As fruit sets, leaf spots follow: Tomato disease refresher

 Marissa Schuh, IPM Extension Educator, mschuh@umn.edu

Despite how dry much of the state is, we are still starting to see some common tomato diseases creeping in on Minnesota vegetable farms.  Seeing yellowing and spots appearing on the older leaves? Read on for a refresher on two fact-of-life fungal diseases in Minnesota tomatoes. 

Early blight attacks the oldest leaves first. Photo: Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo, Bugwood.org

Two fungal diseases, similar life cycles

As tomatoes start to set fruit, growers often notice brown spots on the lower leaves.  This can be caused by one of two fungal diseases: early blight and/or septoria leaf spot. They both like humidity and splashing water: any time there is water on the leaf surface, these diseases can spread.

Early blight causes larger, amorphous brown spots with concentric rings. Photo: Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo, Bugwood.org

Early blight is the one most commonly seen tomato disease across Minnesota. The disease spores survived in crop debris and weeds, then infects this year's plants via splashing water. Early blight first infects the oldest leaves, with infected areas turning brown and developing concentric rings.  Brown spots, also with concentric rings, can also be seen on stems.  As time passes/leaves are wet, the disease can spread upward. Infected fruit develop leathery, dark spots and fall off the plant.

Septoria leaf spot causes many small leaves. Photo: Paul Bachi, University of Kentucky Research and Education Center, Bugwood.org

Septoria leaf spot has a similar biology to early blight.  The disease survives in the field on solanaceous weeds and infected plant material.  As fruit start to set, septoria is able to infect the oldest leaves.  Septoria causes many small spots on the leaf, which are darkly colored, but as they expand can have tan centers.  As time passes, the disease will move up the plant, though fruit infection is rare.

Non-chemical controls

The time has passed for cultural controls like resistant varieties and using plastic mulch has passed. 

Things that can be done now: catch up on staking and tying.  This helps plants dry out quickly, making the environment less hospitable to fungi. You can also prune off infected leaves.

Chemical controls

Remember, anything you use will be more about prevention than cure.  

Organic Options

There are organic options that have performed well against septoria and early blight.  See the Midwest Vegetable Production Guide for options.

Conventional Options

There are many options available, and many prevent both diseases.  See the Midwest Vegetable Production Guide for information.