The Australian avocado industry suffers millions of dollars of lost productivity each year as a result of Phytophthora root rot (PRR). PRR is caused by the soil-borne pathogen Phytophthora cinnamomi (Pc), particularly during prolonged wet soil conditions. Pc infects avocado feeder roots, the main source of the trees water and nutrient supply, resulting in gradual canopy decline, shoot die-back and eventually tree death.
To determine if the incidence of Pc could be assessed non-invasively an experiment was conducted where aerial optical and thermal imagery of an avocado orchard was compared against a visual canopy health rating scale and the number of zoospores generated per unit weight of feeder roots.
The canopy thermal characteristics of selected avocado trees were found to be better correlated to the visual disease rankings of tree health than the number of zoospores in roots. Interestingly, the number of zoospores also showed little correlation to the disease three ratings indicating that the concentration of Pc spores themselves were not the sole driver of PRR symptoms.
It is therefore suggested that PRR disease expression is a combination of Pc infection, exacerbated by additional biotic and abiotic constraints such as soil type, orchard management practices (eg. Irrigation) and rootstock susceptibility.
The findings of this initial study indicates canopy thermal characteristics of avocado tree can be used to indicate health status including the progression of decline due to PRR. Additional research will confirm whether this approach can be used to detect PRR non-invasively before it is visually apparent to the human eye, thus allowing remedial action to be implemented before tree decline.