By Erica Faber – Technical Manager
This year, more than ever, due to our exceptionally dry summer, I feel we will need to be even more diligent with regard to Phytophthora Root Rot control. If your orchard is not under irrigation—as is the case for most of our growers—then the dry summer has had a significant impact on root growth and mass. The severity of which will depend on your mulch layer, orchard environment and nutrition programme.
With very dry soils, many roots begin to dry out and shrivel causing root mass shrinkage. Remember that roots do constantly die back and regenerate but with extremely dry soils the regeneration of new roots can’t keep up with the root dieback. The drier than usual soils over summer would also result in less uptake of solid fertiliser. This means going into winter we have trees with both a root and nutrition deficit so to speak and so the cold, wet winter soil conditions and effects on root health will be more significant. A heavier crop load and / or flower bud intensity will also add stress to the tree and we all know through COVID-19 that a host, be it person or plant, with compromised health or additional stress will be more susceptible to disease!As we have had many new faces to our grower group and some new to avocados, let’s first recap what Phytophthora Root Rot (PRR) is …
Phytophthora root rot?
Phytophthora Root Rot is a disease that affects close to 5000 plant species across the globe and is the most serious avocado disease world wide limiting production. PRR is the result of root infection caused by a soil-borne oomycete, Phytophthora cinnamomi (Pc).
After infection by Pc, the feeder roots start to decay and turn black and brittle as the root tissue rots – giving rise to the name Phytophthora Root Rot. This restricts water and nutrient uptake by the roots and leads to branch-dieback, tree decline, and if left untreated or is severe enough, eventual tree death.
The visual symptoms of a tree with PRR include small, pale green or yellowish leaves that often appear wilted during high temperatures. The canopy is sparse and as branches die back and leaves defoliate, the
fruit and branches become exposed to sunburn. The severity of the outward symptoms depends on the balance between feeder root death and feeder root regeneration.
Often these stressed trees set a heavy stress crop but it is of little worth as the fruit remain small and with a sparse canopy are exposed and become sunburnt. The exposed branches also become sunburnt which exacerbate the tree decline even further as the damaged cambium cannot translocate water and nutrients efficiently.