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

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 Rhizoctonia Damping Off
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 .(JavaScript must be enabled to view this email address) 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.”

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