Late Blight Look-Alikes

Janice LeBoeuf, OMAFRA Vegetable Crop Specialist, Ridgetown

Please continue to contact OMAFRA or your nearest plant pathologist if you suspect you may have late blight symptoms.  It is important to confirm if it is indeed late blight, so that the rest of the industry can be alerted (not with names or specific locations, of course) and so that you can take appropriate control measures.

There are some other tomato issues that can cause symptoms that resemble late blight.  I’ve included some photos and descriptions so that you can compare the symptoms.

Corky Root/Vine Decline Syndrome

We are already seeing symptoms of brown banding on roots in a number of fields.  Foliar symptoms are starting to appear in the last week or so.

Corky root/vine decline above-ground symptoms
Corky root/vine decline above-ground symptoms
Corky root/vine decline above-ground symptoms
Corky root/vine decline above-ground symptoms
Corky root/vine decline above-ground symptoms
Corky root/vine decline above-ground symptoms
Corky root – root symptoms (brown banding)
Corky root – root symptoms (brown banding)

Lightning Injury

Lightning injury – may only be on some leaves, some branches.  Look for a circular pattern, with plants in the centre most affected.

Lightning injury - tomato foliage
Lightning injury – tomato foliage
Lightning injury - tomato foliage
Lightning injury – tomato foliage
Lightning injury - note collapsed stem
Lightning injury – note collapsed stem

Other causes of similar symptoms:

  • oil leaks from equipment dripping on foliage
  • verticillium wilt
  • mechanical damage (broken stem)
  • white mold (dying foliage above affected stem, look for straw-coloured lesions on stem, black sclerotia; in contrast to dark stem lesions of late blight)
  • environmental (wind, heat, air pollution)

A Diagnostic Test

Place suspicious leaves in a sealed plastic bag with a piece of moist paper towel.  Leave overnight.  Examine in the morning for sporulation (fungal growth) around the edge of the lesion on the underside of the leaf.  Note that if you leave the leaves in the bag long enough, you will eventually get some fungal growth (from other organisms feeding on the dead tissue) even if there’s no late blight.  Study the appearance of late blight sporulation in photos.

Sporulation on underside of tomato leaf with late blight (under moist conditions). Photo credit: M. McGrath, Cornell University
Sporulation on underside of tomato leaf with late blight (under moist conditions). Photo credit: M. McGrath, Cornell University

See more excellent photos of tomato late blight at http://www.longislandhort.cornell.edu/vegpath/photos/lateblight_tomato.htm.

More tomato late blight photos can be found at http://oardc.osu.edu/sallymiller/Extension/Diagnostic%20Posters/Tomato%20Late%20Blight.pdf and http://www.ppdl.purdue.edu/PPDL/lateblight.html.

I have included more photos of late blight symptoms below.

Late blight lesion on tomato leaflet
Late blight lesion on tomato leaflet
Late blight lesion
Late blight lesion
Early symptoms of late blight on tomato foliage
Early symptoms of late blight on tomato foliage
Underside of late blight lesion in moist conditions
Underside of late blight lesion in moist conditions
Late blight lesion on stem (dark colour)
Late blight lesion on stem (dark colour)

2 thoughts on “Late Blight Look-Alikes”

  1. How do I find a local plant pathologist. I am in the Eastern Ontario area between the Seaway Valley and the Ottawa Valley.

    If what we have is one of the look alikes,which is what I suspect, as no other farms that I have communicated with have signs of blight, do you have resources for these diseases? We are non-certified organic and have 500 tomatoe plants in numerous varieties. Thanks for the abundance of information.

    Rick and Brenda Kozlan

  2. Hi. Did you ever get a response regarding your late blight question? I am in the Niagara Region and my tomatoes (organic) seem to have late blight. MY neighbour’s tomatoes have the same symptoms.
    Help!

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