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