Potato Blight
This year was the best of times and the worst of times for growing potatoes here in Northern Germany. Solid weeks of sunshine requiring heroic bouts of watering were followed by a string of weeks of unrelenting grey, cold, and rain. I assumed that the potato plants would be angry about the miserable conditions of July and August, they must have liked having an abundance of water. We ended up with our best potato harvest ever. I’ve never grown such giant potatoes! Yet the plants themselves turned brown and died a couple of weeks before the normal harvest time. Even more strangely, they quickly decayed into a slimy, stinking mess. And then the potatoes started rotting immediately… some even within a few days of being harvested, as they were still airing out in our covered patio, before being stowed into storage in the garage.
At first, I didn’t think much of this. It isn’t weird for the occasional potato to get fuzzy and shrivel up over the months they wait in the garage for us to eat them. But by the end of the first week in storage, they had started liquefying like it was going out of style. When it started to feel like there was nothing I could do to stop them, that’s when I started to wonder what was going on. Despite my vigilance with removing the rotten before they could pass their pestilence onto their neighbors, within a couple of weeks, I’d lost 20% of my harvest. In olden, not so golden days, this would mean I’d go hungry. For me, this is just a bummer. But what a bummer it is! Growing those potatoes took a lot of work.
There are many things I can tell you about this experience. Like, when 20% of your potato crop goes rotten in its first two weeks in storage, first you blame the weather. Clearly it was a too wet, not cold enough September for storing potatoes in the garage. If only we had a cellar!
Spouse being the chatty sort, he mentioned my potato troubles to his mother. She’s been growing potatoes since before she was born. (Seriously... her mother would have planted that year’s crop while 8½ months pregnant.) MIL’s first instinct was to blame me, surprise, surprise. (My in–laws are nice people save for the fact that their default setting is Christina doesn’t know what she’s doing.) While I rolled my eyes out of sight of Skype, MIL insisted to her son that I’d failed to sufficiently dry the potatoes before storing them.
But then she mentioned that some years—especially with summers as wet as this one—this just happens. (It’s not just that I’m useless.) Even though they always grew their own potatoes for eating, her mother used to order several Zentner (a unit of about 100 lbs) to feed to the nutria she raised for their pelts. One year, the whole lot of bought potatoes went bad, liquefying in the cellar within two weeks of having been delivered. Now that was a bummer. Pity the poor soul who must clean several hundred pounds of stinking, liquefying potatoes out of a cellar. And the loss of several hundred pounds of food for the nutrias must have been a bitter expense to have to swallow (not literally, luckily) at a time when money was tight.
I feel like, by this point, I should have had an inkling what was going on. But the light bulb did not turn on until, while internet sleuthing for what other heirloom varieties of potatoes I might grow next year, I ran smack dab into the Irish Lumper. As in, the nefarious potato that was involved in the Irish potato famine. The problem I was having, and that MIL’s mother had had with her unfortunate several Zentners, wasn’t damply stored potatoes. It was blight. (Or, to be more specific, late blight.)
This was shocking to me, but also a little bit thrilling, sort of like meeting a celebrity. I’d thought potato blight was something historical, like the Black Death. Blight wiped out 1 million people during the Great Irish Famine and drove another 2 million to flee the country to avoid starving to death, but that was in 1845 to 1852. Incidentally, blight broke out in Scotland at the same time, making life even harder for Scottish crofters. A good portion of them emigrated to places like Australia. But hardly anybody starved, mainly because the British government stepped in in exactly all the ways they failed to step in to stop people starving to death in Ireland. Potato blight also caused a famine that killed 700,000 Germans during WWI. But that was still more than 100 years ago. Like the Black Death before it, potato blight was done, right, more or less disappeared from the face of the Earth...? Or so I had thought.
These King Edward potatoes (developed in England in 1902) should have been super susceptible to late blight, but for some reason, I had much more trouble with what I’m pretty sure were Puikula potatoes (developed in Lapland, Finland, in 1953) (but can’t say for sure because I forgot to write down what I planted and someone threw the tags away). But then, I grew the King Edwards on the plot up the slope, near the house where the soils were dug out and replaced by sandier earth. The potatoes that had a bigger problem with infestation were grown down at the bottom of the garden, where the water table is notably higher, the soil almost pure clay and notably soggier (waterlogged much of the time, you could say).
Well, just as the bubonic plague is still with us, occasionally rearing a minuscule version of its ugly head, so is potato blight. This late blight is still a major cause of food insecurity in some parts of the world and it costs the global food industry about $6 billion a year in crop losses and mitigation efforts. As I realized as I kept reading about blight, I’ve also been struggling with the pathogen that causes potato blight it every summer for years... on my tomatoes. This was just the first stupidly damp summer when blight infested a noticeable number of my potatoes, too.
Since more of us are tomato growers than potato growers, let me ask you this, especially if you’re trying to grow tomatoes in a wet climate. At some point, usually in late July or early August, just before the tomatoes start to ripen, do your plants break out in brown patches on the stems and leaves? What about the tomatoes themselves? Do they also ever go brown, not all at once, but from a growing blob or blotch?
Yes? Then... um... congratulations? Vicariously, through your tomatoes, you’ve experienced the same pathogen that causes potato blight.
It is tempting to call this pathogen, which goes by the name of Phytophthora infestans, a fungus. But it is actually an oomycete. If you don’t like using scientific names in a social setting, you can call P. infestans “water mold”, which is its common name although it is not a mold. Like all oomycetes, P. infestans is a heterokont, not a fungus. Not that this tells you much, I suppose. And, not that this will help much either, but my favorite life form ever, the silica–cell wall making unicellular phytoplankton known as diatoms, are also heterokonts. No one would ever mistake them for fungi.
Since the famine it caused in Ireland was such a momentous event, and the first notable blip potato blight made in history, you might be tempted to think that it originated there. But, thanks to some genetic sleuthing on herbarium specimens, we now know that P. infestans probably originated, not in Ireland on domesticated potatoes in the 1840s, but in central Mexico on a couple of different species of native, wild potato sometime between 500 and 200 years ago. In the early 1800s, it may have spread throughout the Midwest of the USA, moving on to northern Europe in 1842 or 1843, to cause the Great Irish Famine and the potato crop failures in Scotland beginning in 1845. In the early 1900s, a slightly different strain spread also from the Midwest, this time, not just to Europe, but also to South America, Africa, and parts of Asia.
Call it a pretty major downside to global trade. Especially as ever since then, P. infestans has had a fairly global distribution and causes problems for potato farmers all over the place.
P. infestans spread globally because we ship potato tubers all over the world. More locally, the spores of P. infestans fly in with the wind, or drop out of the air in dew or rain. Soils can also become contaminated, which is a good part of the reason why they say give plots a 4 year break in between potato crops (and don’t use any of those years to grow tomatoes there).
When the stalks or leaves of potato plants get infected with P. infestans, the infected areas first turn dark green, before going brown or even black. If it’s especially humid, they may even break out with a white mold. In any case, the plant dies quickly back. Never ones not to make hay when the sun shines, “secondary bacterial rots” can set in, accelerating the decay of the vegetative part of the potato plant, turning it into slimy, a foul–smelling mush. (I swear, my rotting plants smelled like sodium hydroxide (lye), as if their pH had shot up to 10 or 11.)
From what I’ve read, if you cut down the green part of the plant at the very first sign of blight, you can prevent the spread of the pathogen to the potato tubers. And I think, because I did (accidentally) act fast (because it’s easier to find the potatoes when you can pull them up attached to their stalks), that is why I only lost about 20% of my crop, instead of all of it. But you have to be really vigilant to succeed with this, because the beginning of late blight is subtle and the spread is fast.
Three what are probably Puikula potatoes showing the first signs of blight on the outside, namely the strange white outgrowths and the patches of damp.
I can definitely vouch for the fact that on the outside, potatoes can look just fine and still deflate into a putrid mess a few days later. Infected potatoes will begin turning brown on the inside before getting wet looking or moldy looking on the outside. Then they go really disgusting. When the entire crop has been infected, you can lose the whole lot to rot within the space of a week or two, like my MIL’s mother did with the couple hundred pounds of potatoes she’d bought, not to mention like all those poor souls in Ireland, who depended upon their potatoes so desperately for sustenance.
Those same three blighted potatoes sliced in half to reveal the rot within.
Where I live, the spores of P. infestans shouldn’t survive the winter in soils. It’s too cold. But… because there is always a but… P. infestans persists because potatoes are sneaky. You’ll never manage to dig all of them up out of the ground. Potatoes are steathy that way. When infected tubers overwinter, P. infestans survives inside the, to rise again the next summer and infect the next set of potato plants.
To get back to the Irish Lumper, I’m actually amazed that the it is still around. Given its deadly reputation (and clear lack of resistance to late blight), you’d think it would have been consigned to the dustbin of history. Who would have wanted to keep growing it? Also, apparently, it is a ghastly tasting potato. But, if your curiosity is morbid enough, you could try to order some Irish Lumper seed potatoes to grow in your garden. But me, I think I’m going to look for some varieties that are highly resistant to blight. Because you never know when you’re going to get a summer as wet and miserable as our last one!