Cucumber cotyledons turn yellow and shrivel too soon

Question

I am growing cucumbers in pots to transplant. Some of them are doing well. But on others the cotyledons turn yellow and shrivel before the true leaves get big enough to grow. The seedlings stay stunted. Some have died.

I don't know if the problem is disease, nutrient lack, or pest. But it seems to be spreading from the sick to the healthy.

Any ideas on diagnosis and treatment?

ADDED INFORMATION:

Whatever it is, it seems to be spreading from seedling to seedling. When it infects the plant, the plant stops growing. That makes me think it is a pest (maybe mites) or a virus. Unfortunately, I think the seedlings will be a total loss, including the ones in the ground. Only three seem to be thriving, and I guess it's too late to just isolate those.

I'll guess I'll have to start over and try again.

ADDED INFORMATION:

Stormy, thanks for your comments. I sprouted the seeds in a wet paper towel and then planted the sprouts in egg cartons using Miracle-Gro Moisture Control Potting Mix. At first I had them in a sunny window and then gradually moved them outside most of the day. I usually water them once a day, but sometimes skip a day.

The seedlings in one egg carton have done quite well, so I planted some in the ground and re-potted the others in 3 inch pots.

Here are the seedlings in the barely alive egg carton:

Here is a top view and side view of a plant just turning sickly.

Here is a row of potted seedlings, some of which seem to be thriving and some starting to sicken.

Answer 1

Oh I wouldn't be so sure your thumb isn't green. You are paying a lot of attention to what you see, thinking about it, and giving the plants all the help you can. Those are the things that will make your thumb get greener and greener.

Here are three more ideas:

1. Cucurbits grow several inches of root depth by the time they have their first true leaves. You'll notice this when you thin them if you plant several seeds per hill. An egg carton is just too shallow. The little pots you have are better, but 6-8" deep would be even better - quart milk cartons with holes punched in the bottom for example. So the seedlings in the egg carton are fighting a serious uphill battle right from the start. Egg cartons really aren't deep enough to give any plant an optimal start. At the start of a plant's life, the roots matter even more than the leaves do.

2. Damping off is a fungus that kills a lot of seedlings and it can spread FAST. It is a sign of overwatering / insufficient drainage, and sometimes also of the seedlings being too cold. It is evident if the plant sort of withers at ground level then falls over. It may not be what it killing your cucurbits but it's something that initially discouraged me and many a person who starts out growing vegetables to transplant. More here: https://www.extension.umn.edu/garden/yard-garden/diseases/damping-off/

3. Cucurbits seem to grow best if they are not transplanted, but sown directly. I just wrote about that here: Watermelon plants not growing.

I realize these are pretty general suggestions, but I hope they will help you going forward. Continue to focus first on the plant's basic needs - water, drainage, sun, healthy soil, non-monocultures. The bugs and diseases are more often a symptom than the true source of problems.

Answer 2

I don't know, but my guess is they're getting too much sun for how quickly the soil dries out outside. They and their containers both seem a little small to be moved outside. In my area, they'd get roasted like that.

Moving from a windowsill to full sun might not be a great idea. Under a tree might be better.

If you have to use that container size you might consider timed watering to ensure they don't dry out too much between waterings.

In light of your comments, it sounds like it's way too cold to be having your cucumbers outside already (if 60° F. is the daily high. That would explain why you have faith in small containers outside, too. When it's hotter, they can dry out a lot faster).

If those temperatures are going to last a long time, I would recommend using a greenhouse for your cucumbers, and getting varieties that are both parthenocarpic and gynoecious (so they won't need pollinated and so they set a lot of fruit). Greenhouse cucumbers are probably what you want.

You might consider growing cooler weather crops outside, such as radishes, sorrel, and bunching onions. Cucumbers and other cucurbits probably aren't the best crops for those temperatures.

In my personal experience, cucurbits don't transplant well in cooler weather. Extra maturity can potentially help some against cooler-than-average weather, but I'm not sure about for cucumbers—and your cucumbers aren't that mature, yet. Plus, that's more than a little cooler than needed.