Developing harvest strategies to maximise walnut quality (WN11000)

This is a final research report from Hort Innovation’s historical archives. Please note that as these reports may date back as far as the 1990s, the content and recommendations within them may be superseded by more recent research.

What was it all about?

The Australian walnut industry was undergoing rapid expansion with the potential to supply high quality nuts to local and export markets. Optimizing the quality of nuts was essential to maximising these opportunities. Major factors that determined nut quality were the absence of diseases and/or disorders of the kernel and shell. These factors could be adversely affected during walnut harvest.

The timing of fruit maturity was earlier in the semi-arid climate of NSW, than in the temperate climate of Tasmania, with the progression of kernel and hull maturity distinct between early (Serr), mid (Vina, Lara, Howard) and late (Chandler) maturing cultivars in NSW. In Tasmania, the timing of kernel maturity was earlier in Vina, Lara and Howard than in Chandler; however, hull maturity was similar between all cultivars. Walnuts matured in response to climate, and this may have influenced the temporal development of fruits between different growing regions in this study.

Delays in harvesting reduced the quality of nuts, with fewer extra-light and light kernels, and more amber kernels, yellow stained pellicles and kernel moulds, in nuts that were on soil beneath trees or in grass inter-rows for 4 or more days. Rainfall and/or soil moisture may have contributed to the reduction in nut quality, as the timing of these events corresponded to losses in nut quality. Nuts exposure to a constant 20°C, between harvesting and drying, also reduced kernel quality of Chandler and Lara nuts after 4 and 6 days, respectively. Additional research was needed to provide further insight into the nature of these events.

Non-ventilated storage of walnuts for 24 or 48 h after harvesting reduced nut quality in Vina and Lara, compared to drying immediately after harvesting; however, nut quality of the later-maturing cultivars, Howard and Chandler, were less affected with storage. Temperatures in storage bins were higher, and elevated for longer in earlier- than in later-maturing cultivars i.e., maximum temperatures and time at 20°C or more with 48 h storage were 34°C and 34 h in Vina, and 19°C and 0 h in Chandler. In contrast to non-ventilated storage, temperatures with ventilated storage reduced or remained relatively constant. As prolonged exposure to elevated temperature could reduce nut quality, further investigations into ventilated storage were warranted.

Nut quality was maintained when nuts were harvested as soon as possible after kernel maturity. Foliar sprays of ethephon reduced the time delay between hull and kernel maturity, and increased the rate of nut-drop in NSW and Tasmania. The concentration of ethephon in kernels was higher, and peaked later in Tasmania than NSW. The use of ethephon was not considered detrimental to tree health, and did not reduce the size and weight of nuts after three consecutive years of use. Nut quality between different ethephon harvest strategies was similar; however, further monitoring was needed to identify cultivar variations and improve this initial description.