Just wondering…how moist was the surface around the plants stem and how many leaf nodes?
It could have been damping off if they were young enough…the plants develop immunity as they mature.
A good read from @garrigan62
Damping-Off is a fungus that is in soil mixtures, it attacks seeds and seedlings before and after it emerges from the shell. Seedlings that get attacked by this fungus usually die, considering the plant doesn’t have a fighting chance nor a system setup to help fight the fungus. A range of growth stages the plant can be attacked, up to 8 sets of true leaves, or 3 leaf nodes. Leaves, roots, and stems can be affected. Damping-off can be easily confused by excessive fertilization (Nutrient Burn), high levels of salts from either water or nutrient solution build up, excessive heat or cold, excessive or insufficient soil moisture. The Cotyledons show on the soil surface can whither and die or healthy looking seedlings may suddenly fall over. Infection first starts right below the soil line; once attacked, the seedling and vegetative stage plants will start to rot, lower part of the stem will be soft, brown and or leaves will turn purplish hue and can yellow, brown and then fall over or shrivel up; can be any one of these or show all symptoms. Seedlings, or vegetative stage plants then can start to fall over from stem being so soft and starting to rot inside out, one item contributes to this problem is over watering, putting seedlings, or small plants into a big pot; when you water the soil down lower soaks up excess water and ends up sitting in the middle to bottom part of the soil in the pot. When this happens fungus begins to build up due to them not having a deep enough or a big root system to absorb all the excess water, then it ends up turning into damping-off. Later stages of damping-off in vegetative stage seedling, the leaves will droop and turn yellow, as if the plant was over watered. Stems will be severely weaken and may fall over from being under developed and skinny. Older plants can build up a resistance to damping-off, but if they do not, older plants show long skinny stems, stunted roots, and stem discoloration. The Soil that is rich in nitrogen, r rich mixture, and soggy soils, soil that stay wet for long periods, and warm soil or temperatures. Even hydroponics systems can get damping-off; mediums that stay too saturated, you have less likely chance of getting damping-off in hydroponics systems.
Prevent and Control
In order to help control damping off or preventing it you should try to start seedlings in “starter cups”, drinking cups you get from the store; putting drainage holes in the bottom of the cups is extremely important in helping avoid damping-off, allowing excessive water drain so the bottom half of the mixture will not stay wet.
This is why cups are great for starter seedlings, you can still over water, but your chances of getting damping off are much less than that of a seedling put into a big pot with lots of soil. Making sure you have holes and a good amount of perlite in your mixture helps greatly, I always recommend cups having drainage holes. Putting seeds into the soil at a low depth helps out, just putting it an inch or 2 at the most helps avoid the fungus.
Avoid starting planting in rich soil mixtures, not only can you burn them when they emerge out of the soil, but too much nitrogen can lack root growth and aggravate damping-off. Try using a seed starter mixture, not watering heavily, and avoid watering everyday, seedlings in cups should only be watered a bit every 2 days, seedlings with 2 to 3 sets of leaves in cups can start to be watered more heavily where water starts to drip out the drainage holes, only then you should water like this every 3 days. Over watering is one of the biggest causes of
damping-off. Also using soiless-mixtures like peat growing mixed with other stuff can help deterrent it. If you decide to get mostly soil with peat moss, you must make sure there is dolomite lime added to adjust the p H, otherwise you will have ph issues that can lead to locking out nutrients that are in the soil, but not able to be absorbed because the ph is off. Using sterile soil helps greatly, Allow great air flow and circulation in your grow room, allow the surface to dry before watering again, using moisture meters, or sticking your finger down in the soil near the edge of the pot to test for wetness down in the soil. Testing the soil near the edge of the pot helps to avoid bothering root development.
Once Seedlings that have already emerged from there seed shell and end up getting affected by damping off has a very high chance they will not survive and there is nothing you can do about it, by the time it shows in seedlings, it will already be to far gone. Using a fungicide on the seed before putting it into soil mixtures helps to avoid post emerge damping-off. Vegetative stage plants with a few sets of leaves has a better chance of fighting it off, using a fungicide can help depending on how far along the fungus has spread. Using any of these products below can help avoid or kill damping-off. (If using on seedlings or seeds, use organic based fungicides.) Chemical control should only be used on older plants, not seedlings.
Garden Disease Control
Multi Purpose Fungicide
Safer’s Garden Fungicide
Concern Copper Soap Fungicide
Serenade Garden Disease Control OMRI
Safer 3-in1 Garden Spray OMRI
Hope’s this helps it sure helped me out when had this problem