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Chicago Urban Gardening

The day to day experiences of a University of Illinois Extension Urban Horticulture Educator in Chicago, Illinois

Keep Tomatoes Healthy

Posted by Ron Wolford -

Tomatoes are a mainstay crop for gardeners in Illinois. The dry, warm June weather was nearly ideal for plant growth and good, clean foliage development, as well as early fruit set. Fortunately diseases, and to an extent insect problems, have been slow to occur under these conditions in most home gardens in central and southern Illinois. However recent periods of rainfall, heat and high humidity have the potential to increase incidence of disease and insect problems, and gardeners should keep a close eye on tomato plants.

Tomatoes can be very productive, and depending on the variety, expect to harvest 10 or more pounds of fruit per plant. To maintain production through the entire season, it is important to maintain good fertility, even soil moisture, and keep the leaves on the plant using good disease and insect control practices. Leaf retention is especially important, as leaves keep tomato fruit nourished and growing, and they also help shade the fruit from direct sun and protect from sunburning.

Plants should be sidedressed with nitrogen containing fertilizer after the first fruit are set. Apply 2 to 3 tablespoons of a general purpose fertilizer such as 10-10-10 per plant when fruit are golf ball size. Make at least two more additional applications at 3 to 4 week intervals to keep late season varieties productive into the fall. Soluble fertilizers such as Miracle Gro™ are a good alternative to dry fertilizers, and they have the added advantage of foliar uptake. Avoid over-fertilizing, which causes large plants with many leaves, and suppresses flower formation and fruit set. Over-fertilized plants also tend to have a higher percentage of blossom end rot, a common tomato disorder related to poor calcium uptake in the fruit. Excess leaves tend to trap calcium, creating a shortage in the fruit. This causes a collapse of cell walls and rot at the bottom of the tomato. Tendency for blossom end rot is variety related, and also seasonal, with the earliest fruit usually exhibiting this problem. Foliar calcium sprays can help reduce blossom end rot, but should be directed toward young, green fruit where it can be absorbed directly. Calcium sprays directed at foliage have no effect on this disorder.

Maintaining adequate soil moisture during the summer is also important. Tomatoes prefer even moisture, and fluctuations between dry and wet conditions should be avoided. Mulching is a useful cultural tool to keep soils moist between rain showers and watering. Steady soil moisture also helps to decrease the incidence of blossom end rot, allowing more even uptake and translocation of calcium into developing fruit.

Lastly, insects and foliar disease should be managed to retain leaves. Insects such as tomato hornworm, Colorado potato beetle, aphids, whiteflies and mites should be monitored and controlled before they damage leaves. Apply appropriate insecticides when these insects are present.

Warm, dry weather helps to reduce disease on tomato foliage, while rainy and humid weather create ideal conditions for fungal blight infection. Most commonly, early blight and septoria spot are the first diseases to appear on tomatoes. Both of these fungal blights begin on the lower leaves of the plant, where spores have been splashed from the soil onto the plant from rainfall. Secondary infection spreads to other leaves from infected lower leaves

These foliar blights require immediate attention, or they will gradually progress upward on the plant, defoliating it by the end of the season. Early infections should be monitored, and spotted lower leaves removed from the plants and buried or composted. Fungicide sprays are effective at preventing initial infections and slowing the spread when the disease sets in. Several materials are available in home garden centers. Look for those which contain chlorothalonil, mancozeb or other fungicides labeled for tomatoes. Alternating fungicide products helps to reduce resistance build-up by the fungus. Product directions should be followed carefully, and applications made on a regular basis (every 7 to 14 days, weather dependent) through the remainder of the growing season. Fungicides are particularly effective when applied prior to a rain event. Mulches may also slow initial infection by preventing soil splashing, however spores can be blown in from outside locations, such as a neighbors poorly tended garden, resulting in infection anywhere on the plant. Bacterial infection can also be a problem on tomatoes, and diseases such as bacterial spot or speck on the fruit and leaves can be suppressed by regular use of copper containing materials.

If you find that diseases overwhelm your plants even with fungicide applications, try varieties that are known to have some resistance to these blights. Many varieties in the home garden trade are older types, sold mostly because of name recognition and known fruit quality. However many of these varieties have limited disease resistance compared to newer varieties. The 'Mountain' variety series ('Mtn. Fresh', 'Spring') are examples of tomatoes known for their blight tolerance, and are a favorite of commercial tomato growers.

Controlling diseases and insects, as well as maintaining adequate fertility and moisture will help ensure a successful tomato growing season. For more information on tomato and other vegetable crop growing, check out the University of Illinois publication "Vegetable Gardening in the Midwest", available from your local county Extension office or visit the U of I Extension website "Watch your Garden Grow" at:

Source: Anthony Bratsch, Extension Educator, Horticulture,

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