I have attached photos of cucumber plants started indoors about 3 weeks ago. They sprouted fine but are slow growing (others in the background on the 3rd file were planted later and are doing great) and have developed rust on the leaves which are shriveling and the plants are dying. Affecting some tomato plants as well, and both most affected seedling are at the edge of the light field; 400W Sodium vapor light about 40 y/o). I have started seeds this way for decades without serious problems. I have had the same large bag of potting soil which I have used for years and these trays are new this year. No change in water and no problems with stem rot. The smaller containers with cukes and squash/melons are new potting soil. Disease?
Thanks for the question. For clarification, when you said you have been using the same potting soil for several years, does this mean you have been using the same brand or actually using the same soil year after year? If the latter, this might be contributing to your problem. Also a seed starting mixture is preferable to a potting mix. The compositions are different as well as the quantity and type of fertilizer present.
However since you have been following a rather successful practice for several years, it is difficult to ascertain exactly what is now occurring. Speaking from my own experience I do know that if all containers are not thoroughly washed and disinfected, fungal diseases can suddenly arise. I suspect that is what might be happening. Over watering of seedlings can also encourage fungal growth. One possible remedy is to remove those seedlings most severely affected and replant. If you do this, cleaning the planting surfaces would be essential.
One other thought comes to mind. Are you using seeds that are more than a few years old? One of the things that is observed with old seeds is a greatly likelihood that they have been infected with fungal spores.
Here are some publications on growing seeds indoors:
Thanks for your response. i suspect your thought about a fungal infection from the soil, container and/or seeds is most likely as I generally don't wash the containers, have been using the same bag of soil for years and many of my seeds are years old. Some of the affected plants are doing much better while a couple have died (see attached). I re-seeded those that died or still appear sickly and will be getting them outside next week. A few of my tomato plants, especially the ones at the other end of the container were affected but are looking much better. Could an opportunistic infection take hold in plants at the margins of my artificial light field and not take hold where the light and heat are not as strong? That would explain why plants at either end of the trays were most affected. I have had the trays outside a couple days and the plants love it. Thanks again.
correction: I meat to say in my response at the end of the 6th sentence "not take hold where the light and heat are stronger?"
Thanks for the additional information. I now suspect that more than one thing is going on. For the time being I would not be overly concerned about any infection possibly occurring around the margins of the light. I now am thinking that the reusing of the same potting soil might be the single biggest influence.
Potting soil is “denser” than seed starting soil. Among other things, it can become rather compacted with time and with reuse. When using such soil to start seeds, the poor seedlings experience greater and greater difficulty in establishing their initial root systems. This in turn can adversely affect the movement of nutrients through the seedlings. This might be the real reason why some of your seedlings lack good color (insufficient chlorophyll) and have a yellowish hue. Interestingly, seeds do not need much in the way of fertilizer to germinate. Until after the first leaf or two develop, the seedling draws its energy from the seed itself. Additionally, potting soil has a much higher nutrient content than seed starting soils. Sometimes these excessive levels of fertilizer will prove detrimental to germinating seeds. But having said all this, I can't explain why this season you are experiencing issues that you've not seen in past years. Just one of those things that make gardening an ongoing challenge.
See the following regarding soils:
Here is another article dealing with disinfecting seed trays. It uses household bleach in the amount of 1 part of bleach to 9 parts of water. Just be sure that you do not increase the amount of bleach above this level (as I did once!!) for this will greatly hinder seed germination.
One final point. Tomatoes are not too fussy about the type of soil you use. This is probably why your tomatoes appear to be doing OK.