The arrival of summer temperatures should be an alarm for tomato growers to carefully check their plants for insect pests and other problems.
Warm temperatures are both helpful and challenging for garden tomato plants. On the helpful side, the warm temperatures motivate the vines to grow quickly. The greater the growth, the more opportunity there is for flowers to appear. On the other hand, increasing temperatures often reduce fruit set, particularly in some of the more sensitive varieties. This challenge often limits fruit production during the warm season. If there is to be any hope of lengthening the time of production and keeping the plants healthy until the fall season arrives, it is important to identify and solve as many of the other non-related problems as possible. Let’s consider some of the more common problems.
First, let’s address plant nutrition. No matter in what growth stage your particular plants are found, now is a good time to provide them with a little nitrogen fertilizer. A moderate feeding will give them a lift and help the plant focus more energy into producing fruit. A shortage of nitrogen can be devastating to both plant growth and fruit production because it is one of the key elements for growth and development. A shortage at this time could stunt the plant and seriously reduce fruit set and development.
Another key problem could be that lack of water. The onset of hot weather is a perfect time to evaluate the irrigation system and the timing of irrigations. Tomatoes do not do well when they are short of water. In warm weather, all plants will be using more water to help cool themselves and to move nutrients around to where they are needed. On the dry side, tomatoes, or any plant, cannot perform these essential functions. In tomatoes, poor fruit production is the usual outcome. If you have not already done so, consider putting in a drip irrigation system with a timer so that water will not ever be a limiting condition.
This is also the time of year when you may find a large caterpillar with a predominant horn on its hind end feeding on the vines and fruit. The tomato hornworm can reach 4 inches long and almost as big around as a finger. The large size of this animal makes it bulky enough to consume entire leaves and small stems. In addition, it is a sure bet that no one wants to find a worm chewing on that nicest tomato in the patch.
Tomato hornworms can be hard to find in the garden. They are dark green in color, which matches the color of the foliage of the vines, and they have silver to white lines arranged diagonally along their bodies which gives them a bit of camouflage to hide them from their enemies. Sometimes it is easier to look for their large, black droppings that may be on the ground on settled onto leaves. If you see the droppings, look around closely because they will be there, somewhere.
The best way to get rid of hornworms is to simply pick them off by hand or to snip them with shears. It is quick and easy to do. When they are small, Bacillus thuringiensis may also give some relief. If they have been a particular problem in the garden, rototilling after harvest will get rid of resting pupae which have burrowed into the soil to wait out cold or hot temperatures.
Aphids can also be a problem in tomatoes. Aphids are soft-bodied insects that remove valuable juices and nutrients through sucking mouthparts. Since almost every aphid at this time of year is a female, and because aphids give birth to live young, populations can explode quickly. The problem is further enhanced because female aphids do not need to mate to produce young. One aphid today can mean thousands tomorrow, their reproduction is that rapid.
Check regularly for aphid populations in the garden. Especially look on the underside of leaves because they prefer the bottom surfaces. It protects them from the environment and enemies. However, they will also be found on the upper sides, so check both sides.
Predator insects like lady beetles and lacewing larvae will clean up an aphid infestation quickly but sometimes considerable damage can occur before the problem can be completely resolved. Help the natural predators along by washing the plants off early in the morning with a strong stream of water from the hose. Once the aphids are off the plant, it is difficult for them to return. It may take several treatments at regular intervals to keep the aphids washed off. Remember, they reproduce quickly.
Another pest that is often prevalent but easy to miss is the tomato russet mite. This mite is not easily seen without magnification. If fact, it is hard for me to see with a 10-power hand lens even when I know what I am looking for. They are best seen under a good microscope such as the one that I have in my office. If you think you might have mite problems, bring in a leaf sample and either I or one of our trained volunteers will help you look.
Tomato russet mites are rose-colored, conical-shaped mites with eight legs. They are good at crawling around and finding fresh feeding sites. When their populations explode, they can suck the life right out of a leaf and eventually the plant. The most common symptom of russet mites are leaves that turn yellow, wilt and then turn tan as they die. Some have described the condition as a plant that is “melting.” For more information, and photos, take a look at the University of California, Davis integrated pest management website. Just type in tomato russet mite into your browser and you will find a wealth of information on this pest. I like the UC Davis site best. Insecticidal soaps are a good first step in controlling these pests.
Sometimes tomatoes in local gardens grow beautiful, full vines but do not set fruit until fall. Even then, fruit set will be sparse. A common observation is to see the plant put out lots of flowers but shortly after see the flower abort or drop off the plant before setting fruit. This particular problem may be a result of that particular variety’s sensitivity to desert conditions. If this problem happens to you, consider planting another variety next season.
Finally, protect tomato fruit from sunburn. The harsh sun can quickly burn tender fruit and leave them with yellow or brown spots in the fruit. Place a good nursery shade cloth, or even a layer of burlap, on a frame above the tomato vines. The shade will allow sufficient sunlight into the canopy of leaves to produce the energy necessary for plant growth while screening out the harshest rays.
Tomatoes are a great garden treat at any time of the year. The marvelous taste of fresh garden-ripe tomatoes can finish off that tossed salad or fresh-grilled hamburger. With a little planning and good care, tomatoes can be a great addition to any garden.
If you have questions, you can reach one of the Master Gardener volunteers at the Cooperative Extension office, 820 E. Cottonwood Lane, Building C, in Casa Grande. The telephone is 520 836-5221, ext. 204.
Rick Gibson is an agricultural extension agent and the director of the University of Arizona Cooperative Extension in Pinal County. He can be reached at firstname.lastname@example.org.