What causes gray to black to brown decay starting in the center of my blackberry leaves between the veins?
Canes are in good condition and I don't see any insect damage. The damage is on my thornless variety primacanes grown in hot and humid central Virginia. Images show the progression.
Certified GKH Gardening Expert
That appears to be a severe calcium deficiency! This can be resolved quite easily, though.
DOLOMITIC LIME will provide, both, calcium and magnesium to the plant. Adding IRON SULFATE will help acidify the pH of the soil from treatment, as well as provide sulfur, iron, and is a good preventative for fungal infections.
This article will help you to care for Blackberry plants: https://www.gardeningknowhow.com/edible/fruits/blackberries/growing-blackberry-bushes.htm