Pathogens and other factors contributing to dark staining on pistachio shells (PS16002)

What was it all about?

From 2017 to 2020, this project investigated dark staining of pistachio shells and nuts – an issue that leaves produce unmarketable and that has been tentatively linked to fungal pathogens Fusarium and Alternaria in preliminary studies. The aim was to investigate the causal organism associated with dark staining, evaluate when pistachios are most susceptible to infection and to clarify if there are other associated mechanisms.

While this project did not identify the cause of dark staining of pistachio nuts, it did add valuable knowledge for the industry. It showed there was no correlation for dark stain from either external hull appearance or any of the inoculated fungal pathogens. The research also confirmed that dark staining becomes worse at later harvest times. Therefore, with current knowledge, it is unlikely that there can be any advance warning of dark stain symptoms from the external appearance of the clusters.

Further research is needed to clarify the relationships between environmental conditions in seasons with different levels of stain, temporal and spatial variation, and physiological variation of dark staining throughout the season.

In 2017, there was increased incidence of dark staining on shells, with up to 50% of nuts affected in some orchards. Laboratory tests showed a range of fungi associated with this dark staining, including Aspergillus, Fusarium, Cladosporium, Alternaria, Penicillium, Rhizopus and Phomopsis. However, as most of these are also effective saprophytes, it was unclear whether they were the cause of the staining or simply hitchhikers on damaged shells.

The success of the pistachio industry depends on the production of quality produce for export and domestic markets. As most pistachios are sold in-shell, a critical quality parameter is that shells are light in colour and free from damage and staining.

Investigations showed that levels of dark staining increased with time, and the longer nuts remained on the tree following optimum harvest, the higher the incidence of dark stain. No correlation was found between external hull appearance and the development of dark stain. However, it was more likely that severely damaged nuts would also have kernel damage.

Field trials with inoculation of the three main fungi identified (Aspergillus, Alternaria and Rhizopus) found no statistically significant correlation between any of them and dark or light staining. The team concluded the fungi are not the cause of dark staining, but were more likely to be saprophytic on the damaged tissue surface.