Primer on Plant Disease…

Every season has its challenges, but 2013 may go down in the Vermont record books for weather extremes.  The dry spring, followed by ongoing heavy rains, high temps, and crazy humidity has left plants (and quite a few growers!) stressed.

Flooded chard field.

Flooded chard field.

Plants, like us, have immune systems that respond to disease pressure, but under challenging conditions, their capacity can be diminished.

Ann Hazelrigg, UVM Extension Specialist in Plant Pathology, is keeping tabs on both plant diseases and pests that are appearing in Vermont through her work at the UVM Plant Diagnostic Clinic.  She sends out regular updates on pests and diseases through the Vermont Vegetable and Berry Growers Association e-newsletter, “Reports from the Field.

To receive this information regularly, join the VT Vegetable and Berry Growers Association.  Their twice monthly updates include reports about crop conditions from growers in Vermont and surrounding counties in New Hampshire and New York, current disease and pest information, and upcoming educational events for commercial growers.

If you are spotting disease pressure in your fields or greenhouses, you might be able to identify them using Cornell University’s Vegetable MD Online.

Late blight on tomato plant.

Late blight on tomato plant.

If you are having a crop issue that you can’t identify, consider sending a plant tissue sample to the UVM Plant Diagnostic Clinic for analysis.  The form and directions for submitting a sample are online.

Here is an excerpt of Hazelrigg’s latest update in “Reports from the Field.” Check out the useful links to more information about each disease, then take a moment to visit the VT Vegetable and Berry Growers Association web resources, complied by UVM Extension Veg & Berry Specialist, Vern Grubinger.



Late blight: Just confirmed in Huntington, VT.  See:

Bacterial canker: 3 samples of this from GHs in the past month. Usually shows up just as the plant is starting to produce fruit. Can show up just as a scorch symptom or browning on the leaf edges and between the veins so it can look just like the plant isn’t getting enough water. Sometimes long black cankers or streaks will develop on the stems but often you don’t see this. As the disease progresses, the entire plant wilts. Cut open the stem or cut below the green outer portion of the stem to see the diagnostic brown streaking of the water conducting system. If you see this, cut the plant off at the base and get it out of the GH. The bacteria is often seed borne but can be moved from plant to plant easily through wounding from suckering. If you have this in your tunnel, take out all symptomatic plants and watch the surrounding plants for symptoms. This disease can quickly wipe out the entire planting. See:

Leaf Mold: Lots of growers seeing this now in tunnels due to recent high humidity and earlier wet weather. Starts lower in plant as yellow diffuse spot on upper leaf surface. If you turn over the leaf you will see the diagnostic brownish/purplish spores of the fungus. There are resistant varieties to this fungus. Best management is decreasing humidity by opening sides, vents, end walls, pruning off lower leaves, and using lots of fans to reduce relative humidity below 85%.

Blossom end rot: black/brown rot on calyx end due to lack of water movement through plant that transports calcium. Reduce humidity, irrigate sufficiently; usually the next cluster of fruit are fine.

Septoria leafspot (small black spots with gray centers) and Alternaria leafspot (bulls-eye appearance) showing up now on lower leaves. If you are using conventional fungicides or coppers, this will protect tissue from these diseases as well as late blight.

Here is a good site for all tomato issues, with a diagnostic key;

Black rot: bacterial seed-borne disease that shows up as V shaped lesions and yellowing on leaf edges.

Plectosporium blight showing up. Look for white diamond shaped spots on petioles and stems. Damage causes plants to wilt/die quickly.

Fusarium basal rot samples coming in on newly harvested garlic. Look for pinkish cast, rot at base of bulb and white/pink fungus. Seems like I usually find onion bulb mites along with the rot. Not sure if mites were there first causing wounds, then the fungus gets in or it is the other way around.


About Jessie Schmidt

Ag and Community Program Coordinator for the University of Vermont Extension.
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2 Responses to Primer on Plant Disease…

  1. Garland Mason says:

    Reblogged this on What's Growin' On and commented:
    If you’re struggling with pests and disease this summer, read this post from Jessie and don’t forget about UVM Extensions resources, the plant diagnostic clinic and plant pathologist Ann Hazelrigg are there to help people like you!

  2. Pingback: Primer on Plant Disease… | What's Growin' On

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