Disease or disorder: How do I tell the difference? Part 2

Lina Rodriguez Salamanca, Michigan State University Extension - December 22, 2014

Recognizing diseases on vegetable transplants in the greenhouse can improve your scouting. A prerecorded webinar on vegetable diagnostics is available at the Online IPM Academy.

Scouting your vegetable plants in the greenhouse and field will help you identify potential issues early on. In most cases, the earlier you detect an issue, the more effective the management tactics can be. But what are the differences between a disease and a disorder?

Let’s focus identifying diseases in the greenhouse. There are multiple pathogens that cause damping off in seedlings, including Pythium spp., Phytophthora spp. and Rhizoctonia spp. Damping off is a term that describes when a pathogen infects the roots and parts close to the soil line, delaying growth but ultimately resulting in seedling death.

Rhizoctonia Damping Off

Symptoms of damping off. Rhizoctonia damping-off, blight and rot (Rhizoctonia solani) on watermelon seedlings (top left) and Chinese cabbage seedlings (top right). Damping off on cotton seedlings in the field (bottom). All photo credits: Gerald Holmes, California Polytechnic State University at San Luis Obispo, Bugwood.org

Vegetable transplant diseases will depend on the crop. The table below lists the most important diseases on vegetable transplants in Michigan. Click on the disease name and the link will direct you to a symptom image to help with scouting.

Vegetable transplant diseases in Michigan

Tomatoes/Peppers
Disease name	Bacterial pathogen
Bacterial Speck	Ps. syringae pv tomato
Bacterial Spot	X. campestris pv vesicatoria
Bacterial canker	Clavibacter michiganensis subsp. michiganensis
Disease name	Fungal pathogen
Early blight	Alternaria solani
Gray mold	Botrytis cinerea
Leaf mold	Fulvia fulva
Disease name	Oomycete pathogen
Late blight	Phytophthora infestans
Disease name	Viral pathogen
Tobacco mosaic virus (TMV)	Tobacco mosaic virus (TMV)
Cabbage-cole crops
Disease name	Bacterial pathogen
Bacterial leaf spot	Ps. syringae pv maculicola
Bacterial blight	Ps. syringae pv alisalensis
Black rot	X. campestrisi pv campestris
Disease name	Fungal pathogen
Damping off and wire stem	Thanatephorus cucumeris
Alternaria leaf spot/head rot	Alternaria brassicae
Dark leaf spot	Alternaria brassicicola
Disease name	Oomycete pathogen
Downy mildew	Peronospora parasitica
Damping off	Phytophthora or Pythium
Celery
Disease name	Bacterial pathogen
Bacterial leaf spot	Ps. syringae pv appi
Disease name	Fungal pathogen
Early blight	Cercospora apii
Anthracnose	Colletotrichum acutatum
Disease name	Oomycete pathogen
Damping off, root rot	Pythium spp.

Preventative management tactics are the best approach to manage diseases. Planting pathogen-free seed and managing irrigation will keep fungal and bacterial diseases to a minimum for vegetable transplants.

To eradicate seed-borne bacterial pathogens, use hot water and chlorine treatments on vegetable seeds. “Hot Water and Chlorine Treatment of Vegetable Seeds to Eradicate Bacterial Plant Pathogens” and “Hot Water Treatment of Vegetable Seeds to Eradicate Bacterial Plant Pathogens in Organic Production Systems” by Ohio State University’s Sally Miller will provide the steps necessary to treat seed properly for conventional and organic production systems. Treatment is a function of time and chlorine concentration. Care must be taken to avoid seed damage. It would be best to practice on small seed lots and understand the procedure before moving into larger batches.

Early morning irrigation and increased airflow will minimize leaf wetness conducive to disease development on high-density plug trays. Once transplanted in the field, avoid overwatering vegetable seedlings.

If symptoms arise in the greenhouse or the field, make sure you consider the fertility program and have learned to recognize disorders on vegetable transplants in the greenhouse. If you have additional questions, find an MSU Extension expert or collect and send samples to MSU Diagnostic Services. Remember, correct diagnosis is the first step to effective disease management.

Learn more about IPM basics, scouting guides and more at the Online Integrated Pest Management (IPM) Academy. Among the webinars Michigan State University Extension is offering at the Online IPM Academy website, the webinar titled “What is wrong with my vegetable plants?” discusses the differences between vegetable diseases and disorders. Accommodations for persons with disabilities may be requested by contacting Erin Lizotte with MSU Extension at taylo548@msu.edu to make arrangements. Requests will be fulfilled when possible.

For Part 1 of this series, see “Disease or disorder: How do I tell the difference? Part 1.”

Additional resources

Managing Diseases of Organic Tomatoes in Greenhouses and High Tunnels, eXtension.org
Learn to recognize diseases on vegetable transplants in the greenhouse, MSU Extension

This article was published by Michigan State University Extension. For more information, visit https://extension.msu.edu. To have a digest of information delivered straight to your email inbox, visit https://extension.msu.edu/newsletters. To contact an expert in your area, visit https://extension.msu.edu/experts, or call 888-MSUE4MI (888-678-3464).