Critical Control Point #3 – Ground Harvesting Results indicate windfall fruit has at least three times more Aspergillus than fruit on the tree (Figure 1), indicating greater exposure risk to fungi when fruit are on the ground. With the current harvesting techniques relying on ground harvesting and no known practice to avoid natural windfalls, this risk is challenging to minimise. Nevertheless, management options such as: harvesting windfalls separate to the crop remaining on the tree, segregating and separately delivering the two piles, and shortening the duration between shaking and pick-up by increasing investment in more equipment, are all available. Critical Control Point #4 - Stockpiles The current study indicates certain sections of the stockpiles have the potential to provide ideal conditions for both Aspergillus growth and aflatoxin production. Aspergillus growth and aflatoxin production are influenced by temperature and relative humidity (RH). RH affects water activity (a w ), which is a measure of the availability of water in a food product, and is the dominant factor governing food stability or spoilage, or in this case Aspergillus growth and aflatoxin production. In lieu of more controlled climate storage facilities (currently being investigated by a separate R&D project (Figure 2)), the presence of covers and type of covering has a significant influence on product spoilage.
Despite the high levels of Aspergillus inoculum on mummified whole fruit, the data indicates conditions were not conducive for the fungus to penetrate new season’s fruit and infect the kernel. Only 0.06% of kernels extracted from new season’s fruit were infected with Aspergillus , even though the inoculum may be present on the hull and or shell. Interestingly, it would appear that removing the hulls on-farm prior to stockpiling, which is another R&D project that’s recently begun, would also aid the control of Aspergillus growth and aflatoxin production. Nevertheless, it appears abundantly clear there is a vicious cycle occurring and if you prevent hull rot infection, this will be a critical step in reducing mummies and the inoculum source of Aspergillus spp. Management of hull rot is easier said than done at this moment in time, as it’s a complex of factors involving variety susceptibility, hull-split rainfall events, irrigation management and nitrogen fertiliser management. I won’t go into too much detail about the best management practices other than to mention the following key points from research out of California: • Nonpareil (50% of the Australian industry) is the most susceptible variety to infection. • There is a linear relationship between seasonal nitrogen applications and hull rot incidence. • Seasonal nitrogen applications beyond 275kg/ha produced severe infections. • Incidence was higher in low crops years and consequently nitrogen applications need to be adjusted for crop load. Critical values for January leaf nitrogen percentage should be approximately 2.45%. • Nitrogen applications after kernel development should cease. • A slight to moderate water stress event at the onset of hull split can reduce hull rot incidence. This would normally occur for approximately 2 weeks and the water stress should not exceed -14 to -16 bars as measured by the average midday stem water potential (using a pressure chamber). • DMI (e.g. propiconazole) and strobilurin (e.g. azoxystrobin, pyraclostrobin) fungicides sprayed at the onset of hull split are reported to have some efficacy, but due to the variation in hull split timing within a tree and across orchards, the benefit can be variable. It’s also important to note that these sprays are only to be used sparingly through the season due to their high resistance risk. Refer to chemical permits or labels for further information. Critical Control Point #2 - Carob Moth The role of carob moth in Aspergillus growth and aflatoxin production is not definitive. Results show Aspergillus contaminated fruit in both the presence and absence of carob moth damage, but larvae of carob moth have also been shown to carry Aspergillus spp. What’s made it more difficult is the low presence and seasonal fluctuations of carob moth infestation at the trial sites. In the stockpiles, the identification of the insect damage has also been difficult as there are several other insect species at play, and it is difficult to differentiate the type of damage on mouldy kernels. Whilst it appears carob moth may not have the solid link with Aspergillus growth and aflatoxin production as does Navel Orange Worm (NOW) in the Californian almond industry, the results suggest “where there’s smoke there’s fire”. If you have carob moth (or other insect damage), you expose the kernel to infection and in some instances it could be acting as a vector. Managing carob moth with crop protection chemicals, or at the very least minimising mummies (inoculum sites for Aspergillus and infestation sites for carob moth) you will go some way to managing Aspergillus growth and aflatoxin production.
Four treatment types have so far been investigated: (i) outside uncovered; (ii) shed; (iii) outside clear cover; and (iv) outside white over black cover (B&W) (Figure 3). To date, data suggests the clear covering easily allows more Aspergillus growth and aflatoxin production in comparison to B&W cover and shed storage. In the absence of rainfall, clear covers are also worse than outside uncovered stockpiles. Fortunately, clear tarps are rarely used by industry with sheds and B&W covers the most common methods. The clear covers are worse as the top layers of these stockpiles exhibit a greater fluctuation in temperature and RH that leads to condensation and moisture accumulation under the covers (Figure 4. These environmental conditions ultimately give rise to increased Aspergillus (and bacteria) growth and aflatoxin production. Figure 2: Air permeability rig developed to model air flow in almond storage (Project AL12003: University of South Australia)