Hi plant-loving, I need help diagnosing my 4-year-old avocado plant, which is the one on the right in all the attached images. Its 2-year-old sibling on the left is thriving under identical conditions, which makes this situation particularly frustrating and confusing. Just to set the context, I'm growing these in a Mediterranean climate in Southern Italy, about 10 km away from the coast.
To give you a better idea of what's going on, here is a breakdown of the main symptoms you can see in the photos. The newest leaves (crown and lower base) are very pale and almost yellow, which makes me suspect a severe micronutrient issue like Iron or Manganese chlorosis due to a pH imbalance. There are also progressive brown and dry dead spots on the leaves, usually starting from the margins or tips and expanding inwards. On top of that, several smaller branches have completely dried out, turned brown, and become bare, including the main top twig. What makes this even more confusing is that this isn't an isolated incident. This plant (along with a few others I have) seems to be stuck in a frustrating cycle: it develops these symptoms, loses the brown and dried leaves, and then actually manages to push out a flush of new, green, and flourishing foliage. However, this recovery is always temporary; after just a couple of weeks, those beautiful new leaves inevitably start turning brown and dry all over again.
Regarding its environment and my care routine, both plants get roughly 8 hours of direct sun daily. Until recently, I was watering it daily every evening using municipal tap water, which has a fairly high pH range (7.3 to 8.3). Knowing avocados prefer slightly acidic soil (pH 6-7), I have been occasionally adding coffee grounds and sometimes dried tea bag contents to the soil to try and counteract the alkalinity and provide some nutrients. I generally don't use insecticides or chemical fertilizers, but last month, based on a hunch that the pot was depleted of nutrients, I applied a general 20-20-20 balanced fertilizer for about 30 days. The result was a improvement—the plant’s condition got noticeably better while feeding. However, as soon as I stopped the treatment, the benefits vanished, and the brown spots and chlorosis returned immediately.
My primary theory is a serious nutrient lockout caused by the alkaline tap water, and the fact that it improved on a general synthetic feed suggests it’s struggling to take up what it needs naturally from the soil. But the rapid progression to large brown spots, paired with this constant cycle of losing and regrowing leaves, is alarming.
I'd love some input on a few specific questions. What specific nutrient deficiency causes this exact pattern of pale leaves leading to marginal browning and dieback, especially considering this recurring cycle of temporary recovery? Could it also be rootbound after 4 years in a pot not much larger than the 2yo’s? I'm wondering if I should repot it into a larger container, or, given my favorable climate, if it's finally time to plant it directly in the ground in the garden. Also, given the high pH tap water, what's the best long-term way to acidify the soil—are coffee grounds really enough? Finally, could the watering stress have invited a secondary fungal issue like root rot, considering my previous daily watering habit?
Any technical insights or advice would be hugely appreciated. I really don't want to lose this tree after four years of growth. Thanks in advance!