We have been seeing walnut leaves showing browning (necrosis) on the leaf area between the veins in some orchards since about mid-May. Sometimes the leaves will then twist and defoliate in very severe situations. We call this problem mesophyll collapse and believe it is a physiological problem associated with sudden temperature changes, i.e. heat following cold or vice versa.
At the Verona CIMIS station, from around May 10-13, daytime temperatures were in 80s with a high of 91.5°F, followed by daytime maximum temperatures only reaching the 60’s from May 15 -21. May 22-25 temperatures were back up in the 70’s, down to 63°F on May 26, then back up in the 80’s.
This problem may have to do with transpiration and may occur on leaves that are mildly low in zinc, where chlorotic spots become necrotic, and then coalesce. If you see this, check the zinc levels on your July leaf sample report.