Bacterial spot, Xanthomonas arboricola pv. pruni, infects all Prunus species including almonds. It is present worldwide in almond and stone fruit growing areas and can lead to reductions in yield, tree vigour and nut quality. It’s occurrence on almonds is relatively recent and was first recognised on Australian almonds in 1994/5. During a two-year survey we found 2% (2018/19) and 8% (2019/20) of trees with fruit or leaves affected by bacterial spot. In recent wet years, higher incidences of the disease have been observed and industry concern over potential resistance to copper products, led to in vitro tests being conducted.
Bacterial spot isolates were collected from almond orchards in NSW, SA and Victoria from 2018 to 2023. Forty-six isolates (22 from NSW, 3 from SA and 21 from Victoria) were examined for sensitivity to four copper products Blue Shield®, Tri-Base Blue®, Copper Oxychloride WP® 50% and Copper Sulphate. Products were tested at 2, 1 or ½ the recommended label rate.
Four-day old cultures were used to create lawn plates on nutrient agar for each isolate. Four sterile filter paper discs were applied to the plates in a square formation and 10μl of the copper product was added to each disc representing the 3 rates as well as a negative control (10μl water). Plates were incubated at 27 °C and after 3 days the zone of inhibition around each disc was measured to give an indication of the bacterial isolate’s sensitivity to the product at each concentration.
There was a range of different values for sensitivity with some isolates reacting with a very large zone of inhibition, up to a 5 mm radius at the label rate and others only producing an inhibition of less than 1 mm. This could be natural variation in the population, or it could indicate a reduction in sensitivity in these isolates.
Most isolates tested retained some sensitivity to copper products when treated at the label rate or above. Many were also sensitive at half the label rate. However, some isolates were poorly inhibited at the label rate.
Overall, Copper Oxychloride WP® 50% and Copper Sulphate, which contained a greater concentration of copper at the rates used, were the most effective of the products evaluated, with larger zones of inhibition. All isolates except one were sensitive to Tri-Base Blue® at every rate. Thirty-eight isolates were sensitive to Copper Oxychloride WP® 50% at every rate while 43 were sensitive at the label rate or above. All isolates were sensitive to Blue Shield® and Copper sulphate at the label rate and above with the majority also sensitive at the lower rate. It would benefit the industry to carefully monitor and adhere to recommended application rates and implement good chemical management practises to avoid the development of resistance.