Impact of the Polyphagous Shot Hole Borer beetle (PSHB, Euwallacea fornicatus) and its symbiotic fungus (Fusarium euwallaceae) on deciduous fruit trees in the Western Cape Province of South Africa.

PSHB might harm and occlude a plant’s vascular system, stressing the trees. What would be the impact of PSHB on the deciduous fruit industry in the Western Cape? We first set out to do an early assessment of the impact of PSHB on a Packham’s Triumph orchard. Subsequently, we assessed the impact of PSHB infestation after another year of infestation—to see what has changed, or not—also adding more sophisticated measurements and analyses of plant stress for a more comprehensive understanding.

Data were collected from 17 control and 77 variably infested Packham’s Triumph trees in the Strangmans Orchard, Lourensford Estate, Somerset West. A suite of quality parameters used in industry were measured. Carbon and nitrogen stable isotopes were used for the leaf analysis. Secondly, plant stress measurements were performed on 45 trees comprising three categories of PSHB attack density. We then used multispectral drone imagery to see if we could correlate any stress to tree greenness indices, as an early stress-detection method.

PSHB induced significant variation in some quality parameters, most notably, increasing starch conversion. Fruit quality is affected by infestation severity. We saw increased leaf conductance of PSHB-affected groups toward midday (with increased temperatures), although being a weak trend. There were no significant differences between different PSHB attack categories for any leaf stress parameters considered. Multispectral images revealed healthy plants.

While the infested trees do not appear water-stressed at the moment, at least at the leaf level, fruit quality was clearly highly variable, indicating the plants are responding to PSHB presence, appearing to ripen the fruits earlier. It might become difficult to predict the expected quality in affected orchards. Overall, pear trees in this orchard appear resilient against current infestation levels, although some might experience increased water use under extreme drought events.

The pear trees showed resilience against PSHB attack in orchard, with no dieback observed. However, fruit quality would vary according to infestation severity.