RESEARCH & EVENTS
Bacterial Spot in Almonds: Is Copper Still Effective?
Simone. Kreidl, Shona Armstong and Tonya Wiechel Agriculture Victoria Research, AgriBio Centre for AgriBiosciences, Bundoora
BACTERIAL spot, Xanthomonas arboricola pv. pruni affects all Prunus species( Figure 1). It is present worldwide in almond and stone fruit growing areas and can lead to leaf and fruit drop and reductions in yield, tree vigour and nut quality. Bacterial spot was first recognised on Australian almonds in 1994 / 95. Surveys conducted in 2018 / 19 and 2019 / 20 found between 2-8 % of trees affected by bacterial spot during drier seasons( 1). Recently, wetter growing conditions have seen increased incidence of the disease. Management is usually achieved using copper products. A lack of information on potential resistance of the Australian bacterial spot population to copper, led to laboratory tests being conducted by Agriculture Victoria researchers as part of the integrated disease management project AL22002. Bacterial spot isolates were collected from almond orchards in NSW, SA and Victoria from 2018 to 2023( Table 1). Forty-six isolates( 22 from NSW, 3 from SA and 21 from Victoria) were tested 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. Laboratory testing was done on nutrient agar culture plates, spread with individual bacterial isolates. Filter paper discs treated with three product rates plus a negative control were added to the plates. The inhibition of bacterial growth was observed after three days incubation at 27 ° C by measuring the zone of inhibition occurring around the paper discs( Figure 2). Most isolates were sensitive to copper products when treated at the label rate or above. There was a range of sensitivity with some isolates having a very large zone of inhibition, up to a 5mm radius at the label rate and others only producing a small zone of inhibition of less than 1mm. This may indicate a reduction in sensitivity in these isolates.
Continued page 42
Table 1. Number and origin of bacterial spot isolates tested.
Figure 1. Bacterial spot fruit and leaf symptoms.
Figure 2. In vitro test plates showing bacterial growth of two different isolates. A- isolate 23-0037 Copper Oxychloride sensitive at all rates, B- isolate 19-0046 Blue Shield sensitive at the label rate and above but with reduced inhibition.
industry. australianalmonds. com. au
41