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We have a row of 3 different heirloom potato varieties, 50 originally, of which we've lost about 15 to a problem we'd like to identify.

The following photos show the start of the issue, through to a completely wilted and near dead plant over the course of about 3 to 7 days.

1 2 3

Neighbouring healthy plants don't appear to have the issue, which seems randomly placed along the row...

Some more info, hopefully helpful:

  • Our region, north of the North Island of New Zealand, is currently experiencing very dry conditions. We've kept the plants well hydrated around every 3rd day, and while we have lost a couple to lack of water the result of that is more obvious (they dry up and die much faster and as a complete plant and the issue above often appears to affect half of the plant initially).
  • The potatoes were planted later in the season than normally. The same varieties at another location nearby, but planted earlier, are thriving.
  • No obvious caterpillars or insects on the plant or damage to the base.
  • They've been fed liquid seaweed twice (as per recommended concentrations and duration between feeds).
  • We've mounded once so far.
  • We garden organically.

Any identification of the cause, tips for next year and/or remedy for this year greatly appreciated.

EDIT: We dug up some of the plants yesterday, the badly affected ones had slimey sacks left of their tubers, and interestingly none presented with a black insides. We also pulled out some that were only just on the turn, but their tubers appeared quite fine.

  • what do the roots of the dead plants look like? – Graham Chiu Jan 3 '18 at 23:27
  • Blackleg looks to be a possibility, judging by the colour of the stems,which get darker towards the base. Dig a plant up and check the tubers - if they're soft and rotting, its probably black leg. – Bamboo Jan 4 '18 at 0:21
  • @GrahamChiu, haven't looked yet, but will do next time we're up at the farm. – Lamar Latrell Jan 4 '18 at 0:46
  • @Bamboo, Ok, thanks for that, I just looked at blackleg online and it does look very familiar! Will be interesting to see the state of the tubers next time we're up there. Some of the seed potatoes did look a little worse for wear... – Lamar Latrell Jan 4 '18 at 0:48
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    If its blackleg,you need to get those plants out and burn or dispose of them asap - and rotate other crops in the area where its been this year, don't grow potatoes there again next year – Bamboo Jan 4 '18 at 0:57
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caliche clay raised beds first season

pumice soil after 2 seasons new greenhouse built over

Raised beds. No matter the soil. Forget about sides. This is all done at the beginning with a simple shovel. Piles of decomposed organic matter nearby to throw into the bed you are double digging. Clay, loam, sand or pumice...this is the only way to go. See the trenches? That really improves drainage and control of where you want extra water to go. Cleaning the trenches out twice a year throws more soil on your bed that is also twice per year covered with 2" of decomposed organic matter. The causes a live soil continually being mixed, aerated and fed with decomposed organic matter.

I always add fertilizer. Each bed of plants, each type of plant needs very specific additions of fertilizer. Minuscule additions but critical. I also know the pH of the soil; the garden beds and pot soils. Potatoes love very acidic soil. Those are potatoes in the second picture. Tomatoes are in pots. There will be no tomatoes, potatoes or peppers or egg plant planted in those beds for two years. Pots really enlarge your garden. The control over soil in pots sure make this rotation stuff easier. If you want a great potato harvest you will need to check the pH and make sure your soil has 'tilth', friablity, fluffed...or your tubers won't be happy and well formed. I never dig my raised beds after the first time. Just cleaning the trenches out helps to recycle the soil, feed the soil (lots of decomposed matter washes off the top where decomposed organic matter should be applied AFTER the plants are up from seed or starts), should never grow in decomposed organic matter alone. Soil has to be incorporated.

Black Leg disease

This is as bad as diseases get and are quite normal when gardeners try using non certified seed potatoes. Frustrating for sure. Without hands on examination with my little microscope it would be safe to assume you've got bad bacteria or bad fungus in that soil. I would not even try to plant potatoes or tomatoes or peppers or kale or broccoli or cauliflower...in that soil for at least 2 years.

Hope this helps. These raised beds make a world of difference with success growing any plants. Healthy plants are able to resist disease and insect problems. I think this would help you out an awful lot...done just once. It just takes one bad bacteria or one bad fungal spore to undo all of your work. We all have to learn this lesson for ourselves. I've h.r of mistakes to learn this stuff.

One other thing you must do is remove all the plant material and tubers! Whether they are rotting or not they will be harboring this bacteria. Or fungus. Do not leave a single potato tuber if you can possibly manage. Easiest to start double digging, pull out tubers and make some good plant beds for next year. You do not want to use a rototiller for your clay soil. Just the shovel, turn over each shovel full, chop chop a bit, pile up as you dig down at least a foot if not 18 inches. Depends on your soil, environment and your plant association...

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