Tomato Leaves Spotty and Yellowing? Could Be Bacterial Leaf Spot.

by | May 7, 2020 | Crop Rotation, Disease, General Gardening, Integrated Pest Management, Irrigation, prevention, Tomatoes, Vegetable Gardening, Watering

Early BLS symptoms appear on lower leaves. Photo courtesy of the author.

Tomatoes are among the most notoriously difficult vegetable species to grow in Florida.  Even when you do everything right (choose disease resistant varieties, buy clean seed/healthy transplants, plant early, rotate crops, scout for insects, prune and support vines, etc), things can go wrong in a hurry.  Bacterial Leaf Spot (BLS) caused by the pathogen Xanthomomas perforans, is one of those things.  BLS often hitches a ride into gardens undetected on seed and transplants and presents itself as warm and humid weather arrives in late April/early May.  Outbreaks typically happen in concurrence with the frequent heavy rainstorms that accompany these months and quickly wreak havoc on tomato plants.

BLS occurrence, much worse in some springs than others, has exploded this year in home gardens.  First noticed on the plant’s lower leaves, BLS pathogens enter through natural openings in leaves called stomata, manifest as small, “water-soaked” leaf lesions and then develop within days into dark circular spots, sometimes surrounded by a yellow halo.  As conditions worsen, all these individual infections cause a general yellowing or “blighting” of entire leaves.  Though the issue begins on tomato leaves, BLS can affect all parts of the plant, including stems, flowers and fruit too! It’s at this point, when blighted leaves become obvious and lesion developing on flower stalks cause both flowers and small fruit to drop, that most home gardeners notice that something is up.

Advancing progression of BLS with “blighted” leaves. Photo courtesy of the author.

Unlike with many other pathogens, gardeners don’t have an abundance of options to control BLS.  However, taking the following preventative measures can reduce the pathogen’s impact and stave off disease progression until ripe fruit can be harvested.

  • Don’t irrigate overhead. Utilize drip/microirrigation or hand water beds and containers at soil level. This prevents the leaves from becoming wet; wet leaves become a perfect host for BLS.
  • Never work tomato plants when leaves are wet. No staking, pruning, tying, or anything else until plant tissue is dry.  Working wet tomato plants is an excellent way to spread bacteria!
  • Sanitize pruning tools often and wash hands after working infected plants.
  • Maintain airflow in plants by planting at correct spacing and thinning interior leaves as necessary to facilitate quicker drying of the plant.
  • Prune lower leaves to at least 12” from the soil’s surface to prevent disease transmission from soil to leaves through splashing water.
  • Remove infected leaves (when plants are dry!) to slow disease progression.
  • Rotate crops and destroy infected plants to prevent buildup of BLS organisms in soil.

    BLS lesions on tomato flowers. Photo courtesy of the author.

  • Preventative measures are best, but if your garden has a history of BLS and local weather forecasts call for warm/wet/humid weather, it is best to spray fungicides proactively to protect your crop. A weekly spray with a tank mixture of copper and mancozeb is effective at keeping BLS at bay.  This is the only chemical spray combination proven to be effective in controlling BLS.
  • Unlike many common tomato pathogens that can be reliably avoided through planting resistant cultivars like ‘Big Beef’, ‘Celebrity’ and others, no varieties are resistant to BLS.

While BLS is an extremely destructive pathogen of both home and commercially grown tomatoes, growers can lessen the disease’s effects by following the preventative measures outlined above and spraying with a copper/mancozeb mix if necessary.  Growing tomatoes this year hasn’t been easy, but it’s worth it to fight back and realize a harvest despite difficulties like BLS!

If bacteria have given you the blues or other garden issues are bringing you down, give your local UF/IFAS County Extension Agent a call!  We’re here to help you be successful in all your horticultural endeavors.   Happy gardening!

Daniel J. Leonard
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