Get tomatoes until end of the season

August 09, 2008|By BOB KESSLER

About this time of year, most gardeners' tomato plants have heavy fruit loads in many stages of ripening, says Steve Bogash, regional horticulture educator.

This heavy load takes a toll on the plants as nutrients move rapidly from the plants reserves and surrounding soil into the ripening fruit.

In addition, our generally hot, humid conditions are ideal for the development of many fungal and bacterial diseases that can greatly shorten the life of your plants.

With a little care, home gardeners can harvest great tomatoes right up to our first frost.

Ripening fruit uses a tremendous amount of calcium (Ca), magnesium (Mg) and potassium (K). Tomatoes, peppers and eggplant all do best when supplied with additional nutrients as they set and develop fruit.


Look for soluble fertilizers that are low in nitrogen, but high in potassium. Fertilizers are labeled with their percentages of major nutrients as N-P-K. Commonly available fertilizers are 10-10-10, 20-30-20 and 5-10-10. That third number becomes our primary concern as the plants turn from creating foliage to ripening fruit.

I prefer a finishing fertilizer with close to a 1-2-3 ratio such as 9-15-30. Some fertilizers are packaged as complete and have lesser amounts of calcium and magnesium along with other trace nutrients. There are organic sources available as well to meet these requirements.

Three organic sources that come to mind immediately are Fertrell Co. products (, which are available in many independent garden centers, as well as Gardens Alive ( and Peaceful Valley Farm Supply (

Fruit maladies such as cracking, blossom-end rot and uneven ripening can be substantially reduced by paying attention to the changing needs of plants carrying heavy loads of fruit.

Evening out your watering through the use of soaker hoses and timed drip irrigation will also improve your plants' health and the quality of the fruit. Fruit that grows evenly will crack less and the root system will scavenge more of the nutrients required to create high-quality fruit.

Blossom-end rot looks exactly like it sounds, as the flower end of the fruit will turn black and rot. Sometimes these rots occur on the sides of the fruit as well.

Our generally hot, humid conditions are ideal for the development of fungal and bacterial diseases.

It's very easy to miss the most common disease, Septoria leaf spot, as it occurs on the lowest leaves first before rapidly moving up the plant and killing the leaves. If you see small black spots that do not brush off, you've got this very common disease.

Protectant fungicides are the best method to keep Septoria leaf spot under control. Chlorthalonil, packaged as Daconil, and Fungonil are readily available and work pretty well at slowing the development of the disease as well as many other tomato and pepper leaf and fruit diseases.

Organic growers should look for copper-based and Neem oil-based materials. As septoria leaf spot progresses up the plant, it gets harder to slow its progress, so treat the plant when you see the first signs.

The next pest that causes summer decline of perfectly healthy tomato plants are spider mites. These minute pests literally suck the juices out of the plants and are especially bad as things get hot and dry.

Our succulent garden plants are an oasis for sucking pests as weeds and grasses die back in the heat of summer.

Their damage appears as light flecks on the upper leaf surface. You can see small dark dots on the underside of the leaf that move slowly.

To be sure, tap a suspect leaf over a piece of white paper and see if the dots move. If so, you've got mites.

While there are a number of pesticides labeled for spider mite control, I've found that insecticidal soap applied weekly at the maximum label concentration will keep this problem at bay with little damage to beneficial insects and environment.

The insects tend to wash off in heavy rains, so thunderstorms will provide some control of mites as well. With a little care, you can keep your tomato plants producing great fruit right up until our first frost and even a little after if you are willing to throw some floating row covers over the plants on cold nights.

Bob Kessler specializes in consumer horticulture and energy for Penn State University. He can be reached weekdays at 717-263-9226 or by e-mail at

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