Plant Doctor Archive

Yellow leaves on puka

WE live on a semi-coastal section (within 500m of the sea) with well-drained soil. I have planted puka around our section, but over the past year most of them have succumbed to what I suspect is phytophthora. The leaves turn yellow, droop and the tree appears to be rotting. I have not overwatered or fertilised the trees. The age of the tree does not seem to be important - some of them are several years old. Most of our other plants are natives and they are very healthy and thriving.

THE symptoms do suggest root rot (phytophthora is the most common form), but I recommend you do some further checking, as your soil sounds relatively freedraining and root rot generally doesn't occur in freely drained soil. Nor would I expect a lot of pukas of different ages to succumb at the same time, unless there had been a flood or significant waterlogging, which you would no doubt have been aware of.

Dig up one of the smaller affected pukas and check out its roots. Healthy roots should be white and firm all the way from the trunk out to the root tips, with healthy branching side roots. Roots infected with root rot will be at worst brown and soft, often with the outer tissue coming away, leaving a thin central threadlike portion. Less affected roots may have brown lesions or soft damp areas in them and the root tips may be dying back.

If the roots are healthy, the problem is probably from some other cause, perhaps drought, frost, severe wind damage or damage to the base of the trunk from collar rot, rats, possums or rabbits.

If you decide root rot is the culprit, then I suggest you try to improve drainage around the root system. One way is to plant on a raised mound so excess water drains away quickly. You could also try treating the remaining trees with the fungicide aliette, available as No Root-rot.

Reproduced with permission from the former Weekend Gardener magazine. The views expressed here are not necessarily those of the RNZIH.