What’s wrong with my potatoes?

View all images of my poor, sad potatoes

We got a couple successive spells of hot, dry weather in early July (interspersed with some rain and low-80s temps) and my potatoes kind of keeled over. At first, I just thought it was just the end of the cycle for the taters…they’d bloomed, blossoms faded, and plants started to die back.

Then I noticed kind of slimy “lesions” on some stems yesterday, and started to panic about blight. The plants aren’t significantly worse than yesterday; in fact, the sliminess of the lesions has dried to dry, dead spots on the stems – mostly tan, but some black. Below the dead sections, the plants are sprouting new healthy growth. Black dot, verticilium wilt, and fusarium wilt are my prime suspects.

The spots on the leaves are brown and crispy, and they’ve been that way for a couple weeks. I’m wondering if that might be leafhopper damage; you can see even the “healthy” leaves have a lot of holes chewed in them.

So, all you Nightshade Ninjas…what’s going on here? Can I at least rule out blight?



  1. EdgeWiseInAnnArbor said,

    July 20, 2010 at 11:46 am

    Hmmm… Did you look at http://www.potatodiseases.org/
    You might get lucky and the tubers won’t be affected.

    I had a similar problem with mine (leaf chlorosis), but no slimy spots, or stem damage to speak of. I have mine in buckets so I could dump one out, and 2 out of 3 were spoiled (cracked surface with brown/black discoloration). In all the rain, I think they got over-watered. I’m letting them go for now. Only one five gallon bucket seems completely unaffected out of 18. This is my first year growing potatoes, and it was just for fun anyway.

  2. July 20, 2010 at 3:22 pm

    It were late blight, the leaves should have all been wiped out and as much rain as we have had, you’d have seen white mycelium on them.

    The pictures were extremely useful. I can diagnose the problem exactly. It is Sclerotinia sclerotiorum (name of fungus), or white mold (name of disease). See http://www.potatodiseases.org/whitemold.html.

    You should remove all the affected material immediately and put it in the trash. Do not compost it. The fungus survives by hard-walled resistant sclerotia. It is extremely hard to eradicate from soil once you get an infestation. It also has a very wide host range and once in the soil can affect many different crops.

    (The part of the potato that is unaffected is safe to eat and you may leave the plant in the ground if you can remove all the affected part.)

    I see in the pictures that you used some type of mulch. It may have come in on that.

    I get one or two affected plants (not potatoes yet, thankfully) from this a year. It commonly is seen on green beans. It is important to be watchful and remove diseased material immediately and dispose of it away from the garden or anywhere plants will be grown.

    Two good diagnostic symptoms and signs:

    1. Symptom: white bleached-looking tissue. (Usually accompanied by wilting of the top of the plant above it.). This is because the fungus produces oxalic acid which macerates the tissue and bleaches it out.

    2. Sign: irregular blobs of white mycelium that become blackened in part – the sclerotia. (This is visible in several of your shots – see DSCN1813.)

    I’m not sure about the leaf spots – S. sclerotiorum produces ascospores under some conditions and these may be leaf infections, but they won’t go very far.

    • Emily said,

      July 21, 2010 at 8:23 am

      Oh my! That’s very helpful. Thank you so much!

  3. basil said,

    October 6, 2010 at 11:01 am

    Why are my potatos hard and dry? The plants seemed healthy and cropped well, the tubers are of a good size and are not diseased. However when I come to cook the potatoes they are almost inedible. If boiled they go tpo mush and any other cooking leaves them hard and dry. This is the second year in a row I have had this problem, with different varieties and in grown in different parts of the garden. Please help – I feel likee giving up………..

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