Assuming that's not the result of a "burn," I'd be carefully inspecting the leaves - including their undersides - for tiny little insects with voracious appetites.
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The chart does not take into consideration at least 3 things. Plant species( lettuce and cannabis don't have the same optimal growing conditions)Leaf surface conditions, and the ability of a plant to manipulate it's own environment through stomata. The chart omits one very impactful variable---air movement(wind)!
Here's my greenhouse which is often under "Dangerous" conditions.
I got it about 10 years ago from Farmtek.I agree 420% with the above. Just use the chart as a "guide" not a gospel and you will do well. Plants are pretty versatile. The unknown is pests.
@Bush Doctor 77 what brand tunnel are you using for your greenhouse??
Nice - what size is it??
10 years is long whats the material?? The ones I'm looking at only carry a 4 year warranty on the material.