Late Blight in Our Garden

– Posted in: Pests, Plagues, and Varmints, Vegetables

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.

The empty area is where he dug potatoes. You can see blackened leaves hanging from the surrounding plants.

The empty area is where he dug potatoes. You can see blackened leaves hanging from the surrounding plants.

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

Tomato plants infected with late blight

Tomato plants infected with late blight

All of our tomato plants were grown by us from seed. We didn’t use any purchased plants.
Our very first tomatoes, picked August 3, 2009. They may be our only tomatoes.

Our very first tomatoes, picked August 3, 2009. They may be our only tomatoes.

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.
  • My husband dug these from the garden. The brown patches are infection. We trimmed off the brown parts, cooked, and ate them. The raw potatoes were very crisp, like the crispest apple.

    My husband dug these from the garden. The brown patches are infection. We trimmed off the brown parts, cooked, and ate them. The raw potatoes were very crisp, like the crispest apple.

  • 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.

The End

The End

About the Author

Kathy Purdy is a colchicum evangelist, converting unsuspecting gardeners into colchicophiles. She would be delighted to speak to your group about colchicums or other gardening topics. Kathy’s been writing since 4th grade, gardening since high school, and blogging since 2002.

In the end, this may be the most important thing about frost: Frost slows us down. In spring, it tempers our eagerness. In fall, it brings closure and rest. In our gotta-go world–where every nanosecond seems to count–slowness can be a great gift. So rather than see Jack Frost as an adversary, you could choose to greet him as a friend.

~Philip Harnden in A Gardener’s Guide to Frost: Outwit the Weather and Extend the Spring and Fall Seasons

Comments on this entry are closed.

Andrew February 20, 2013, 8:42 pm

I grew potatoes successfully for the first time last year and still have some potatoes from that harvest that I’m planning on using as seed tubers. I didn’t have any potato blight last year so hopefully these will work fine. Seed potatoes aren’t the cheapest either, so I’m going to give it a shot either way. Very informative post tho. Now I’ll know if I have trouble with them.

Uncle B September 16, 2009, 9:55 am

Do Purple potatoes show blight resistance? my early Yukon Golds are all canned and safe! My cherry tomatoes and most my Early Saskatchewan Tomatoes were already green and gold salsa and chutney before I got in trouble! Very wet, dark summer didn’t help at all, but even this long warm fall, I’ve managed to get a few tomatoes! Cabbage was all ‘krauted long before , and turnips are waxed or frozen in freezers already! Peppers did poorly but we ate a few , salsa’ed a few too! I like to can potatoes with red skins on, at just under golf ball size, so I beat the game that way this year! I can’t look forward to a big crop of bakers though, they are in bad shape! I got squash! now, if the ripen a bit in the fall sun, we will have good pork dinners with sweet yellow winter squash all winter long! Carrots -OK as usual, and beans – a disaster! First pi8ckings were very small immature and canned fast, next bunch yuccch! Beets did OK. Variety seems to be the key, I still have tomato juice from last years bumper crop of tomatoes in storage, good thing!

Organic guy August 25, 2009, 10:34 am

I got hit by blight also all 24 tomato plants are infected, what a bummer. The peppers are doing ok but with all the rain in upstate NY the cucumbers also got powdery mildew.

I am not going to give up on gardening ever but how do we treat the infected soil for next year any ideas?

Teresa August 23, 2009, 6:35 pm

We have lost all of our tomatoes because of late blight, we also had to dig up all of the potatoes as well. My husband and I spend alot of time in our garnen and take alot of pride in it , we just cant get over how fast it affected the garden ” over night “. As most people we have alot of rain and cool tempertures this summer. Cant wait for next spring. But hope for better results. Thanks for all the infromation.

Scott Supak August 23, 2009, 11:30 am

Well, I think I got it. But I’m not sure. The tomato leaves just turn brown, wither, and die. I don’t see spots, or flaky mold-like stuff at all. Just brown, wither, death. They still smell like tomatoes (the leaves)… So I’m wondering if these plants are just unhappy with the cool weather and lack of sun, or do they have the blight? Large dark brown spots spread on the green leaves, but no spots or circles, and it doesn’t look like the pictures…

Oh, and the big plant that volunteered from the compost pile right next to the house and the electric meter has no signs of it: it’s taller than me and healthier too.
.-= Scott Supak´s last blog ..One way to beat a short summer: speed! =-.

Donalyn August 15, 2009, 8:37 pm

We are hard hit as well – my freezer will not be crowded this year. Guess we have to count the blessings of the crops that have done well, and hope for better things next year.
.-= Donalyn´s last blog ..A Winner! =-.

kerri August 12, 2009, 2:06 pm

It’s a real bummer, isn’t it? As I mentioned before, this will be our first year since growing a veggie garden (and that’s been a LOT of years) that we won’t have tomatoes to can. The little early cherry tomatoes have been a treat, but they won’t be around much longer.
Thanks for the good info, Kathy. Seems like there’s a lot of us in NY state with the same tale of woe.
We’ve had a couple of feeds of lovely new potatoes so far. I’m off to check them now.
.-= kerri´s last blog ..Daylily Delight =-.

Margaret Roach August 9, 2009, 9:46 am

I am celebrating my beans, salad, garlic and greens…all of which look OK, at least….telling them “thank you, thank you” for their good behavior in a lousy year. The potatoes got to good enough size, I think, before the foliage got awful, so I am guardedly optimistic that the crop is at least so-so. The tomatoes…well, that’s another story. I think I will have enough paste to put up about half my annual stock of sauce, but not much else. What a strange year. Sorry to see we are sharing the non-bounty.
.-= Margaret Roach ´s last blog ..garlic harvest and curing: i did something right =-.

bibliochef August 8, 2009, 5:40 pm

I live in upstate too and know a gardener who lost 40 or so tomato plants to a blight he has bene fighting for a few years. Alas. And for oyu, I am so sorry — I love new potatoes.

Eventually (I am behind some ) I will do an entry on my blog that notes yours — because we’re both in upstate. My blog focuses more on cooking and food, but we do have murder mysteries/China Bayles in common!
.-= bibliochef´s last blog ..Fox Run — Garlic Festival =-.

Beegirl August 7, 2009, 9:58 pm

Hi Kathy – so glad you came by via Dee. So sorry to hear about your experience too. It happened so fast!! I think it started on the tomatoes, but now the potatoes looking like they are starting. We are digging them up asap. The plants don’t look too bad, so I am hoping the potatoes are okay. We planted Katatdin and Kennebec. We purchased our tomatoes from a local nursery, but who knows where they came from or what the neighbors have planted. Thanks so much for stopping by. I really appreciate it. Your post is fantastic!
.-= Beegirl´s last blog ..Chinese Takeout =-.

June August 7, 2009, 9:00 pm

Oh, I am sorry. It is a dreadful plague. I got hit by a hailstorm that wiped out forty tomato plants last year. I was hoping for a better go this season. But it’s not to be, is it?
.-= June´s last blog ..Hollyhocks! =-.

Judy August 7, 2009, 7:57 am

I am located in NE PA and I got hit with late blight also. I started all my own plants from seed (some saved, some purchased) and planted certified potatoes (Kennebec, Katahdin, Red Pontiac, Yukon Gold). I don’t know of one person in the area that still has tomatoes. I gathered some tomatoes to make green tomato relish. The ones I didn’t use quickly turned brown. I dug up all my potatoes. I thought I had read I could store them but not use them as seed next year. HA! My county ext. office said to use them ASAP. There is just no way to do that! They did tell me I could can potatoes which I might have to do so I can atleast get something out of this years garden.
.-= Judy´s last blog ..Memories =-.

greg draiss August 6, 2009, 8:54 pm

I thought I had the blight beat! I sprayed Daconil at first signs about a week ago. However it has rained 50 of the last 60 days here in the Albany, NY region.
Today the tomatoes are toast. I posted pictures on my blog of the disaster. All heirlooms all started from seed in March.
.-= greg draiss´s last blog ..Thyme to Rethink Hybrids and All Things "Modern" =-.

JoeLand August 6, 2009, 12:31 pm

That really sucks, those spots don’t look tasty one bit. Hopefully you can slow down the infection, and enjoy whats left.

Linda Lehmusvirta August 6, 2009, 11:47 am

Kathy, thank you for this helpful information, even though at your expense. Seems like we’re both struggling. . .you’ve got ideal conditions for fungus; we have ideal conditions for garden fires after 45 days of over 100. I wonder if anyone anywhere ever has it perfect?!
.-= Linda Lehmusvirta´s last blog ..From Linda: July 30, 2009 =-.

Joene August 5, 2009, 9:36 pm

I too am sorry to read of your late blight woes. I check my tomatoes daily, and so far have been lucky. Others in Connecticut have not been so fortunate. I found a lot of good info out of Cornell. Visit my late blight posts here,,
for the Cornell links and others. The Cornell site may have a link to the webinar they did on late blight about a month ago … it was very informative.
.-= Joene´s last blog ..The People’s Garden Initiative =-.

Kathy Purdy August 5, 2009, 9:44 pm

Thanks for the tip, Joene.

Garden Junkie August 5, 2009, 2:44 pm

Ugh, so sorry to hear about your poor tomatoes and potatoes. So far we’ve been spared but I’m checking the tomatoes daily and plan to yank them at the first sight of late blight – I figure it’s not a matter of “if”, but “when”. All of this rain (it’s raining again as I write this) has really messed up the entire gardening season this year.
.-= Garden Junkie´s last blog ..Coreopsis ‘Redshift’ =-.

Paul Harris August 5, 2009, 2:29 pm

Wow, sorry about your potatoes. Had no idea bout the 10 mile thing. This is making me want to run out of here and check on my potatoes at the community garden!
.-= Paul Harris´s last blog ..Rose ‘Glowing Peace’ =-.

Commonweeder August 5, 2009, 12:16 pm

Kathy, I am so sorry to hear about this. We got local warnings about the blight and I know local farmers have been hit and are suffering. I am watching my tomatoes carefully. Your post was very clear and helpful.
.-= Commonweeder´s last blog ..Beautiful for a Day =-.

Scott Supak August 5, 2009, 11:53 am

Luck so far, I’ve heard reports in the village of Cherry Valley, about three miles from here… We’re pretty isolated, and up pretty high, so I’m hoping we’ll be spared. Even so, the tomato plants are small from lack of sun…
.-= Scott Supak´s last blog ..Peppers Need Sun =-.

Bromeliad Plant Care August 5, 2009, 11:23 am

I’ve had the same problem with my garden too. It was discouraging but what can you do. Its nice to have your information on this problem too.

Layanee August 5, 2009, 7:53 am

I am so sorry to hear that you have late blight. I will watch more closely for symptoms and hope for the best. Great information on the problem.
.-= Layanee´s last blog ..Wordless Wednesday – August 5, 2009 =-.

Dwayne August 4, 2009, 10:08 pm

Sorry to hear about your Late Blight ordeal.

There have been a few commercial potato fields in Maine that have been affected. I hope they get control of it before it gets out of hand.

Did you know potatoes, tomatoes, celery, petunias and a weed called bittersweet nightshade are all susceptible to Late Blight? Keep your eyes open if you grow any of those.

How about the new found threat to Blueberry plants? It is a fungus called Valdensinia leaf spot. Seanna Annis, the blueberry pathologist at the University of Maine who discovered the fungus that is affecting blueberry plants in Maine stopped by the blog and left a comment. She believes the fungus has been caught early enough and can be controlled.

Weird stuff going on huh?

Kathy Purdy August 5, 2009, 8:06 am

According to the University of California, late blight on celery is caused by a different organism than that on potatoes and tomatoes. I did not know that petunias could get the same late blight as tomatoes and potatoes. Apparently this can be a problem when tomatoes and petunias are grown in the same greenhouse. I will have to read up on the blueberry fungus. We have both wild and cultivated blueberries on our property and U-pick blueberry fields are a local industry.

Dee/ August 4, 2009, 9:14 pm

Kathy, I’m so sorry this occurred in your garden, but you’ve given us very good information for the future. I understand that late blight has devastated the East coast. Again, I’m sorry. Glad you can eat the potatoes though. I can’t eat them raw. They make my mouth itch.~~Dee
.-= Dee/´s last blog ..Dear Friends and Gardeners Week 22 =-.

Kathy Purdy August 5, 2009, 8:17 am

Dee! We don’t eat them raw, either. I did say we cooked them. It’s just that the raw ones seemed unusually crisp when they were cut into. The only reason we could eat them is because we cooked them immediately upon digging. Infected potatoes will rot in storage.

Carol, May Dreams Gardens August 4, 2009, 8:59 pm

So far, I haven’t seen any blight, but I’m being watchful. Sorry you have it on your potatoes and tomatoes!
.-= Carol, May Dreams Gardens´s last blog ..Ritual Of The First Tomato – Revisited =-.

Lynn August 4, 2009, 8:57 pm

Sorry, Kathy. We have it, too, and I don’t expect any tomatoes this year. I hope it does not take out your whole giant potato patch.
.-= Lynn´s last blog ..beets beets beets! =-.

Mr. McGregor's Daughter August 4, 2009, 8:22 pm

Just when you thought it was safe to grow your own tomatoes & potatoes…that’s so discouraging.
.-= Mr. McGregor’s Daughter´s last blog ..Mishmash Monday =-.