Because the pathogens are nearby and spread pretty easily, it’s likely that late blight will appear in gardens earlier this year than last. In 2009, Clark County had a confirmation of late blight on tomatoes.
Unfortunately, this disease is a big deal for potato and tomato growers as once it infects the plant, it has the ability to wipe it out in no time at all, especially if the right conditions exist.
The pathogen, Phytophthora infestans, is the same pathogen that caused the Irish potato famine of the 1840s and literally wiped out the main food source.
Late blight appears on the leaves as pale green, water-soaked spots. These spots often begin at the tips or edges of the leaves. These lesions are surrounded by a pale yellowish-green border that merges with the healthy tissues.
These lesions enlarge rapidly and turn dark brown to purplish-black. The stems develop these lesions that appear brown to black.
During periods of moist weather, plants die quickly, and an entire field can be wiped out in no time. Disease development is favored by cool moist weather such as nights in the 50s and days in the 70s, along with rain or heavy dew.
Though the pathogen thrives under cool, moist conditions, it is still around during hot, dry weather. It doesn’t go away entirely.
What can you do for late blight? The first thing is to be vigilant and inspect your plants on a regular basis.
There are a few fungicides labeled for control of late blight but there are varying opinions on the effectiveness. Products containing chlorothalonil or copper-based fungicides are labeled for this disease.
However, when using fungicides, you must have the fungicide on the leaf surface prior to infection. The pathogen enters the leaf and begins to destroy cells before you see the symptoms.
Therefore, if you are going to try to spray to prevent this disease, you must have the fungicide on the leaf surface before the infection occurs.
WHAT TO DO
If you suspect your tomato plants have blight:
• Get a confirmation from either your local OSU Extension office or the Diagnostic Clinic.
• Dispose of plants immediately.
• Don’t throw them on the compost pile. Remove plants during the heat of the day when foliage is dry to reduce spread. However, if it’s identified during a wet period, remove as soon as possible.
• Seal plants in a garbage bag, and leave in the sun for a few days to kill the pathogen.
• Alert your neighbors, and encourage them to be on the lookout.
• Watch other plants in the tomato family. Some strains of late blight can affect weeds such as nightshade and flowers such as petunias.
For more information
Send suspected samples of late blight to the Ohio State University Plant and Pest Diagnostic Clinic. Go to this website for information on fees, forms and how to submit the sample: ppdc.osu.edu
For more details on late blight:
• extension.osu.edu
• vegetablemdonline.ppath.cornell.edu.
Source: Ohio State University Extension Office
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