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This can be either underwatering or a fungus.

If the plants are growing in the ground, they need at least 2 inches of water a week. If they are in a container, they need to be watered when the soil is dry to the touch on the top. If the weather is warm where you live, this can mean daily or even twice daily.

If you feel the plants are being watered properly, it may be a fungus. Treat the plants with a fungicide.

Covering plants with leaves is a great way to mulch them for the winter and keep them safe from fluctuating temps that can kill them. You can take the leaves off after the weather starts to get relatively warm, 40 degrees F or higher (more often than not). After you remove the leaves, don't worry about occasional cold snaps or snow that may happen. A few of these will not harm the plants.

As long as the bed does not have a history of plant disease, planting them together is fine.

The strawberries are ok to plant out now. Lettuce should be ok in a week or two, as long as you are not expecting cold. I would wait until the end of April or even a week into May for cucumbers and corn and 2 or 3 weeks into May for Tomatoes and Peppers. Tomatoes and peppers do better if planted outside when the soil has warmed some. They grow faster that way.

The plants depleted the phosphorus in the soil. This helps with flowering and fruiting. Bone meal is a good organic ammendment to fix this. You can also use a high phosphorus fertilizer (typically sold as a "bloom booster" fertilizer).

If this is a newly planted strawberry, I would not worry too much about the sun damage. They will toughen up pretty quickly. You can trim out any brown leaves as these are dead and will not recover.

As for the too much water, where you live, I doubt it will cause any issues. As long as they are not left in standing water for more than a few hours. Actually, if you are keeping them in pots, you may want to be watering 2X a day, especially on days where the temps are over 80F.