LSU AgCenter horticulturist
I’ve been getting a great deal of questions about tomatoes lately. ‘Tis the season. Most folks who planted vegetable gardens in this spring are now reaping the fruits of their work.
And with this season of harvest comes the challenges of the weather of the season, which is favorable for many of the diseases that affect tomatoes. But not every problem is caused by disease. Some can be because of fertility issues, lack of pollination, inadvertent herbicide damage and pests.
Let’s look at some of these issues and how to deal with them.
Tomatoes grow best in most types of well-drained soils. That being said, tomatoes should be rotated out of an area or container it’s been grown in every year for a period of three years and replaced with another crop such as squash, beans or corn. Avoid using related vegetables such as potatoes, peppers and eggplant. This helps control soil-related diseases.
Unfortunately, tomatoes suffer from many diseases caused by bacteria, viruses and fungi that are helped along by several pests. So, if you like a challenge, tomatoes are your crop. But do not be discouraged. Researchers continue to work year after year to understand these diseases and find ways to prevent and treat them.
Because the list of diseases is so extensive, we’re going to cover just the common tomato disorders that aren’t related to disease. You can check the 2020 Louisiana Plant Disease Management Guide for a complete list of disease symptoms and treatments with corresponding photos online at https://bit.ly/LaPlantDiseases.
The first disorder that often affects tomatoes is blossom end rot. The telltale sign is a sunken black spot on the bottom of the fruit where the blossom once was. This indicated you need to adjust your watering. The spots become enlarged by decay-causing organisms that infect the compromised fruit. Try using a soaker hose or drip irrigation to help with consistent water delivery to allow the plant to efficiently process the calcium that is also associated with blossom end rot.
Calcium deficiency in the developing fruit can be an issue. In combination with fluctuations in moisture caused by over- and under-watering, high humidity and rainy or cloudy weather. The problem can be exacerbated by excess ammonia forms of nitrogen (ammonium sulfate and ammonium nitrate) in complete fertilizers such as 10-10-10, potassium or magnesium because they compete with calcium for uptake in the plant.
Now let’s look at two garden practices that have been going on for ages: Epsom salts and eggshells. It’s true that eggshells are a source of calcium, but it won’t be available in time to help with issues going on right now. Honestly, you are better off composting eggshells and then amending your soil with compost with each crop rotation. However, it will never hurt to throw eggshells in the garden. I get it. Grandma always did it, so it has become a tradition you do to honor a family member. But the science tells us that you will not receive enough calcium in time to alleviate any problem with this method.
Many people follow the common practice of adding Epsom salts to the garden, but this practice actually may promote blossom end rot. Potassium or magnesium will compete with calcium for uptake by the plants. Do not add Epsom salts to garden soil unless a soil report indicates a deficiency.
You can help treat blossom end rot by using limestone applied three to six months before planting or adding gypsum (calcium sulfate) applied to the soil at 1 to 2 pounds per 100 square feet. The soil pH should be 6.5. A calcium nitrate sidedress fertilizer is usually the best choice and is applied monthly at 2 pounds per 100 feet of row. You can also apply gypsum at 1 to 2 pounds per 100 square feet as a sidedress treatment.
Another concern with tomatoes is growth cracks. Tomatoes crack when we have uneven watering, just as with blossom end rot. Drought followed by heavy rain or watering encourages rapid growth during ripening, leading to cracks in the fruit. Some cracks may be deep, allowing decay organisms to enter the fruit and cause fruit rot. You can help prevent this by providing even moisture with regular watering.
Another common problem is poor fruit set, and it can happen for many reasons. Extreme temperatures and drought can be the cause and are conditions we Louisianans are accustomed to. The blossoms drop off without setting fruit when temperatures are below 55 degrees or above 90 degrees for extended periods.
Plant heat-set tomatoes to help fight the heat. And speaking of heat, you should look for varieties with names that are associated with heat as those that are heat tolerant. Look for Solar Fire Hybrid, Florida 91 Hybrid (very reliable), Phoenix, Sun Chaser, Sun Leaper, Solar Set and Sunmaster. Bella Rosa is also recommended for heat tolerance.
Other situations that effect fruit set include not enough sunlight — specifically, less than six hours of sun a day — and excessive nitrogen that promotes leaf growth at the expense of blossoms.
Growers who use herbicides around the garden can also see herbicide injury from the drift from nearby sprays of non-specific herbicides. This can cause whitening or yellowing on leaves. If you use herbicides, be sure to work with them on days with the least amount of wind. Also try targeted sprayer nozzles to minimize drift.
Finally, another temperature-associated issue is leaf curling. This upward curling of lower, older leaves happens during prolonged high temperatures and drought. Not much you can do here. These plants have come to the end of their life due to heavy yields and increasing temperatures. It’s just time to plant heat-set tomatoes like those listed above.
Tomatoes are a challenging crop, but they are well worth the effort for enjoying the fruits of your labor. Search tomato on the AgCenter website at www.lsuagcenter.com for additional information about tomato growing.
Improper watering and calcium deficiency can be the cause of blossom end rot in tomatoes. Photo by Heather Kirk-Ballard/LSU AgCenter
Tomato fruit split is often caused by excessive rain following dry periods. LSU AgCenter file photo by Dan Gill
Drift from non-specific herbicides such as glyphosate can affect tomato plants. Photo by Carol Pinnell-Alison/LSU AgCenter