When the plants were being watered, were they only watered once the soil became somewhat dry to the touch a few inches deep? We ask because over-watering can cause symptoms similar to these, though there is a lot of overlap with other causes as well, which makes diagnosis difficult.

Was the soil tested by a laboratory in this planting bed, at least within the past 3 years? If not, submitting a sample for testing might reveal certain nutrient deficiencies that could use supplementation and perhaps a pH (acidity level) adjustment as well to better help roots absorb what they need. There's no reliable way to judge if the soil is deficient without lab testing, and while applying a general-purpose fertilizer sometimes works, it might also be adding nutrients the plants don't need (excesses of which sometimes cause problems).

At this point, it's time to begin seed-starting for a fall crop of most brassicas, like kale, so you may want to harvest what you can from these struggling crops and start a fresh batch, especially since high heat will really stress kale (less so collards).

Miri