Symptoms are not seen on older leaves. Some have speculated that application of sulfur is causing Mn deficiency. Actually, just the opposite should be true. Mn availability is greatly affected by soil pH. As soil pH declines Mn becomes more available. One means of increasing Mn availability to plants would be to reduce the soil pH by applying sulfur.
Heat is likely a contributing factor.
We have seen yellowing of old leaves commonly in years with high temperatures. Some growers have applied a micronutrient product containing Mn and have found some relief from the symptoms, but the relief has been short lived. I don't think so. In one set of experiments we fed radioactive CO2 to new leaves above developing fruit, old leaves below developing fruit or to leaves on a non-fruiting upright that was adjacent to a fruiting upright along the same runner. We then examined the fruit for the presence of radioactivity that would indicate that sugars had moved from the leaves into the fruit.