot at or near the blossom scar of green fruits. The skin remains unbroken because it is the tissues beneath that have dried out and collapsed.
The disorder is caused by changes in soil moisture or sudden water shortages. It is most severe when rapidly growing plants in well-watered soils are suddenly exposed to a hot, dry spell. The best way to prevent blossom-end rot is to keep your plants well watered. Mulch your plants to help conserve moisture and keep the soil cool. You need to correct problems relating to poor soil drainage before you plant tomatoes, because a lack of aeration contributes to the problem.
Hot weather, especially when temperatures exceed 100 degrees for several days, also can interfere with tomato fruit set. A hot spell may interfere with fruit set for a week to 10 days, even after temperatures become cooler. This is especially true with tomato varieties that are not well adapted to the hot, dry weather of the Central Valley. Along with the heat, strong winds may dry pollen, preventing pollination of the blossoms and decreasing fruit set.
Another example of a hot-weather problem is a disorder called pit burn, which occurs in the fruit of some apricot varieties. Pit burn affects apricots between the time the fruit begins to lose its green color to about the time it's ripe enough to pick. After a few days in the high 90s or above, pit burn may begin to show up, especially in a heat-intolerant variety like Blenheim (Royal). The first evidence of the disorder is a softening of the flesh around the pit, as if the pit had been heated, causing the flesh in the interior to ripen faster than the exterior. The soft area around the pit turns brown in a few days, but there is no evidence of trouble on the outside of the fruit. When cut, fruit in the late stages of pit burn often is affected by one or more of the rots that attack ripe fruit.
Because pit burn is caused by hot weather, there is little you can do to prevent it in some varieties. Keeping the soil moist during the time the fruit is maturing may help somewhat. Use moisture-conserving mulches whenever possible.
Ed Perry, a farm adviser with the UC Cooperative Extension, can be contacted at 525-6800 or firstname.lastname@example.org.