Cornell University InsigniaCornell University New York State Agricultural Experiment Station

 

 

 

 

 

Ask the Plant Doctors


"What's wrong with my tomatoes?" is a phrase gardeners ask themselves every summer.
Often, with a little common sense and some keen observation, it is easy to find the answer.
The following is a guideline that home gardeners may find useful in diagnosing problems.

Diagnosing Tomato Problems
By: By: Dr. Helene Dillard, Dept of Plant Pathology &
Dr. Steven Reiners, Department of Horticultural Sciences

 

Q: Every year, I seem to have lots of green tomatoes on my plants when the first frost comes. What can I do with these unripe fruit?

A: Often, the first killing frost of the season may be followed by weeks of mild weather. First, try protecting the plants from frost. When a frost is forecast, cover the plants. Try using an old sheet, a plastic tarp, or even a floating row cover material like reemay. Place it over the plants in the late afternoon and take it off the next morning after temperatures have risen above freezing.

This will also be a good time to root prune the plants. Root pruning means taking a spade and pressing it into the soil about 6-8 inches. Do this in a circle, one foot from the base of the plant. By doing this you will sever parts of the root which will usually hasten ripening.

When frosts are becomoing more regular, it's time to cut the plants at the base and take them to a cool location, like a basement or garage. Hang the plants from the rafters and you will see the fruit slowly ripen. The fruit will not be as tasty as those in the summer, but they will be better than what you will find in the supermarket.

Instead of removing the whole plant, harvest all tomatoes of good size and place them in paper bags in a cool location. Tomatoes do not need light to ripen and should never be placed on a windowsill! Place the tomatoes in a single layer in the bags. Tomatoes stacked on top of each other are more likely to be damaged. When placing the fruit in the bags, try to separate them based on ripeness (i.e., put all fruit of a similar ripeness in the same bag). Check the bags regularly and remove fruit as it ripens.

Q: What causes a big black spot on the blossom-end of my fruit?

A: The problem is called Blossom-End Rot (BER) and is caused by a lack of calcium within the fruit. When calcium reaches low levels in the tomato, the fruit simply dies back at the bottom leaving the characteristic black or brown spot. Calcium is present in most soils in adequate amounts. The problem is that the plant cannot absorb enough calcium through its roots because of dry soil conditions. That's because calcium is taken up by the roots along with water. If, for some reason, water uptake is restricted, calcium uptake will stop.

What can you do about it? Have your soil tested to make sure you have enough calcium in the soil. If you don't, add either limestone (if the soil's acidity level or pH is below 6.0) or gypsum. If calcium levels are adequate, the most important control measure is to maintain optimum soil moisture. Whenever tomato plants are allowed to experience the slightest bit of water stress, you are causing BER. To maintain good soil moisture, try using a mulch of straw, leaves, or even grass clippings to prevent evaporation.

Tomatoes need about one inch of water per week as a minimum. If you do not get this through rainfall, you will need to apply the water As a rough estimate, each tomato plant needs approximately 3-5 gallons of water each week. If you use a mulch, this amount may be cut in half. When watering, don't lightly sprinkle the ground. Apply the water so it enters the ground and moves through the soil to a depth of 8-12 inches. Shallow watering only encourages shallow rooting which leaves tomato plants more prone to wilting.

 

Q: My tomato plants look great in the morning and then they wilt in the afternoon. I have a lot of green fruit that need to ripen...will these plants live to produce ripe fruit?

A: There are two pathogenic fungi that can cause wilts in plants: Fusarium and Verticillium. When you purchase seed or tomato plants, look at the variety name and the letters that follow. Most varieties are VFN resistant, which means they are resistant to Verticillium, Fusarium, and nematodes. Use varieties that are VFN resistant.

The other possible cause for late afternoon wilting is known as walnut wilt. The roots of walnut trees secrete a chemical that is very toxic to tomato plants and will cause them to wilt when they are stressed in the heat of the day. Planting tomatoes in locations where there are no walnut trees is the only answer.

 

Q: In May and June, dark brown to black leaf spots developed on the leaves of my tomato plants. Some were as large as an inch in diameter. What caused these spots?

A: Leaf spots can be caused by fungi that are parasites on your tomato plants. The two most common diseases are early blight and Septoria leaf spot. Spots caused by early blight can get as large as an inch in diameter and will have a bullseye appearance. Septoria leaf spots are dark at first and then become more gray in the center as they age. Both of these disease will cause the leaves of the plant to fall off prematurely. These fungi are mostly a problem during prolonged periods of wet weather.

 

Q: My tomato fruit looked beautiful but after I harvested them, several developed sunken circular areas that eventually rotted. What caused this to happen?

A: Sunken circular areas on tomato fruit are typical symptoms of a disease known as anthracnose, which is caused by a fungus. Although this fungus may attack both green and red fruit, symptoms do not develop until the fruit ripen. A perfectly good looking tomato will often develop these sunken areas during storage. This disease is typically a problem on tomato fruit that are sitting on the ground and when there have been prolonged periods of wet weather.

 

Q: How do I control diseases in the future on my tomatoes?

A:

  1. The first control measure is to purchase or produce disease-free transplants. Don't purchase tomato plants that have spots on them or that are yellowing or wilting.
  2. Consider the location in your garden: it is best not to plant tomatoes in the same area in consecutive years.
  3. Research has shown that placing straw, plastic, or paper mulches around the base of the tomato plants will help reduce development of disease. Mulching will provide a physical barrier between your tomato fruit and any disease organisms that may be lurking in the ground.
  4. If disease is present on your tomato plants, spread of the disease can be reduced by using fungicides that are available from home garden stores.

 

Q: Should tomato plants be suckered at the first sight of blooms? If so, why and how is this done?
A: Pruning is the selective removal of "suckers", those sideshoots that grow from the crotch above a leaf branch. If these are not removed, each sucker will become a main stem with flowers, fruit and even more suckers. By pruning tomatoes, you are reducing some of the potential fruit the plant could produce. Rather than a plant producing 80 fruit, a pruned plant may produce only 40. Since there are fewer fruit each will weigh more. This doesn't mean that a cherry sized fruit will turn into a giant beefsteak type but it will typically add an ounce or two, depending on the level of pruning.

Tomatoes have three different growth habits: determinate, indeterminate, and semi-determinate. Determinate types are relatively small, often referred to as bushy or compact. Each short branch ends in a flower cluster and the plant does most of its growing before any fruit are set. The tomatoes tend to ripen at once, usually over a short 2 to 3 week period. After most of the tomatoes are harvested the plant yellows and additional production is limited. If grown upright, the plants are seldom more than 2 - 3 feet tall. Some examples of this type include Pik-Red, Early Girl, and Super Bush. In general, many of the earliest varieties are determinate types.

Contrast that growth habit to indeterminate types. These are the traditional, large home garden varieties. They produce plants as large as you will allow them to grow. They have many widely spaced branches, numerous suckers, and produce tomatoes all season long. These are the varieties that can be manipulated in all sorts of fashion to make the plants conform to your needs. Examples of this type of growth habit include Big Beef (most of the beefsteak types), Supersonic, and Big Boy. In between these two are the semi-determinate types. They will produce suckers like indeterminate types but not as many and the plant will grow between 3 and 5 feet. Examples of semi-determinate types include Celebrity and Mountain Pride.

Knowing the growth habit of the variety you are growing is critical in determining whether a plant can be pruned and the level of pruning. Indeterminate varieties will have many suckers and branches, each producing many flowers and eventually fruit. These can be pruned and pruned severely. Determinate types, however, are pruned slightly if at all. Any pruning done on a determinate removes a finite number of blossoms and fruit. If you prune all the suckers on a determinate type you will have a small plant, few fruit and lots of sunscald due to a lack of foliage and shading. You will also dramatically reduce your yield. Semi-determinate types can be pruned but not nearly as much as indeterminates.

What's a gardener to do? Remember, you can grow perfectly fine fruit without pruning your plants. But if you want to prune, here are a few guidelines. For determinate types, there is no need to prune at all. For indeterminate types, allow one, two, or three suckers to grow from the base of the plant. Each of these will become a main stem with lots of flowers and fruit. Prune off all the others suckers and provide the plants with strong support. Research has shown that the best time to remove suckers is when they are about 3 to 4 inches long. For the semi-determinate types, limit your pruning. When the plant is 8 - 10 inches high, look carefully and observe the first flower cluster on the stem. Remove all the suckers below the flower cluster except for the one immediately below the cluster. You may have to go back and give these a second pruning 7 to 10 days later. Remove no more than that or you run the risk of pruning too much. The amount of pruning among these varieties to produce optimum yields varies. Some varieties would do better if you left 2 suckers below the flower cluster. Experiment and see which works best for the variety you are growing. Hope this is helpful. Any questions, please let me know.

Q: I see in my parks Catalog that they classify their tomatoes as "Indeterminate and Determinate". What does this refer to.
i.e..hybrid non hybrid?
Does it make any difference to where I plant my tomatoes?

A: Tomatoes have three different growth habits: determinate, indeterminate, and semi-determinate. Determinate types are relatively small, often referred to as bushy or compact. Each short branch ends in a flower cluster and the plant does most of its growing before any fruit are set. The tomatoes tend to ripen at once, usually over a short 2 to 3 week period. After most of the tomatoes are harvested the plant yellows and additional production is limited. If grown upright, the plants are seldom more than 2 - 3 feet tall. Some examples of this type include Pik-Red, Early Girl, and Super Bush. In general, many of the earliest varieties are determinate types.
Contrast that growth habit to indeterminate types.

These are the traditional, large home garden varieties. They produce plants as large as you will allow them to grow. They have many widely spaced branches, numerous suckers, and produce tomatoes all season long. These are the varieties that can be manipulated in all sorts of fashion to make the plants conform to your needs. Examples of this type of growth habit include Big Beef (most of the beefsteak types), Supersonic, and Big Boy. In between these two are the semi-determinate types. They will produce suckers like indeterminate types but not as many and the plant will grow between 3 and 5 feet. Examples of semi-determinate types include Celebrity and Mountain Pride.


At one time, all tomato plants were indeterminate types. In the early part of the 20th century, a compact, determinate plant was field growing in a field of tomatoes - a natural mutation. From that one plant has come all the determinate plants,

So it has nothing to do with hybrid or open pollinated. There are probably more non-hybrid, open pollinated indeterminate types than determinate types but you can find hybrids in both vine types. As far as where you should plant, they both have the same requirements but the indeterminates will take up more room.
Hope this helps!

 
For more gardening resources visit:
Diagnosing Vegetable Problems
Cornell Gardening Resources Web Site
 
Do you have more tomato questions? Try these links:
Try our searchable database of tomato questions and answers
Watering tomatoes drip by drip