Friday the garden looked fine. Saturday we were gone, Sunday it rained. On Monday I asked my husband to dig up some new potatoes for our supper, and he discovered the garden was infected with Phytophthora infestans, commonly known as late blight.
Late Blight Factoids
- Ideal conditions for late blight are days in the 70sF (~20sC) and nights in the 50sF(~13C). (Those are ideal conditions for me, too.) Even better if there is moisture in the form of rain, fog, or heavy dew. “Four to five continual days of such weather are an open invitation for an outbreak.” It just so happens when it isn’t rainy, we usually have both fog and heavy dew, and we’ve had these conditions for weeks.
- Infection is spread by sporangia from moldy leaves. The sporangia can’t survive in dead plant debris or in the soil. It needs to winter over on potato tubers. This is why you are always admonished to use certified seed potatoes. One infected potato, given the right weather conditions, can take down your whole crop. However, “use of certified seed can reduce the amount of infestation from infected seed pieces, but it will not prevent foliar infection from other sources, such as neighboring fields.” And apparently a “neighboring” field can be up to 10 miles away.
- The sporangia come from oospores, which apparently can survive in soil. But the sporangia, not the oospores, are what cause the major outbreaks.
- “A rule of thumb: if rainfall or irrigation water exceeds 1.2 inches in a 10-day period, good conditions for late blight exist.” We had over 2 inches of rain earlier last week.
Tomatoes Can Also Get Late Blight
All of our tomato plants were grown by us from seed. We didn’t use any purchased plants.
What Can Be Done?
- “When late blight appears in isolated sections of fields, spread of the disease can be slowed considerably by quickly destroying infected plants. Killing the living potato tissue halts further spore production.” “Sanitation is the first line of defense against late blight.…Volunteer potatoes, solanaceous (potato family) weeds, and any infected plants should be destroyed as soon as they occur.” (What’s growing in your compost pile?)
- Potatoes infected with the fungus can start rotting before being harvested. They will continue to rot after being dug and can spread the fungus to good potatoes.
- Storing potatoes in a cool, dry location will slow down the infection. Our basement is cool, but damp.
- The following varieties show some resistance: Kennebec, Elba, Onaway, Rosa, and Sebago. Kennebec is one of the 10 varieties we are growing.
One More Thing
Late blight stinks. Literally. The infected plants smell really bad. If infected potatoes get a secondary bacterial infection, they smell really bad, too.
I used these two sources for information and quotes. If you have more accurate or detailed information, I’d love to learn it.
- Late Blight of Potato and Tomato (conventional approach)
- Organic Alternatives for Late Blight Control in Potatoes