The Lenticel Damage Picture becomes Clearer

The state of play in avocado production

In avocadoes, lenticel damage used to be seen as mainly a Fuerte problem, given that it shows up so clearly on the variety's skin that remains green when ripe. Hass was deemed less of an issue since lenticel damage is hardly visible once the fruit has fully coloured and ripened. However, as more Hass fruit with lenticel damage reached markets, buyers made their displeasure known.

In response, the SA Avocado Growers' Association (SAAGA) published a detailed document of picking and packhouse protocols in 2005 to assist growers and packhouse managers to reduce or prevent lenticel damage.

The search to unearth the cause of lenticel damage has led scientists to the mineral content of fruit skin and pulp. While not much is known for avocadoes in this regard, information is available for other subtropical crops. For example, several studies have highlighted nitrogen content as a supplementary tool to predict and manage rind disorders in mangoes, grapefruit and oranges.

Due to its role in cell-wall structure and membrane function, calcium has also been implicated in the physiological disorders of many fruits.

The possible influence of calcium levels in fruit skins on lenticel damage in avocadoes has not been investigated, but it is well known that nitrogen can negatively affect the absorption of other immobile nutrients, such as calcium and micronutrients. Low nitrogen leads to low iron absorption, which in turn, influences black-cold-damage in Fuerte.

Given these clues, it was agreed that research into the relationship between the mineral composition of avocado skin and pulp and lenticel damage was needed. The result was a comprehensive three-year study, jointly funded by the Postharvest Innovation Programme and SAAGA, and carried out by ExperiCo under the project leadership of Danie Lemmer.

Year 1:

2019

The project's first order of business was to determine the extent to which avocado producers follow the industry protocol for the reduction of lenticel damage. This entailed farm visits before and during harvest to document the practices that were being followed.

Importantly, the study found that most producers did not adhere to the protocol that specifies soil moisture at harvest, despite existing proof that fruit picked when soils are drier are less susceptible to lenticel damage.

In one instance, a producer who was forced to stop irrigating – one month prior to harvest due to water shortages, obtained 70% sound fruit, 25% Grade 1 and 5% Grade 2 damage. No fruit showed Grade 3 or extensive damage, according to the PPECB lenticel grading protocol.

In addition, the farm visits confirmed that lenticel damage was directly associated with handling during picking, transport and packing, and increases along the handling chain. Grade 3 damage only occurred on fruit sampled from the pack-line.

Handling fruit with care during harvesting, transport and packing – in accordance with the industry protocol – is therefore essential to reduce the development of damage during storage.

The project's first year also included a nutrition trial to establish the relationship between the incidence of lenticel damage and the nutrient composition of avocado skin and pulp. This entailed three foliar applications of various calcium nitrate concentrations to a low-nitrogen orchard, in an attempt to increase the nitrogen and calcium content in the skin of Hass.

Late applications of calcium nitrate increased nitrogen but not calcium levels, and should rather be avoided, since lenticel damage and anthracnose incidence increased in fruit that received a pre-harvest application, especially when they were subjected to jostling (manhandled by tumbling) prior to packing.

Conclusions

1. A need seems to exist to redefine the present protocol for suitable soil moisture to reduce lenticel damage; however, soil type should be considered along with irrigation cycles.
2. Further research is needed, especially to determine if, and by how much, irrigation should be reduced before harvesting, and whether the industry impulsively over-irrigates.
3. None of the factors associated with lenticel damage can or should be seen as the sole cause. The data indicates several contributing factors that all need to be addressed.

Year 2:

2020

The second year’s irrigation study compared an orchard that received the full irrigation treatment to one in which irrigation was reduced five weeks before harvest. Both were high-nitrogen Hass orchards. The results showed that less irrigation did not reduce the incidence of lenticel damage.

The nutrient trial was designed to ascertain if foliar applications of calcium, boron and silicon – three months before harvest – show up in the fruit skin, and help to reduce lenticel damage in a high-nitrogen orchard.

A 2019 orchard with optimum leaf levels of nitrogen, calcium, magnesium and boron and that accumulated boron in the fruit skin with a value that was ± 100% higher than the leaf boron value, showed the least lenticel damage. This partly explained this orchard's remarkable resistance to developing lenticel damage.

In contrast, a 2020 orchard with leaf levels of nitrogen, calcium, magnesium and boron very similar to the 2019 orchard that showed the least lenticel damage, accumulated only ± 6% more boron in the fruit skin than the leaf value.

The difference indicated optimum photosynthetic output in the first orchard, while in the second it had been inhibited during fruit development.

The three foliar spray combinations of calcium and boron applied with two silicon and boron foliar sprays three months before harvest, reduced the total incidence of lenticel damage on Hass avocadoes by ± 13%, despite the fruit having come from a high-nitrogen orchard.

Conclusions

1. Further research is needed to verify the important boron-level finding, given that photosynthetic output possibly caused reduced fruit skin integrity, which would explain the higher sensitivity of the fruits to developing lenticel damage. If confirmed, this will highlight the importance of optimum irrigation during the first 6 – 7 weeks of fruit development to ensure that calcium and boron accumulate optimally in the fruit skin.
2. The foliar application combination findings warrant further research to work out the best combination, timing and number of applications that will allow silicon treatments to improve calcium and boron absorption into the pulp and skin of Hass avocadoes.
   • The beneficial effect of silicon soil applications preceding silicon foliar sprays also needs to be studied.

Year 3:

2021

In its third year, the study further refined its scope with more detailed irrigation and plant nutrition investigations.

The importance of optimal irrigation in the early stages of fruit development to ensure fruit skin integrity was established in the 2020 studies. The follow-up trials investigated the effect of optimum and sub-optimum irrigation on lenticel damage in Hass avocadoes produced in high- and optimum-nitrogen orchards respectively.

Results showed that orchards with optimum leaf nitrogen levels exhibited less lenticel damage, compared to high-nitrogen orchards. As more than 80% of South African Hass orchards are high-nitrogen environments (nitrogen is applied to increase fruit size and yield), this alone complicates the objective to reduce lenticel damage in this variety. Producers would therefore be well advised to find alternative ways to increase fruit size and yield.

Trials run in orchards used in 2019 and 2020 respectively, confirmed the previous year's hypothesis that skin integrity influenced susceptibility to lenticel damage, and that the latter was the result of optimal photosynthetic output, which depended on root uptake of calcium and boron during the early stages of fruit development. And this, in turn, depended on optimal irrigation.

In addition, sustained optimum irrigation in the months until harvest is needed to obtain maximum boron levels in the fruit pulp and skin to contribute to ideal fruit skin integrity at harvest.

An orchard newly selected for the project's third year and that had optimum leaf levels of nitrogen, calcium and magnesium, but lower boron, showed an almost 143% increase in boron in the fruit skin in the presence of optimum irrigation. Lenticel damage in this orchard came to 39.5%, largely due to the impact of wind damage.

The nutrition spray trials focused on ways to improve the absorption of calcium and boron into avocado skin and pulp, given that this emerged as a problem area in the 2020 research.

The new tests included a wetter with additional penetrating function (WetCit), as well as a wetter (PANAF5) that contained a lipid-amino acid, already proven to be a good carrier of fungicides into plant cells.

The results indicated that both WetCit and PANAF5 were able to significantly increase boron and silicon absorption into the fruit skin. However, neither increased the skin calcium content.

Conclusions

1. The irrigation trials demonstrated the importance of optimum irrigation to maintain phloem transport of boron to the fruit skin, for optimum fruit skin integrity when it comes to managing lenticel damage.
2. Penetrators like BREAK-THRU® SP 133 (Alchem Group) and Tronic (AECI), which can effectively move active ingredients through the plant's wax layer, needs to be tested on avocado in future research.
3. None of the spray trials done in 2019, 2020 or 2021 succeeded in increasing calcium in the fruit skin during the late season. Improving calcium absorption into the fruit skin during the first six weeks of fruit development, therefore remains a priority for future trials.

Despite the information gathered in this extensive study, the exact cause or causes of lenticel damage remains a mystery. For the time being and while studies continue, the solution to lenticel damage lies in pre- and postharvest practices that limit and control the problem.