What is destroying part of my home garden potato crop?

Wailuku, HI(Zone 14)

I have been growing organic potatoes successfully for many years but suddenly just one raised planter full of an heirloom fingerling potato (La Ratte) has wilted prematurely, with silvery diseased appearing leaves and weak stems. If I pull up a diseased looking plant, potatoes are about grape size or smaller, though the number of potatoes is about what I would have expected to see. These shouldn’t be ready to harvest for another 4 to 6 weeks.

Planters right next to this one, with same batch of organic potato seed, type of dirt, mulch and amendments have very healthy plants. I am growing these at 4000 feet in Hawaii. Weather this summer has been untypically cold (50-60 degrees), with more rain than usual, but all the plantars are subject to the same conditions, and generally potatoes grow well through all our seasons in Hawaii - hot, cool, dry. The only thing they can't take is too much water. We also have pheasants and franklins that occasionally will try to nest in the vegetable beds and manage to break a few potato stems, but that has never affected the whole plant like this.

I would appreciate any ideas of what is going wrong, and what I should do to protect the rest of my home crop. Should I pull out all diseased plants before this spreads further? Is it most likely a pest or a disease? I've posted 4 photos of the diseased plants and one for the healthy plants.

Thank you, Jean Tessmer Hawaii


This message was edited Sep 15, 2012 4:17 PM

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Homeworth, OH(Zone 5b)

looks like a fungus of some type. I would not take a chance of this spreading. Personally i would pull the plants and burn them, (don't compost them), and use an organic fungicide on the healthy plants.

Shawnee Mission, KS(Zone 6a)

Here are a couple of websites articles to check out.

http://pubs.cas.psu.edu/FreePubs/pdfs/agrs75.pdf

http://www.potatodiseases.org/foliardiseases.html

Wailuku, HI(Zone 14)

BCH521, I will take your advice and burn the plants and apply the fungicide on the healthy ones. I just saw your post today, pulled the plants but noticed it has already spread to the healthy ones, too. It seems to be moving very fast. If spraying doesn't help, I'll destroy it all and consider this year's crop a loss.

SusanKC, your references were a terrific help, and this does appear to be Late Blight. The PDF booklet in your first link from Penn State College of Agricultural Sciences describes the ideal conditions for late blight, and this is just what we are experincing in upcountry Maui right now: "cool nights (50 to 60°F) and warm days (60 to 70°F) accompanied by rain, or long periods of leaf wetness. Conditions must remain moist for 7 to 10 hours for spore production to occur". This site suggests fungicides too, vine kill, continuing to apply fungicides until plants are completely dead, and only harvesting when plants are dead. Penn State also suggests using resistant cultivars, and I'll try that next time.

Helpful advice, thank you both. One other quick question - it sounds like it is still possible to eat the potatoes since Penn state talks about 'harvesting'. Are the potatoes safe to eat?

Jean Tessmer Hawaii

Homeworth, OH(Zone 5b)

Quote from Jean_Tessmer :
BCH521, I will take your advice and burn the plants and apply the fungicide on the healthy ones. I just saw your post today, pulled the plants but noticed it has already spread to the healthy ones, too. It seems to be moving very fast. If spraying doesn't help, I'll destroy it all and consider this year's crop a loss.

SusanKC, your references were a terrific help, and this does appear to be Late Blight. The PDF booklet in your first link from Penn State College of Agricultural Sciences describes the ideal conditions for late blight, and this is just what we are experincing in upcountry Maui right now: "cool nights (50 to 60°F) and warm days (60 to 70°F) accompanied by rain, or long periods of leaf wetness. Conditions must remain moist for 7 to 10 hours for spore production to occur". This site suggests fungicides too, vine kill, continuing to apply fungicides until plants are completely dead, and only harvesting when plants are dead. Penn State also suggests using resistant cultivars, and I'll try that next time.

Helpful advice, thank you both. One other quick question - it sounds like it is still possible to eat the potatoes since Penn state talks about 'harvesting'. Are the potatoes safe to eat?

Jean Tessmer Hawaii


That one I can't answer, good luck and hopefully someone here has more experience w/ growing potatoes.

Shawnee Mission, KS(Zone 6a)

It might be best to call the local extention office. Usually they are associated with a state university.

Ayrshire Scotland, United Kingdom

I would guess this is called potato Blight (Phytophthora ) this has been a huge problem here in UK with Potato and tomato also, it is noticed when developing between May- August but is worse between June July, the later months are when my tomato's got all those blackened or dying foliage.
It has been noticed to start off in the West here and spreads Eastword across the country. The spores in dry weather are spread by wind and the yellowing on the leaves turns brown and they start to shrivel up, in wetter weather, the fungus can be seen on the underside of the leaves. when you water the plants the spores also get into the soil and this in turn spreads to the haulms and this gets onto the new foliage or tubers.
IF the bacteria gets to any damaged or growing soft tissue, Tomatoes, it causes the fruits to either drop off or are un-edible and they don't ripen. IF it is on a stored potato, it spreads to all the others while in storage over winter. You should never use potato tubers as seed potato or you spread the blight all over again and even further afield.
As far as I'm aware, the way once used to help with this problem was to spray the foliage every10- 14 days, particularly in wet seasons (and we have had a lot of rain this summer) spray with a copper fungiside you can get from the garden store, however you may also have more up-to-date things for blight now.
next year use seeds that are certified Blight free.
Hope this helps WeeNel.

Wailuku, HI(Zone 14)

WeeNel, thank you, your information does help. I did find out after spraying, as you suggest, to kill the foliage, it is still safe to eat the potatoes if they look normal. And I confirmed this was late blight. Thanks for helping, all the way from Scotland. Kind of you.

Jean Tessmer Hawaii

Ayrshire Scotland, United Kingdom

Your very welcome Jean, the Blight has been a huge problem world wide it seems, it has been trouble in Canada, USA, UK and many other places where there has been nothing like this for many years so it's nothing you have done really, once your aware of it, there are several things on the market to help either prevent it or reduce it spreading.
As far as I know, this disease will not kill us ha, ha, ha, the potatoes should be fine to eat, BUT if you store them, make sure you keep an eye on the ones you have stored as any tubers that have the slightest bit of damage will very easily pass it onto the other tubers they are stored with.
I would go by the rule that even IF we but potato's, we often get one or two that dont look just right and we discard them so I feel sure you will do the same with your crop,
As you lift the tubers, you will soon notice if there are several that have any infections, i WOULD deffinately burn all the foliage from the crop as you start to lift the potato's as the foliage will have the disease on it,
My friend had no flowers on her crop of potato's this year and she uses the flowers as a guide when she should begin lifting the tubers, she had the dreaded blight but her potato's have been fine so far, she does look at the stored ones every week as this is a new problem for her and is unsure how, when or IF it will get to the stored tubers. Make sure you lift and burn ALL the potato foliage as the spores can lay dormant on top soil but dont know for how long, maybe the winter frost don't kill it. just take precautions.
Hope this helps you out Jean.
WeeNel.

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