Disease Control Potatoes Tomatoes

Late Blight – Know Your Enemy – Tomatoes – Potatoes

I spent most of the summer fighting Early Blight.  Then all of a sudden the leaves on one of my Window Box Roma tomato plants got funny little dark  holes in them and blotches all over the leaves.   One day the plant was lush and full and the next day it had all these blotches all over the leaves. That is also what happens with Late Blight — so right away that’s what I thought it was.

I kept looking for the fuzz or mildew like substance on the leaves that is one of the tell-tale signs of Late Blight, but never saw any.  The leaves that were effected just dried up and disappeared. I never saw the “water-logged” leaves that are usually mentioned when symptoms of Late Blight are being described.

I also looked for brown/grey patches on the stems, especially at the joints.  These patches can be one of the earliest symptoms of Late Blight. Fortunately — never saw those either.

I had also read that my plants would be dead within two weeks if I had Late blight.  That didn’t happen.

I could see a problem starting on the larger regular tomatoes  as well but it was spotty and holding — rather than rampantly spreading as Late Blight would have done.

In looking back I realize now that from the little information I had read on Late Blight I was not getting a full understanding of what this disease is like. Even the pictures I found and used as a guide were not adequate. Finally I came across some great pictures that helped me know that I did not have Late Blight.

To Help you Identify Late Blight

Before I go any further let me give you the link to an excellent site that has great pictures that should allow you to identify Late Blight without having to use a microscope. You can click the address and it will take you there, but I’m putting the entire URL in just in case something happens to the link in the future:  http://www.longislandhort.cornell.edu/vegpath/photos/lateblight_tomato.htm

The pictures in the link above will show you just what to look for should you need to.

  • The nickel to quarter size olive green/brown spots on leaves that can appear greasy or water-soaked.
  • The white powdery growth — usually on the underside of the leaf. (This comes after spores form in about 7 days.)
  • Brown/grey/black lesions on the stems
  • On green tomatoes look for brown to black bruise-like spots.
  • On red tomatoes it looks like a bruise that will soon cause the skin to collapse

Within a week or two of seeing the first symptom your plants would be a mass of black tissue. (A creepy thought.)

Why I was so Alarmed About Late Blight

Until this summer—- the last I really gave a lot of thought to any tomato disease was back in the late 70s and early 80s.  I was a new gardener back then and worried about every little “garden problem” that came along. I stopped all that as I explained in my post Gardening – Keep it Simple Because it Is.

But when I had Early Blight hit my garden this summer —- and then in August I thought I had Late Blight  — believe me —-I did more than just give it some thought.  I hit the computer and did more research than I’ve done since I’ve been gardening. And I gave a lot of thought as to how I could protect myself in the future from both Early Blight and Late Blight — just in case.

As almost all of us know by now – Late Blight is the same devastating disease that caused the Irish potato famine of the 1840s.   Since 1990 this disease has been wide spread across US and Canada.  And as recently as 2009  it hit farmers and home gardeners from Maine to South Carolina — and as far west as Ohio— more ferociously than ever and wiped out a lot of major tomato crops for both organic growers and conventional farmers.

In 2009 it got its start from hundreds of thousands of tomato plants sold through Wal-Mart, Lowes, K-Mart and Home Depot stores.  Almost ALL of those plants came from one source in Alabama. (Another great reason to start your own seed.)

Believe me, this is nothing we want in our gardens because there is no cure! Once a plant is infected — it’s as good as gone. Good healthy soil and following good gardening principles are your best bets.  (I’ll cover prevention in a minute.)

What is Late Blight?

Most everyone refers to Late Blight as a fungus, but it’s not a true fungus.   The name of the pathogen that causes Late Blight is Phytophthora Infestans. (In Latin – Phytophthora means “plant destroyer”.)

“Protists” — the group to which this organism belongs are commonly called”fungi”. It is also called a “water mould” because they thrive in moist environments.  Free water must be present on the plants for it to become infected.

How it Spreads

In most cases in the United States the pathogen overwinters only in living tissue.  Most commonly in the Northeast “living tissue” is in the form of seed potatoes, unharvested potatoes and potatoes damaged and meant to be thrown away.  It could also overwinter in a greenhouse on a living plant even in the north.  (The 2009 epidemic that I mentioned above – came from tomato plants grown in Alabama.)

Although there are new strains that can develop in hot weather, most strains of the disease are favored by cool moist weather. The spores are transferred to a new host plant on the wind or in blowing rain.  Rain, fog, or heavy dew also make it possible for this disease to transfer. In some cases 90% humidity alone has been enough to enable transfer of the disease. Spores can readily spread by irrigation water, run-off, and equipment not cleaned with disinfectant.

Some accounts say spores can travel 10 miles; some say 20; some say 50.

Spores usually die within hours in sunlight or dry conditions.

When it Appears – It Won’t Necessarily be Late

This disease will manifest itself whenever conditions are right for it.  In most cases that would be lots of moisture and temperatures between 60 and 80 degrees. However, there are strains that develop in warmer temperatures.

(Just keep in mind that even if you have those conditions — but the pathogen is NOT present — your plants will NOT be infected.)

What to Do with Infected Plants

If you have this disease – and if in fact your plants look like what is shown in Cornell University pictures, it’s so repulsive you’ll want to pull the tomato plant, bag it and trash it.

And that is exactly what you need to do — and do it quickly before spores multiply even more!  If possible do it on a sunny day when the spores (that are going to be dislodged when you take up the plant) will be exposed to and killed by UV radiation.

Staked tomato plants can be difficult to remove without jarring spores loose. Cut the stem at the bottom first and then work your way up, cutting stems and ties.  It’s not always possible, but if you can cover the plant with a plastic bag or tarp while you are working on it or at least  before you remove it — it will help contain some of the thousands of spores that are going to dislodge when you take it up.

Some sources I saw suggested leaving the bags of debris in the hot sun for an hour or two to kill plant tissue and the pathogen.  If for some reason you are unable to place debris in bags for removal, pile the debris and cover with plastic or a tarp and leave in the hot sun for an hour or so to quicken the death of the pathogen.  (The tarp will keep the spores from spreading before they are killed.)

Burn Debris Where Permitted, Disinfect hands – clothes – tools – equipment used, and NEVER compost

NEVER compost the plant debris or the fruit, because the spores will survive even a hot compost pile! Bury the debris and all fruit deeply. Most sources recommend 12 to 18 inches deep.  And many recommend 2 feet.

(I have read accounts that recommend composting, but I feel this would be a grave mistake.)

Where permitted, diseased debris and fruit can be burned. Propane flamers would kill foliage quickly, but obviously they would not be suitable where plastic or straw or other inflammable items are close by.

Wash your hands, clothes and all tools and equipment with a detergent or disinfectant like peroxide after handling diseased or unusual looking plants.

Notify your neighbors growing susceptible crops so they can protect their crops. Notify your state extension office so they can be aware.

Diseased Plants Cannot be Saved

Late Blight is systemic and  spreads through the entire plant. Once the disease is contracted the plant cannot be saved.

Saving the plant by cutting off infected leaves is not an option with this disease because the spores are said to reproduce so quickly. And as I mentioned, it is systemic — all through the plant.

The disease cycles very quickly. From the time the plant is infected it can produce more spores to infect other plants within 7 to 10 days. In other words it takes 7 to 10 days from spore germination to symptom.

There are those who have had Late Blight symptoms start on the leaves, recognized it — and have checked twice a day to remove the infected leaves so that spores would not reproduce and spread.  They do this of course — especially in tomato plantings to try to bring the fruit to maturity before pulling up the plant. Sooner or later they can’t keep up with the disease and go ahead and pull the plant.

This is a judgement each individual would have to make.  I think taking this approach is little scarey and risky.  As much as I want my tomatoes I don’t know that risking the health of all my future tomatoes, potatoes, and possibly eggplant and peppers — is worth a few tomatoes in a year that Late Blight attacks.

What if Late Blight is Found in an Isolated Area of a Planting?

Whether or not you can save the surrounding crops will depend on a lot of conditions:

  • Were you able to remove all the diseased tissue and do so before more spores are produced?
  • Are weather conditions working for you (bright sun and clear) or against you (humid, rain, fog, cloudy, etc.)?
  • How close are you to other farms and gardens that have been infected?
  • Were you able to start and continue applying organic fungicides at least every 7 days and every 4 days in conditions that favor the disease?

Take the necessary disinfectants into your garden/ or field to clean your tools and equipment as you go.  This will help eliminate more contamination.

Danger of Leaving Late Blight Uncontrolled

The danger of leaving the diseased plants in the garden is the speed with which the spores multiply.  Spores build up in the soil and increase the likelihood of the disease in future years. Also, the higher the spore count – the more potential for new strains to be created.

Even one lesion on a plant can produce hundreds of thousands of spores to carry more Late Blight.

By leaving diseased plants in your garden you could have trouble in years to come because of the build up of pathogens in your soil.  In addition, the disease could spread throughout your area in epidemic proportions by blowing on wind and rain to other farms and gardens.

Some CAN survive the winter

Some extension offices are telling folks that late blight requires living host plant tissue to survive and that once the plant is dead the pathogen is dead.  If this is true — you would not need to worry about spores surviving the winter in your soil.

Others with a little more wisdom — say “currently” it is thought not to be able to survive the winter.

The fact is there are various strains of this disease that reproduce in different ways.  For many years the strain that produced oospores — which allowed it to survive in the soil during the winter — was not in the U.S.  That strain is now present in the U.S.and Canada.  It could be just a matter of time before various extension offices have to change their information.

I would suggest treating the disease as if it would survive the winter. To me the disease would be a nightmare and I had rather take every precaution and do everything I can to make sure — to the best of my ability — that I don’t get the disease or at least lower the chances of getting it.

And by the way — this current strain will survive the composting process.

How About Fruit Brought in from Somewhere Else that Might End Up in OUR Compost Pile?

Many sources bring up the fact that fruit from an infected field can develop symptoms after harvest. They recommend the fruit being checked before marketing — BUT — they do say it can be marketed. They also recommend telling the customer so the customer will know it might have a shortened shelf life. They go on to say it might be wise for the customer to put the fruit in the trash rather than a compost pile since there is a possibility that spores of Late Blight could be produced before the fruit completely rots.

I don’t know how that information effected you, but I didn’t like it.  What do you think the chances are of the customer (you) being told this?

I don’t buy much produce from the store, but this makes me want to be even more cautious than I already am. I don’t want to take a chance — no matter how remote —- that spores of Late Blight could be on what I am being either given by someone or buying at a store and then have them survive even a hot compost pile! It’s not worth it to me.

What About Fungicides to Protect Unaffected Plants?

I’ve never liked to spray even with organic products.  I have used them.  But it’s seldom.  It takes so much time and I seemed to have managed pretty much without them for 35 years.

However, I think an immediate threat of Late Blight could change that — especially if I wanted Tomatoes and Potatoes — which I do. And fungicides can protect unaffected plants from the disease. (I intend to call the State-Coop Extension office every year to see if any outbreaks of Late Blight have been reported — then I’ll make the decision — and hopefully it will not be to late.)

If you decide to start a program of spraying fungicides there are a few things  I’ve found that I think will save you a lot of time.

  • In order for a fungicide to be effective and prevent the disease — you start spraying the day your transplants go into the garden.  You spray every 7 days.  If weather conditions warrant it — spray every 5 days.  And you don’t stop until you’re ready to call it quits.

Obviously this would be especially applicable if Late Blight is already known to be in your area.

  • The general consensus among those who collect data and do the research is: When conditions are extremely favorable for Late Blight OR a highly aggressive strain is present  — fungicides may not work.
  • Data collectors report that those who have been successful in preventing the disease alternate the fungicide they use.

An example to relate to:

This made a lot of sense to me. I related it to something that happened to me many years ago.  When I was in my 20s and 30s I had horrible psoriasis on my hands. It lasted for years and years.  Anyway — I would use a cream — say  — with vitamin e in it.  It would work for a while and then not be effective.  Then I would use aloe.  That would work for while — then not be effective.  I would use something else until I ran out of things to use.  By the time I got back to the cream with vitamin e — it worked again.

Recently Bill had some type of rash on his hands.  He experienced the same thing.  He’d use one cream for a while with great success and then it would stop working.  He’d use another — and on it went —- just like with me many years ago.

So this business about alternating the fungicide makes perfect sense. I strongly suggest trying it if you are going to use fungicides.

What Organic Fungicides Should You Use?

Here is a link that lists all the fungicides that are approved for organic gardens. http://www.omri.org/simple-opl-search/results/fungicide I am showing the complete URL in case something happens to the link in the future.

Important: Spray must totally cover the plants.  Under the leaves, the stems, everything.

Which Ones are Best?

The answer to this cannot be a definite because a lot depends on your garden and which fungicide is going to work best with your set of variables. So, not surprisingly, I found many different opinions.  But after reading hundreds of opinions certain ones start to surface as being the best.

My findings were almost identical to the findings of Eric Sideman, PhD. He is an organic crops specialist for the Maine Organic Farmer Gardener Association (MOFGA). After the growing season in 2009 (you’ll recall that Maine was one of the States that had Late Blight Spread by tomato plants shipped from Alabama) —he surveyed the farmers and received more than 100 responses.

Here’s the consensus of that survey: “The only clear information I (Dr. Sideman) got was that growers who began spraying early with an organically approved copper product and sprayed weekly all season escaped major loss. Some other people reported some success with other products, such as Serenade, Sonata, Oxidate and Actinovate , but for each product reported as successful, I had other reports that that product did not work.”

One of the things that I noticed in my consensus that Dr. Sideman did not mention, many farmers and gardeners use copper and alternate the copper spray with Serenade or Actinovate with much success.

Note: Organic Gardening website mentioned using bacillus subtilis which is a good bacteria that helps fight bad fungi.  The fungicide product Actinovate contains bacillus subtilis.

About Copper:

If you decide to use copper as a fungicide, please be very careful and do your due diligence.  Copper can be very harmful to humans AND if used frequently can build up in your soil causing harm there as well. It kills disease organisms, but also damages beneficial organisms and your beneficial insects.  Frequent application can stunt plants.

Since it is approved for Organic Gardens — its use is a decision that each individual has to make.  But I for one would not use it.  It’s just too dangerous.

Fungicides with more natural Ingredients Can be Used

  • Cornmeal is a fungicide.  (Not the mix — but real cornmeal.) Ground corn is about the same.  You’ll see conflicting opinions about this online.  Some people — especially conventional gardeners — say its a myth.  Other sites give a lot of backup and support for this line of thought.

My opinion: I am seriously considering trying to get organic cornmeal in bulk and using it on my tomato and potato beds next year – not only for because of Late Blight, but the more immediate threat of Early Blight.

  • Some folks use a baking soda spray. (2 heaping tsps of baking soda, 2 drops of cooking oil (to make it stick to the leaves) and a drop of dishwashing soap in a spray container) You usually see this concoction for powdery mildews and rusts, but baking soda can be used as a fungicide as well.

Compost tea, fish oil or seaweed/kelp spray can also be used to boost nutrition.

Many folks use compost tea or seaweed spray or fish oil spray all the time.  I think if your soil is healthy and providing everything your plants need, you don’t usually need this extra boost in nutrients.  It’s not necessary.  BUT — if you know there has been an outbreak of Late Blight and you stand a good a chance of having it reach you —- that’s when the boost in extra nutrients would be beneficial.

The key in using these sprays successfully is also consistency and total coverage—- just as with using more commercial fungicides.

Other Things You Can Do

Good healthy soil and following good gardening principles are always your overall best bets for any garden problem. In order for a disease to manifest – it has to have the right environment.  You want to make your garden the most inhospitable environment for disease.

If your garden soil is healthy — your plant will be much stronger.  A human that is in good health and has a strong immune system can fight off all kinds of diseases.  Plants are the same.

Most of these good principles of organic gardening have been addressed many times on this website:

  • Rotate your crops.  Once every 4 years is best. Once every 3 is ok.
  • Cover crops. If your garden is small and rotating members of the same family like potatoes, tomatoes, peppers and eggplant is almost impossible — rotate the crop itself with a cover crop. Cover crops suppress disease as well as enrich your soil.
  • Continually replenish your soils organic matter.  The stronger and healthier your soil is the more your plants can ward off and fight whatever diseases come.  If you have a big problems with diseases — I can almost guarantee — it’s your soil.  This doesn’t mean you can’t get Late Blight if your soil if perfect — but it does mean — you have a much better chance of not getting it.
  • Mulch to conserve water and keep soil pathogens (like Early Blight) from splashing onto your plants.
  •  If you’re set up to water — don’t over-water.  Too much water is just as bad as not enough. And make sure you water from  the bottom, not overhead.  You want to make sure the foliage stays dry.
  • Start your own seed if possible.  If not — know where your seedlings come from.
  • Take any diseased plants OUT of your garden and do not compost them.  Trash them or burn them.
  • Control insect pests that can transmit diseases from plant to plant. (Like aphids and cuke beetles.)
  • Make sure your plants are spaced to allow good air flow. This is a simple but very important principle of good gardening.
  • Be THERE — (in your garden that is.)  Visit and check everyday to see what’s going on.  Catching something before it takes hold is the key to success in most things.  Late Blight is no exception.
  • Avoid working in your garden when it’s wet.

Other Things you Can do That Specifically Address Late Blight

  • If you allow volunteer potatoes to come up, watch them very closely.  If there are any signs of Late Blight take them up immediately following the instructions for disposal given in this post.
  • Do not allow cull piles of potatoes (throw-aways) to sit out in a pile whether you’ve had Late Blight or not.  You don’t want to invite trouble.
  • If you have had Late Blight in your garden, do not allow volunteer tomatoes or potatoes to come up.  Take them out of your garden.
  • Plant your tomatoes and potatoes in areas that receive the morning sun.  The dew on the foliage will dry more quickly this way.
  • Good drainage is imperative.  Soils that hold too much moisture and are too wet are much more likely to have Late Blight.
  • When buying seed potatoes, make sure they are certified disease free.
  • If you’ve had Late Blight do not compost any potatoes or tomatoes or plant debris from these plants.
  • If you’ve had Late Blight on your potatoes, let the foliage die back totally (at least two or three weeks) to keep the spores from transferring to the potatoes.  (This assumes they have not transferred already.  If you have had them well covered with soil — that should have protected them from the spores from the foliage.)
  • Store your potatoes at cool temperatures in dry conditions.  Make sure no water is on the surface of the potatoes.
  • If you cut your seed potatoes plant them immediately.  Holding them for long periods of time can increase your chances of disease.
  • Control weeds that can host Late blight.  You might have to do a little research on this.  Basically weeds in the Solanaceous family — which is the same as tomatoes, peppers, potatoes, and eggplant — can hold Late Blight spores, keeping them close by. Jimson weed is one I have a lot of.  Also petunias can host Late Blight.  (What a shame!)

ID Late Blight on Potatoes: Here is a website that will show pictures of how Late Blight effects potatoes: http://www.longislandhort.cornell.edu/vegpath/photos/lateblight_potato.htm

One more thing to consider:

Various sources I’ve looked at says that Late Blight is not carried on Tomato Seed (Early Blight can be.)  I can’t see how that could be true and I would think no one would want to save seed from a plant that had been infected with Late Blight.  I know I wouldn’t.

Final Thoughts

New strains of this blight are constantly developing. From what I’ve read, seen, and experienced – the Extension Offices don’t always keep up with the most current developments.

I would say your two best defenses against Late Blight are knowing your enemy and the health of your garden soil.  I think this post and others on the health of your garden will help strengthen both lines of your defense. Act on the information and you’ll have a far better chance of dealing successfully with any disease that comes along.


Related Reading:

Early Blight on Tomatoes – There’s Hope


Organic Gardening is easy, efficient, and effective — and it’s a lot healthier.


All content including photos is copyright by TendingMyGarden.com.  All Rights Reserved.



  • After reading this, Theresa, I’m fairly confident my tomato plants did NOT have late blight. I’m not sure what they had, but they definitely didn’t look like the pictures from Cornell U. And while I’m thankful I don’t seem to have late blight, I’m also thankful I decided to pull the plants. Being a new gardener, I’d rather lose a tomato crop now than risk infecting future years’ crops.

    Thank you again for your wealth of information and your time and energy spent researching these topics.

  • WOW! I’m going to forward this to my husband for the potato info. I think my tomatoes are okay but they share a “greenhouse” with cucurbits that are infected with powdery mildew every year. In fact, I harvested all the pumpkins and hubbards yesterday because their stems and leaves are spent; I guess I should do the cucumbers today. So this leads to a question or two:

    1) Can powdery mildew affect tomatoes?
    2) Will you be doing as indepth an article on powdery mildew?
    3) Is it safe to burn infected plants in the open air, or do you need to be contained somehow? Can they be burned “green” or should they be left to dry in the sun first?

  • Hi Connie,
    I have not had powdery mildew affect tomatoes. I don’t know if it effects tomatoes, but my guess (and it is only a guess) is that anything could get it if conditions were just right.

    Sooner or later I will probably to something on powdery mildew. I always get it on cukes and squash. To tell you the truth — I don’t pay to much attention to it.

    With things the way they are in this world today — I never think open air burning is a good thing. However I don’t think spores would transfer, but rather die — if that is what you mean. If you let them dry in open — spores will transfer.


  • Theresa, This is a very well researched and presented article. After studying the information here, (and reading more online) I have also come to the conclusion that this is NOT what is causing my tomatoes to rot. They’d be dead by now as this has been going on over the last several weeks.
    The rot seems confined to the Black Krims and Cherokee Purple, but my Dafel has rebounded from early blight and is starting to give me some very nice tomatoes again.

    US Pest has this map available http://uspest.org/risk/tom_pot_map to track infestation risk for late blight. You can zoom in on your particular area to get a more detailed analysis of the latest plant disease forecasts based on models which consider degree days and rainfall, to estimate spore formation. I looked at it, but I think I glean more from just being outside looking at my plants. If I were farming, it would probably be more of a help. It’s updated daily.

    Are your tomatoes looking healthy again? Now that you have probably ruled out having late blight, have you pinpointed what you think they may have been showing signs of?
    Again, kudos for this article. It is very well explained, and I really hope I won’t encounter this dreaded disease!

  • Theresa, Another couple of questions. Do you discard the straw around your tomatoes from year to year to prevent infection?
    And, how on earth do people with small gardens practice crop rotation of tomatoes? If I do this, and I always try to, my tomatoes end up being only about 3 or 4 feet from where they were the year before, just because of the limits of my garden and the size of tomato plants. I guess I may be helping a little, but the benefits can’t be that great. What do you think?

  • Sandra, I was glad to hear that your Dafels have rebounded. I had been so disappointed with mine this year because they had not produced anything like Dafels usually do. Then all of sudden they’ve started looking beautiful and producing great tomatoes. So I’m pleased and hope yours will continue along the same line.

    Regarding “They’d be dead by now.” From what a lady in Pennsylvania — who had the disease on her tomatoes — indicated on a forum I frequent — hot and dry conditions will halt the spread of the blight — not kill it — but halt it and allow the plant to look like it’s going to make it for a while longer.

    I guess I would have to see it for myself (which I hope I never do), but I’m not sure I would consider leaving the plants in the garden at all.

    I liked your map. There is also another http://www.usablight.org/?q=map — the only thing about this one — I don’t know how often it’s updated.

    My tomatoes are looking much better and giving lots of fruit — but I still have not determined what I thought was Late Blight but wasn’t.

    I hope we never have Late Blight in our gardens. Good soil is our best hope and knowing what to look for. If spores surround us I don’t know what else we would do.

    Regarding your other questions:
    1. No I do not discard the straw around my tomatoes from year to year. I will address that more in the post on Early Blight — as several people have asked me about it.
    2. It is next to impossible to really rotate things a lot in a small garden. I still think you need to try to do what you can. One of the things that I will recommend to everyone in the Early Blight post is planting cover crops. I think this can go a long way toward suppressing diseases.

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