Janesville65.2°

Not all tomato plants have dreaded blight

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Catherine W. Idzerda
August 24, 2009
— Reports of late blight have some gardeners in a panic.

Tomato lovers are pulling out plants before they confirm the disease or are mistaking it for other more common diseases such as early blight or septoria leaf spot.


In other words, plenty of people are mistaking a common cold for swine flu—or tuberculosis.


In an e-mail, Mike Maddox, Rotary Gardens and UW Extension educator, expressed his concern that “people are pulling up their tomatoes for the ‘other blights’ that are out there.”


“If people want to know if they have late blight, they need to have it confirmed by the lab,” Maddox wrote. “Otherwise, as with the community garden, I’ve seen plenty of sad plants, but it doesn’t appear to be late blight.”


Before home gardeners pull their plants, they need to make sure they have a correct diagnosis, Maddox said.


Here’s an easy way to distinguish between the deadly blight and more common varieties: Early blight and septoria leaf spot generally do not affect the fruit of the plant. The leaves might look awful, but the fruit looks OK.


Late blight spreads rapidly, turning tomatoes into “furry mush bags,” Maddox said.


Late blight also can turn stems a grayish-brown.


Other identifying factors:


Early blight: Small spots on leaves with tan center, concentric rings and—this is key—yellow markings.


Septoria leaf spot: Leaves are pale green and yellow and sickly and have small brown spots that look like water marks. The blight can progress to produce dark gray spots sometimes with black dots in the center.


Late blight: Lesions usually begin pale green in color, eventually turning brown to black and appearing somewhat greasy. Stems can turn grayish-black.


The best way—short of lab testing—to determine if you have late blight is to go to a Web site and look at photos of all three diseases. Here’s a small selection:


-- http://learningstore.uwex.edu/pdf/A2606.PDF: Information and photos about early blight and septoria leaf spot.
-- www.longislandhort.cornell.edu/vegpath/photos/lateblight_tomato.htm: Great selection of late blight photos.
-- http://www.datcp.state.wi.us/arm/environment/insects/late-blight/Late Blight.pdf: State’s Web site comparing early blight, late blight and septoria leaf spot.

Still have questions?


Maddox or members of the Rock Prairie Master Gardener Volunteers hold office hours from 10 a.m. to 2 p.m. Tuesdays, Wednesdays and Thursdays from Memorial Day to Labor Day at Rotary Gardens, 1455 Palmer Drive, Janesville.


Questions can be e-mailed to mike.maddox@ces.uw ex.edu.

To leave a message for Maddox or a master gardener, call the Extension office at (608) 757-5696.



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