Phytopthora and nutrient deficiencies

by Laurie Meadows   
August 28, 2019, edited 7 September 2024   

Avocado leaf nutrient deficiency
Some seedling avocados seem to tolerate Phytopthora better than most. But they are rather rare. Some avocado seedlings are very badly affected.

Root damage and death from Phytopthora infection reduces nutrient availability throughout the tree. One consequence is nutrient deprivation in leaves.

Various mitigation techniques can be used, some improving conditions for the roots, some attempting to address the nutritional deficit at the leaves.

Phytopthora infection is almost inevitable, in my view.

Fiddling round on home-scale approaches, I am trying a top-down approach. A winter leaf spray with phosphoric acid, calcium nitrate +solubor sprays, micronutrients, MgSO4 sprays, and KSO4 spray. Leaves are re-greening.

But these are bandaids, really. The main challenge is to improve the soil conditions, especially in shallow soils overlying clay (which is what I have). This means making sure that minerals (NPK) are applied regularly in the right amounts  (not too much). I find 'Avogain' NPK plus trace element mix is a suitable blend to use.

The soil needs to be well supplied with phosphates to promote root growth. Roots actively grow in spring and on into the warmer months - if the soil stays moist enough. Superphosphate is a good base dressing, and as it enters the soil matrix it leaves behind calcium sulfate, a good source of calcium that doesn't shift the pH.

Probably most important of all is woody mulch.  Avocados are relatively surface feeding, and decaying mulch is supected to be important in helping suppress Phytopthora. It also helps retain moisture in the vitally important spring root growth period, as well as eventually supplying additional nutrients to the trees.

Leaf fall eventually supplies good levels of mulch, but, paradoxically, phytophthora affected trees have much reduced foliage.

The ultimate solution is to buy trees grafted onto Phytopthora resilient rootstocks - but these are generally unavailable to the home gardener.