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Originally Posted by Cole_Robbie
The organic matter in those fertilizers is there to feed bacteria. In warm weather, it is the beneficial, aerobic bacteria that grow. But in cold weather, anaerobic bacteria like pythium take over. I get stem rot, mostly from fish emulsion. Using any mix that contains worm castings, like Light Warrior, and letting the plants get below about 50 or so, especially when in six-packs, will make them turn yellow and purple, and become permanently stunted. Microdwarfs and PL varieties seem to get it the worst. I toss out several flats of plants every season due to these problems.
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50°F or so? Don't you use heat mats or something to keep the soil temperature up? Soil temperature is what matters.
Pythium and other common damping off pathogens are not bacteria, they are not even fungus, they are Oomycetes (water molds). They don't care what kind of nutrients you use or what kind of medium you use. Pythium is the scourge of hydroponics water culture too with no organics. It's true that
Pythium may do better in anaerobic conditions which is one reason why over watering should be avoided Temperature has little to do with whether there are aerobic or anaerobic conditions, it's the level of oxygen in the soil. Yes beneficial bacteria are more active in warm soils but they don't shut down and go dormant unless the soil temperatures are really cold.
Spores can come from anywhere, your water source, on your shoes from walking outside, dirty tools or containers, previously infected dead roots laying around, even through the air. Their spores (called zoospores) germinate in water in the presence of plant roots, they are not immobile like fungal spores, they have flagella similar to what some bacteria have which allows them to swim to plant roots. That's where they infect inside the root, feed, grow and reproduce. They don't eat your fertilizer, whatever that may be.
I've only had damping off once some years ago in a so-called sterile mix. How the spores got there, I have no idea. Haven't had it since I followed the following program. I think your best bets since it is a persistent problem for you is to keep the environment clean and disinfected as possible, avoid bringing in any possible outdoor sources of spores and then inoculate at first with the seed beneficial rhizobacteria and fungi (trichoderma and mycorrhizal fungi) to give the growing roots a defensive shield against those pathogens. Reinforce the microherd with more inoculations maybe in 2 week intervals.
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