Yellowing foliage (particularly on old growth) with yellow veins, is a symptom of nitrogen deficiency, although it can sometimes affect the whole leaf. However, potassium deficiency can cause the foliage (also primarily the old growth) to yellow around the edges of the leaves. If the plants are weak and easy to break, they probably need potassium. If the leaves are a dull color, they probably need nitrogen (the leaves tend to get dull before the leaves start to yellow—not after). Plants with plenty of nitrogen tend to have shiny (or at least shinier) leaves. Plants with plenty of potassium tend to be very tough to break, and don't bend as easily (stems, roots, and to a lesser degree, leaves). It's possible your plants might need both nutrients.

I don't think you have an infection here. I think it's just potassium and/or nitrogen deficiency. Other things are possible, though. Some infections can cause nutrient deficiencies. I do see some minor leaf curling; so, maybe.

For potassium, I've had the best success with the following (you don't need to use all three):

* Potassium sulfate
* Wood ash (provided the soil isn't too alkaline; it can raise the pH)
* Monopotassium phosphate (this also provides phosphorus)

For nitrogen, most general purpose full fertilizers that are fit for vegetables are probably a good source, but I tend to use ammonium sulfate and/or urea and/or monoammonium phosphate (which only has a little nitrogen and a lot of phosphorus), if I need something water soluble. I'm not sure what I would recommend in the way of a quick organic solution off-hand.

If you're getting nutrient deficiency symptoms in your outdoor soil, you might have an issue with the pH of your soil. Testing it might be a good idea.

Here are some other nutrient issues to look out for in the future:

If the leaves are unusually small or if they are purple underneath, they may need phosphorus. Phosphorus is especially useful for plants when it's cold.

If the fruits are particularly small, that may indicate a potassium or phosphorus deficiency.

If the leaves (primarily on older growth) are yellowing, but the veins are green, then it could be magnesium deficiency.

If the undersides of the leaves look kind of purple-ish and maybe a little translucent, it could be sulfur deficiency. Sulfur deficiency can also cause yellowing of leaves on new growth, much as iron deficiency.