Heat Damage to Raspberry Foliage

In case you haven't heard or been outside lately, it's been real warm in the Watsonville-Salinas area the last few days.  A lot of effects, like sunburn and softness of the fruit, have been pretty common but I also spied the damage on raspberry leaves depicted below in photos 1 & 2 below at a mite trial I was spraying this morning.

The damage is happening on some recently matured leaves of emerging primocanes (vegetative non-fruiting canes) and looks to be coming from the plant not quite keeping up with the water needs of those leaves and subsequently they burn and curl up.

What is notable and worth sharing however is that the negative effects depend on whether the plant is outside or inside of the tunnels that cover the large majority of our raspberry and blackberry culture on the Central Coast.  All the burning and leave decline I observed was outside of the tunnels and none was seen on the inside (photos 3 & 4).  Maybe this is not so surprising because of the reduction in the intensity of the Sun's rays by the tunnel's plastic makes it unusual to find sunburned fruit there and we must be seeing a similar situation now for the leaves.

Also recognize that the plastic on these tunnels isn't always super clean and could be reducing light penetration even more!  The plastic is often used for three years or longer, and accumulates a lot of dirt, as you can see in photo 5.  But perhaps this is not all a bad thing, one hypothesis for the consistent outperformance of raspberries in tunnels is that they are plants which originally thrived in the filtered light at the edge of forest clearings, so this enhanced reduction of light could, beyond preventing fruit and leaf burning during intense light and heat, be just right.