I've got young feijoas in my garden, and I'm in Wellington, NZ. I will have a look and see if there are signs of disease - if I have something similar then it might make it easier to identify. If it was a root problem, the plant would probably be wilting (despite having enough water). I think that this is probably a leaf fungus based on your description. It is quite likely that the plant had it when you got it, just not so obvious. It may be made worse by the plant being in a protected environment in the greenhouse, although the weather lately has been encouraging to fungal disease. Unless you see the black leaf spots obviously getting larger and joining up, it is probably something that the plant can live with. If it was a root problem, it would be worse. In general feijoas don't have too many problems, so I wouldn't be too concerned. If you can, get it outside so there is more air movement - hopefully not gale force winds though. While feijoas are considered wind-tolerant, they aren't "Wellington wind-tolerant". Try and protect it a bit, especially from the southerly, which is the more damaging wind to plants. Also, don't overwater - if the drainage is good and there isn't a saucer underneath, overwatering probably won't be an issue, just don't let the pot stand in water. Don't overfeed as that can make plants vulnerable to disease as well. While I'm greatful that Wellington doesn't have the temperature extremes of North America and Europe (and I'm especially grateful not to be in the snowstorms I've seen on TV lately), Wellington is having pretty extreme weather right now. My rain gauge recorded more than 80mm of rain over the weekend, with gale force winds and temperatures more typical of winter. Unfortunately, it's just a bad growing season for everything except fungi, and there is a lot more disease around. I've attached a link to my garden blog in case you are interested. It is largely just my personal record of what I plant and what I eat from the garden, but you might find something useful there. Here is a link that might be useful: Wellington edible garden blog

Thank you very much, Fruitnut. Unfortunately, not only I can't do anything to aid the drainage (I planted them on my lawn which is level), I just mulched the darn things this past weekend. I'll remove the mulch tomorrow and start praying. Thank you for your advice on rootstocks. I have not paid much attention to them before. I will do now. I just plan to plant a new peach tree (TangOs) on a mound next year following Olpea's system. It looks like I may need to plant a few more new peach trees if it does not stop raining anytime soon.

I was following the directions given by the nursery, but I think I will water every other day this week and then next week only 3 times. Thanks for the advice! LOL...a huge downpour is just starting, so Monday it is.

You need to keep them watered in the heat of Qatar. Also- its possible the water you have is too salty? minerally,hard? You might use bottled water for the young mangoes and later they will have enough roots. Also,if not the water? Then its still residual damage from the salts in the manure. Its going to take more time than a few days. It might totally defoliate...but if the trunk stays firm? You will see new buds. Success! You might whitewash the trunk..Mango wood will burn if exposed to high desert heat. That would kill the tree also.