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Cool, wet spring has led to Grey Mold and Bacterial Spot

Marissa Schuh, IPM Extension Educator,

This long, wet spring has made transplant production a challenge.  We got more reports of disease issues from young plants in the greenhouse than usual, and as these plants are getting ready to be planted, let’s think how these issues may or may not carry over.

Gray Mold (botrytis) in Transplants

Gray mold is a fungal disease that thrives in cool temperatures (60-75°F) and high humidity conditions (80% or greater).  This means it is a disease we don’t often see in outdoor production, instead we typically only see in greenhouse transplants and hoop house vegetables.

Gray mold on pepper transplants. Photo: Natalie Hoidal, UMN Extension

Gray mold spores are fluffy and can be seen with the naked eye. Photo: Natalie Hoidal, UMN Extension.

The cool, humid spring means this disease has shown up on all kinds of vegetable transplants.  Gray mold has a huge host range, which includes vegetables and ornamentals. 

  • In the greenhouse, one thing you can do is remove infected leaves.  The fuzz you see are spores that can infect neighboring plants.  

  • There are also fungicides available for treatment, see the Midwest Vegetable Production Guide. Filter for “greenhouse uses allowed” to quickly narrow in on viable options.

Gray Mold Outside

Gray mold needs high humidity to develop, so often the issue resolves itself once plants are outside.  The plants may be weakened from the disease/leaf removal, thus more vulnerable to other problems. 

Gray Mold in Hoop Houses

Gray mold can continue to be an issue in high humidity environments, making dealing with it in hoop houses a challenge.  Ways of preventing this disease indoors is largely focused on changing the environment to reduce humidity and keep plants dry. This actions include

  • Adequate plant spacing

  • Pruning 

  • Running fans

  • Rolling up sides to increase airflow 

There are both conventional and organic products that can be used in greenhouses/hoop houses for gray mold in vegetables.  The Midwest Vegetable Production Guide has an option to filter by if greenhouse use is allowed to narrow in on options for the crop you are seeing gray mold in.

Bacterial Spot in Transplants

The wet conditions also allowed bacterial diseases to thrive, especially bacterial spot in tomatoes and peppers.  This disease is known to be seed-borne, and infects young plants.  The tight spacing and abundant water in transplant trays allows the bacteria to move from plant to plant.  This movement can extend across multiple trays, depending on the pathogen and greenhouse conditions.

Bacterial spot lesions on the underside of the leaves of a tomato transplant. Photo: Cheryl Truman, University of Guelph.

Ideally, transplants that show symptoms should be removed.  They serve as a source of bacterial spot for their neighbors, who are likely already infected but not showing symptoms.  This disease doesn’t automatically go away when plants are planted outside.

Bacterial spot in pepper leaves. Photo: Paul Bachi, University of Kentucky Research and Education Center,

Bacterial Spot Outdoors

Bacterial spot can make the transfer from indoors to outdoors.  Try not to plant transplants showing signs of disease.

If you can’t avoid it, be prepared for losses and make sure you have a good rotation in place. Things to do include:

  • Minimize working in field when plants are wet

  • Handle plants gently to minimize wounds that allows the disease spread easier

  • Clean tools used in fields with active infections

  • Remove/till in residue at the end of the season

Thinking to Next Year

To avoid seeing bacterial spot issues in future years…

  • Make time before sowing next year’s seeds to deep clean the greenhouse and tools.

  • Don’t have seeds from plants that show signs of diseases

  • Do not reuse trays or media that held diseased plants

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