Anthracnose in Tomatoes

Anthracnose can be a major problem in growing tomatoes.  There are three steps to understanding and managing this disease.  First, you need to understand how the disease cycle works.  Second, you need to know how to recognize the symptoms and third, you need to know how to apply good cultural practices to avoid this disease. 

Anthracnose is a type of soil borne fungus that lives and gets its nourishment from host tissue, in this case, tomato fruit.  Fungi reproduce by spores and are spread by wind, water and even the tools you use in your garden.  Once on the host, the spores will germinate and cause an infection to the healthy plant tissue.  Rainfall (splashing water), humidity and temperature are all involved in determining the development and spread of this fungus in the garden.  

A layer of mulch and a little trimming can help keep tomato plants healthy

In the early stages, infections on green fruit go unnoticed, then as the fruit ripens, depressed, circular, water soaked spots start appearing on the red fruit.  Over time these spots will enlarge and a black center will appear in the lesion.  This black fungus, called microsclerotia, can overwinter in the soil and come back the following year to wreck your tomato crop.

One of the main cultural practices that a gardener can do to help reduce anthracnose is to remove all old or diseased plant debris and rotten fruit.  Fungal spores can overwinter on diseased plants, and that includes weeds in the nightshade family such as horse nettle and ground cherry.  The disease cycle can be restarted the next growing season from fungal spores on old diseased plants that were not removed from the garden.  Do not add infected material to your compost. 

Crop rotation is another way to avoid contamination by spores.  Plant tomatoes in a new location, away from areas where plants in the nightshade family were planted the year before.  Along with tomatoes, the nightshade family includes eggplant, potatoes and peppers.  It is recommended to practice a crop rotation of three years to help reduce soil borne fungi infections.

Tomatoes should be grown in full sun with good air circulation.  Staking or caging the tomatoes will keep the plants high and dry.  I like to do a bit of pruning on my plants. I trim the lower branches off so that I have about eight inches between the lowest branch and the ground.  I will also trim in the center of the plant so the leaves have plenty of circulating air to keep them dry.

Any supplemental watering with a sprinkler should be done in the morning so the plant has all day to dry off.  It is recommended to use a soaker hose to eliminate wet leaves.   Overhead watering splashes and spreads the fungus if any is present. 

Spread a blanket of newspapers covered in mulch to prevent the fruit from coming into contact with the soil.  A barrier of mulch will also reduce weeds, hold in moisture and keep the soil cooler on hot days. 

There are fungicides on the market that can be used to help fight this disease.  Use of a copper based fungicide is recommended when fruit first develops.  Be sure to read, understand and follow all instructions for use.  Certified seeds (disease free) and plants that are marked to have resistance to anthracnose should be used whenever possible.

As always, Happy Gardening!

More information about gardening and related subjects can be found online at www.purdue.edu/ext/garden_pubs  The Purdue University Cooperative Extension Service can be reached at 499- 6334 in LaGrange Co., 636-2111 in Noble Co., 925-2562 in DeKalb Co. and 668-1000 in Steuben Co.

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