First report of Alternaria alternata causing late blight of pistachio (Pistacia vera) in Australia

Pistachio (Pistacia vera L.) is a relatively new crop in Australia and few diseases have been found. However, in other regions where pistachios are grown, as the area of bearing trees increases so does the number of diseases (Michailides, 1991).

In February 2000, leaves on pistachio trees were observed with a target spot, that is, a series of concentric circles within the lesion (Fig. 1).

This type of lesion is consistent with the symptoms of late blight of pistachios. Isolations were made from lesions onto potato dextrose agar. The fungus that was isolated was identified as Alternaria alternata (Fr.) Keissl. (NSW Agriculture accession number DAR 744629). Koch's postulates were satisfied by the inoculation of leaf disks from both male and female plants. Leaves were surface sterilised (2% chlorine solution for 2 minutes) and then placed on 0.5% water agar containing 1ug/mL of benzylaminopurine. They were then inoculated with 10 µL of a spore suspension (10⁴ spores/mL). A control leaf disk was inoculated with sterile distilled water. The leaves were incubated at 25°C for 24 hours then they were placed at room temperature (20-22°C). After 5 days, typical target spot symptoms developed on the inoculated disks. Alternaria alternata was reisolated from the disks. In all cases the symptoms were more extensive on the leaves from female plants.

Alternaria sp. on pistachio was first reported in Egypt in 1974 (Wasfy et al., 1974) and in California in 1985 (Michailides et al., 1995). It is now regarded as one of the four most important diseases of pistachio in California (Michailides et al., 1995). The disease is most severe after fruit maturity and may cause severe defoliation of trees. The fungus may also cause staining of the fruit and may invade the nuts giving rise to mouldy fruit (Michailides et al., 1995). This is the first report of the disease occurring in Australia. At present there are no fungicides registered for the control of late blight of pistachios in Australia.