Not sure if nutrient problem or disease - leaves

[mcsprint71]

Hi all, posting from Melbourne, Australia, wherein we have just had a 40 degree day (celcius). I have wondered for several years why my leaves tend to yellow with necrotic areas (generally starting with the older leaves and working upwards). I haven't noticed any wilting. I've researched this one to the nth degree (well, I guess nth might include sending samples to the lab - so maybe nth-1 degee. The plants seem to lose most of their leaves by the end. Looking at the photos, do you think it is a disease, or perhaps potassium deficienc whereby nutrients are translocating to the fruit? There are several varieties with Tommy Toe appearing the most affected. I'm not concerned about the yellowing of the older leaves - it's the necrotic areas I'm wondering about. Only one variety has had fruit picked (Kotlas). Soil has had dolomite lime, compost, blood and bone and two light applications of potassium sulphate (last one recently). They also get foliar feeds of seaweed, sometimes a small dose of fish emulsion, some molasses and some compost tea depending on the immediate effect as measured by increase in brix. Brix of leaf samples have been OK (10-13 depending on variety). Any thoughts? Same thing happened last year - yield was about 40kgs from an area of about 10m2 so they did quite well although the later maturing varieties did not do as well. The irrigated by a drip system getting around 40mm every 5 days (calculated using crop factors, evapotranspiration etc).