The 2018/2019 mango season saw some extreme weather events, especially in The Tablelands where the region experienced high temperatures followed by extreme rainfall. AMIA thought it a prudent time to give growers a refresher on reducing lenticel damage; both on-farm and in the packing shed.
The following article was prepared by Roberto Marques and Peter Hofman, Maroochy Research Facility (MRF), Department of Agriculture and Fisheries (Queensland). These recommendations are based on overseas reports and research by the Department of Agriculture and Fisheries (Queensland).
Lenticel damage (LD), also called discolouration or spotting, is a skin defect that can severely reduce fruit visual appearance and marketability of many mango cultivars. It is common in most mango producing growing countries.
LD is caused when the tissue in the lenticels (breathing pores) on the skin swells and becomes pronounced, resulting in small round or star-shaped spots scattered over the skin. The cells around the lenticels may get discoloured, producing brown or black spots, or red or green halos. LD often becomes worse with longer storage, and as the fruit ripens and ages.
What causes LD?
The exact mechanisms causing LD are still unclear. It may be through entry of water and/or contaminants into the lenticels, causing dark-coloured (phenolics) compounds to form in the cells around the lenticel as a stress-related defence mechanism. LD may also be induced by sap components (e.g. terpenoids) ‘escaping’ from the resin canals near the lenticels, causing cell membrane rupture that allows the phenolics to mix with the enzymes that cause tissue browning.
Key factors increasing the risk of LD
Cultivar: Some cultivars are more sensitive than others, likely because they have larger lenticel cavities, less wax/cuticle protection of the cells in the lenticel, or more lenticels on the skin. For example, Calypso fruit have more lenticels per cm2 of skin than do KP, R2E2 or Honey Gold.
Wet growing conditions: Cool, humid and wet (due to rain or excess irrigation) conditions, especially around harvest, generally increases the risk of LD.
Larger fruit size: Larger fruit within a cultivar often have more LD. This can happen when there are lots of leaves for each fruit, e.g. after low fruit set, top-working of trees or excess nitrogen.
More mature fruit: Generally more LD develops as the fruit matures (late-harvested fruit).
Harvesting/packing: LD tends to increase gradually as the fruit progress through the picking to packing process. Practices that expose the fruit to water (e.g. de-sapping solution, washing) and excess rubbing (e.g. brushing) can increase LD. Irradiation can also significantly increase LD.
Management practices to help reduce LD
LD can be challenging to manage due to the various factors involved. Practice change at a number of steps might be required. For example:
During fruit growth
Pay special attention to fruit from recently top worked trees, when weather events reduce fruit set, resulting in a high leaf to fruit ratio, and avoid using excess nitrogen.
Adjust harvest scheduling to avoid harvesting fruit that is too mature.
Regularly inspect the harvest aid trough, so both floating and sinking fruit stay in the detergent solution less than two minutes, or one minute if the fruit are very mature.
Do not harvest during rain if at all possible. There are no research-based guidelines for dealing with rain events during harvest, however general advice includes: wait for one (fine weather) or two days (overcast) before harvesting if at least 20 mm falls over about 12 hours, or two to three days if at least 50 mm falls over 24 hours or more. Of course, this can be difficult due to the typically short harvest period for mangoes.
If the harvested fruit become wet, move bins to a covered area as soon as possible, and remove the bin cover to dry the fruit.
Aim to pack on the same day of picking (ideally within 6-12 hours) to shorten the time sap stays on the fruit.
Ensure fruit are removed from wet dumps within five minutes.
Do not use additional water over brushes if a wet dump is used.
Limit brushing to less than two minutes, and just enough to remove dirt, residues etc.
Clean brushes at least daily and replace them when they get worn, stiff or difficult to clean.
Maintain the cold chain at all times to avoid condensation on the fruit. For example, pre-cool the truck to transport temperature before loading and load it as quick as possible.
Avoid excessive (forced) air movement over the fruit during ripening.
Ripen and hold fruit at recommended temperatures.
Minimise the duration between packing and retail.
To hear more from Peter Hofman, be sure to register for the 12th Biennial Australian Mangoes Conference. Click here to register now!