COLUMBIA, Mo. – The scorching heat of a typical Missouri summer can be hard on our garden plants, including tomato, the most popular home garden vegetable in the U.S., said University of Missouri Extension state horticulturist David Trinklein.
Poor fruit set
Hot weather can make it difficult for plants to produce fruit, particularly when humidity is low. Excessive heat may prevent healthy pollen from developing or keep fertilization from occurring. As a result, plants often flower poorly and set fewer fruits when daytime temperatures exceed 90 degrees Fahrenheit and nighttime temperatures stay above 70 F. Research suggests that nighttime temperatures play the biggest role, with the optimal range being 59 F to 68 F, according to Trinklein.
Delay in ripening
Tomato fruit gets its color from two pigments, lycopene (red) and and carotene (yellow). Temperatures above 85 F tend to slow or even halt production of these pigments. Since 37% of the lycopene in a tomato fruit is contained by its skin, it’s little wonder why tomatoes growing in excessively hot conditions produce poorly-colored fruit, Trinklein said.
Yellow-shouldered fruit
Yellow shoulder is a physiological disorder characterized by areas at the top (shoulder) of the fruit remaining yellow as the remainder of the fruit ripens and turns red. These yellow areas never ripen properly, and the tissue below them is tough and poorly flavored.
Tomato varieties that are green-shouldered when immature are more likely to show the trait than varieties that have uniformly green immature fruit. It appears that both temperature and nutrition are involved in the development of yellow shoulder, Trinklein said.
((INSERT PHOTO…WHITE CORE…HERE))
Photo courtesy of Gerald Brust, University of Maryland.
White core
Under stress, tomato fruit often develops a tough, white core. The white tissue might form only in the fruit just beneath the calyx or, in extreme cases, through the entire depth of the fruit. The internal walls of the fruit may also appear pale and “corky.” Once again, excessive heat and improper fertility seem to be related to the formation of white core. Malnourished plants with poor foliage cover tend to bear fruit exposed to the sun, adding to the temperature stress of the fruit
Blossom-end rot
Blossom-end rot of tomatoes is caused by a lack of calcium in the blossom end of the fruit, resulting in tomato fruit with brown or tan areas on their blossom end. These areas start as small lesions and gradually develop to cover nearly the entire end of the fruit.
Although blossom-end rot is caused by a lack of calcium, it is a lack of water that is most often responsible for its development. Since hot weather increases water loss (via transpiration) from tomato plants, blossom-end rot is most common when in hot weather. You can prevent this problem by maintaining proper soil pH, supplying tomato plants with enough calcium and irrigating on a timely basis.
As the saying goes, “Everyone talks about the weather, but no one does anything about it.”
Unfortunately, there is little we can do other than to keep our tomato plants vigorous, well-watered and adequately fertilized. If foliage is sparse, try covering tomato plants with a material that partially blocks the sun, such as cheesecloth or shade fabric, said Trinklein.