Late blight, which is caused by a fungus Phytophthora infestans, is one of the most destructive and contagious plant diseases directly affecting the fruit and killing the plant. (Phytophthora means ‘plant destroyer’ in Latin)
Late blight spreads like a wildfire. This fungus produces spores in abundance and with the help of moist wind they will travel up to 30 miles. It is imperative to destroy infected plants as soon as possible as this disease is easily spread.
This disease shows up in late summer as the hot and warm humid days may be followed by cool and moist nights with accompanying rain, fog and heavy dew. This is a indicator to the home gardener to become watchful for the signs of late blight.
What to look for:
- Leaves will develop water soaked spots.
- The spots will enlarge and quickly turn from brown to purple to black.
- Shoots of the plant will turn black and eventually the plant will collapse.
- As the plant rots there is a foul odor; a whitish mold may be present.
- Tomatoes will show grayish water soaked spots, enlarge and turn dark. They may have additional rot, an unpleasant foul odor and be mushy.
- Potato tubers will show purplish or brown corky spots. There may be additional rot, an unpleasant foul odor and be mushy.
Can this disease be prevented?
- Check your potatoes and tomatoes daily following cool, humid and prolonged wet weather followed by warm/hot humid days. Sign up to receive alerts (www.usablight.org) for immediate notification when late blight is confirmed near you.
- Copper fungicides (Elemental copper as cupric oxide) can be highly effective if applied as a preventative (before infection) and with complete coverage of all plant foliar surfaces, including the undersides of leaves where the fungus typically produces spores. Follow the label and use as prescribed.
- Neem oil is an alternative fungicide. Follow the label and use as prescribed.
- If symptoms are found remove the plant(s) immediately in plastic sealed bags.
- Remove the plant(s) when the leaves have dried to decrease the dispersal of spores.
- Dispose of this diseased material in plastic sealed bags with your household trash. Burning the diseased plant material is equally effective.
- Change your clothing and wash your hands/change your gloves before entering the garden again. (Spores from late blight could be reintroduced from your clothes, hands and gloves) Remember this is a highly contagious disease.
- Hill your potato plants to stop spores from draining down to the tubers.
- Some gardeners will cover their tomatoes with an old tent frame or swing set and place plastic over the frame. The frame must be higher than the planting and permit air movement as there cannot be condensation on the leaves. High tunnels have been very successful.
Are there blight resistant tomatoes?
An article in growveg.com (January 17, 2014) by Barbara Pleasant offers a list of 9 blight resistant tomato varieties offering excellent to very good resistance.
- Defiant PHR
- Lemon Drop
- Matt’s Wild Cherry
- Mountain Magic
- Mountain Merit
- Plum Regal
- Mr. Stripey (AKA Tigerella)
- Iron Lady
Are there blight resistant potatoes?
- Elba, most resistant
- Jacqueline Lee
- Island Sunshine
- If there are symptoms as harvest approaches remove all potato foliage from the garden and wait 2 weeks before digging the tubers.
- Wait for dry conditions to dig your potatoes.
- Remove all potatoes from the soil. ( the spores of this fungus will overwinter in unharvested tubers, in the soil and living crop debris)
- Check your harvested potatoes for spots and cull if spots are present. Do not store spotted (culled) potatoes with unaffected potatoes.
- Any fruit that rot after picking should be put in a trash bag and disposed with household garbage.
- Unaffected parts of fruit can be consumed, but fruit from affected plants should not be canned due to concern that the pH may not be sufficiently low.
- Tomato seed is not a carrier of this disease.
A photo gallery of this disease: http://www.hort.cornell.edu/lateblight
Cleaning the garden of all living debris, tomatoes and potatoes is essential. Burn or bag this debris for disposal. Do not compost. Again, this is a very contagious disease.
Is there a cure?
Late blight has been with us a long time. The potato famine in Ireland of 1845 was caused by late blight. As you have read this disease is highly contagious and your daily observations are necessary to spot the symptoms and act accordingly.