Farm Call: Yellow Leaves on Raspberry in Tunnels

I was called out last week to evaluate the following situation, which I am sure is not at all unfamiliar to any of you who spend time working with raspberries under tunnels. Symptoms, as shown in the pictures below, consist of very obvious yellowing, sometimes half the leaf, sometimes less, sometimes more, started a few days after the tunnels went up and a run of hot days.

Consistent with the work we have done on this issue in the past, it's fairly clear that the leaves are dying from the high temperatures in the newly erected tunnels. The elements nitrogen, phosphorous and potassium are being remobilized from the yellow leaves to newer leaves on the plant, while others, markedly calcium, are left behind or continue to accumulate (look at the large differences between iron and boron in healthy green leaves as compared to yellow leaves). Curiously, copper trends high in both – usually I see that around 2 or 3 parts per million.

Recommendation to the grower last week when I took the samples was to vent the tunnels by moving the plastic up a bit, and happily he reports an improvement in the plants already. 

One half of the leaf yellow, and the other half green.

Yellowing even more pronounced on a single leaflet, discoloration divided right down the midvein.

Whole leaf in decline, only green left in the veins.

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