Growing Green

By Robin Trott, Extension Educator

It’s easy to see why the tomato is a top choice among gardeners. These plants adapt well to most soil conditions, they are perfect for small garden spaces and the tomato itself has many uses. One plant is likely to yield a high number of tomatoes, so just a couple are necessary to satisfy the needs of most families for the entire season. Tomato plants are relatively easy to grow, but may be susceptible to a few different unwanted diseases. Many things may be to blame for our unhealthy tomato plants; however, the most common invader is fungus. The two most common fungal diseases seen around the area are Early Blight and Septoria Leaf Spot.

Early Blight

Early blight usually occurs near the end of the season and can affect the leaves, stems and fruit. You will first notice small brown spots that appear on older, lower leaves, and will spread upward toward the new growth. The brown spots will be surrounded by yellow leaf tissue. As the spots grow bigger, the yellow tissue will begin to turn brown and the leaf will die. Dark brown spots may be seen on the fruit as well. The spots will appear sunken and leathery. Warm and damp weather may lead to many cases of Early Blight around the garden.     

Septoria Leaf Spot

Septoria leaf spot most commonly affects the leaves and stems. It can affect plants at any stage of development.  Round yellow spots will be noticed on the lower leaves.  The spots will enlarge and turn brown to gray.  Tiny black fruiting bodies will eventually form in the center of the leaf spots. These bodies produce spores which may cause secondary infections throughout the plant. Heavily infected leaves turn yellow, then brown and fall from the plant.  Defoliation of the plant will expose the fruit to the sun which may cause sunscald damage. Warm temperatures, wet conditions, insects and even the hands and clothing of gardeners can spread this disease around the garden. 

Preventing these fungal diseases may prove to be a challenge.  It is impossible to control the temperature and weather conditions, but there are a few things that can be done.  Septoria and Early Blight overwinter in debris of diseased plants, in the soil and in weeds of the Nightshade family.  Here are some things to consider when planting and maintaining healthy tomatoes:  

• Plant resistant varieties

• Remove and destroy any infected plant material to prevent the fungi from overwintering in the debris and creating new infections

• Keep your garden weed-free since many weeds are hosts for disease.

• Keep plants off the soil to prevent disease from spreading through water splashes.

• Apply mulch around the base of plants to minimize water splash.

• Rotate your garden by planting tomatoes where no tomatoes, potatoes, peppers or eggplants have been for the past 3 to 4 years.

• Water tomatoes early in the day and at the base of the plant so leaves are able to dry in the sun.

• Stake plants to improve air circulation around the plant.

• Apply fungicides early in the season to prevent the spread of fungus.

For more information on tomato plant diseases, treatment and prevention, visit https://extension.umn.edu/vegetables/growing-tomatoes.