Actually, I believe the organisms in organically rich soils are eating up the roots themselves, I have a florida turpentine rootstock pickering mango and the only dieback I see is on the branches that were exposed to the great ungodly sun. With my Low organic content soils, I get stunning root growth, but growth that has 2x larger leaves than compost grown plants. When i went to feel the soil under my cherimoya, the part that i changed half the soil had feeder roots that were going crazy, the compost area had little to no roots in it, other than the sparse few that were clinging to life. Phomopsis and other root rot diseases are caused by low oxygen, either by compost bacteria and fungi eating the oxygen up, or being submerged for over 2 weeks. Compost tends to decompose faster in heat wave conditions. If you look at peat bogs, you will see that nothing grows there except shallow rooted plants and moss. This stuff is toxic I tell you. Perhaps due to the weakness of Florida Rootstocks, they are more susceptible to low oxygen conditions, while manila mango seedlings do somewhat ok due to their native areas being more claylike and perhaps being stronger due to their adaptation to the Californian climate. I water my pomegranate 3x day and its been nothing but vigorous, as I do with citrus, avocado, lychee, atemoya, etc. If you look at a local farm, you see that they water 18x a day, also if that were the case(too much water) how would things grow in Hilo, Hawaii, or swamps in the first place? Elephant Ears are submerged for over a year in their native environment, yet if you water them 2x in a row, they rot! So that is my Theorem on compost being the sole reason for the dieback. If I were you, I'd totally shake off every bit of that soil, cut back the leaves in half, put them in pure sandy loam or riverbed soil with charcoal infused in it, then putting it in shade for 3 months, and wait to see the beautiful results. Many plants like durian are not feasible due to the compost rotting them out a year or two later. That's why you see many plants like cassias and gardenias not being able to grow anymore. Back in the old days, they used redwood sawdust, which is pretty slow decomposing, compared to the fir bark trash. In nature, compost is only on the top as mulch, not in the soil! Orangeries in France were shown to have 97 percent sand. Also, if you look at ancient Chinese and Japanese texts about growing things in pots, you will see zero mentions of compost! Durians absolutely loath compost, and when digging around the cloudforest forum for some info, I found that they would die in the summer, the winters don't faze them at all! If Avocados hated wet feet, how come you can grow them in straight up water and not have them rot. In Conclusion, I believe you should change the soil to allow more oxygen into it, and if you want a larger tree, perhaps you should try grafting them onto a manila mango seedling for their well known vigor in this area
Sincerely, Matthew Zhou
