SueCT, fertilizing to reduce disease is one of the first things I learned here at T'ville. My plants were always coming down with stuff as soon as they started to ripen their fruit. Fertilizing stopped that, and kept them healthy during the ripe cycle.
My opinion, based on observation, is that the typical tomato plant will start moving nitrogen out of the lower leaves when they run short. These leaves also become susceptible to whatever blight the variety is prone to, whether early blight, grey mold, whatever. Only a rare variety will drain the leaves and let them fall, just yellow, without disease. It just seems that whatever immune support is present, the plant stops sending that when the leaf starts to be used as a source of nutrients.
This year I tried a soluble fert from Promix with low N and high P and K. I was not entirely stoked about the results. Late season leaves are paler than they should be or have even yellowed in spite of being fed. The only clear result I got from feeding, is that there would be a flush of ripe fruit the day after. Compared to the usual ferts which have a decent amount of N (proportionally) I didn't see the renewed health and vigor which I normally get.
Thanks Linda for the heads up about excess potassium in the environment. News to me I'll have to look that up.
