Epidemiology of spinach

With a mascot like Popeye, you’d think spinach would be the burly lumberjack he-man of leafy greens. But you’d be wrong! It’s actually quite the diva in the greenhouse.

that awful moment when you become a plant pathologist and find out everything they told you about spinach was a lie

believe it or not, this cartoon was not entirely truthful about spinach’s magic powers.

Spinach plants’ roots are especially vulnerable to “summer Pythium.” Pythium is usually grouped in with other “damping-off” pathogens like Rhizoctonia and Phytophthora (the one that killed potatoes and indirectly also several million people in Ireland): diseases that flourish in cool, wet conditions. But it turns out there are some funny-looking stepchildren in the Pythium family that like it hot.

Like all plant diseases (or diseases period), there are certain settings where the pathogen grows faster than the host. Most plant diseases really get off on low-oxygen conditions. This is especially true of those that affect the roots. Root can’t make their own oxygen– photosynthesis is the leaves’ department!– so they have to absorb it from the outside environment to function. And, remember that the warmer water is, the less dissolved gas– like oxygen– it can hold. So the warmer the hydroponic solution, the more oxygen-stressed the roots are, the worse the entire plant’s health becomes as a result, and the more fat and sassy the Pythium gets anyway. It’s a recipe for disaster.

And in hydroponics, whenever Pythium gets a good beachhead, it gets massively ugly in a hurry. Pythium is spread by little spores that swim. In soil they can only swim so far. But in hydroponics where the water is constantly whooshing around unobstructed, they can infect the entire system in a day! You go to bed one night and everything’s all normal, and the next morning the spinach is all flopped over and crying out for morphine.

An oomycete zoospore on the prowl!  (I sorted through a lot of photos of horrifying veterinary Pythium infections so you don't have to. You're welcome!)

An oomycete zoospore on the prowl! Look at that sinister Cyclops face. Clearly this microbe is up to no good.

Growing spinach in hydroponics is basically a race between the spinach and Pythium. Even in conventional hydroponics it’s difficult to control Pythium with fungicides– you have to stop using fungicides for at least 1-3 days depending on the fungicide before harvesting the plants, and that’s plenty of time for Pythium to eat them alive. In aquaponics it’s especially tricky because most if not all fungicides are toxic to fish. You can’t use them ever.

So what CAN you do to keep Pythium at bay in hydroponic spinach? Stay tuned…

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