What is known about skin browning in R2E2 fruit

A disorder known as Leathery Skin occurs mainly on R2E2 fruit and has been observed for a number of years on trees grown on light sandy granitic soils in Mutchilba and Dimbulah, west of Mareeba. During the 2016/17 season, a number of additional growers in the region have had the same issue, including growers from several Central Queensland properties.

Symptoms

Skin Browning in R2E2 fruit

Skin Browning in R2E2 fruit

The disorder causes fruit to have a leathery skin appearance with symptoms occurring mainly on the sides or the bottom of the fruit. The flesh is generally not affected as the disorder is only skin deep. Symptoms appear much like a hard cell collapse under the skin. Reports from growers suggest that the disorder occurs not on the sun-facing blush side of fruit but more on the inward facing side.

The Leathery Skin disorder is difficult to visually spot in the field but is more evident after storage and when fruit have been packed. It typically shows up within two days storage and is reported to be more evident when stored at low temperatures.
The disorder Leathery Skin does not appear be fungal-related. According to feedback from growers, postharvest treatments of hot and cold Scholar® had no effect on controlling the disorder.

Locations

Leathery Skin occurs on R2E2 fruit grown in the very light sandy granitic soil types located west of Mareeba, in Mutchilba and Dimbulah in Far North Queensland. It has not been observed on trees grown in heavier soil types. The same issue has also been reported this year on fruit grown in lighter soil areas in Central Queensland.

The incidence of Leathery Skin disorder varies from year-to-year, with the 2016/17 season being a particularly bad year. Some growers who have experienced this disorder for many years have now removed most, if not all, of their R2E2 trees.

Nutrition

Some initial thoughts have been that the disorder is caused by a nutritional imbalance or a deficiency in the fruit, such as low calcium or boron levels. In the past, the Queensland Department of Agriculture and Fisheries (QDAF, formerly QDPI) had apparently found lower magnesium levels in fruit from badly affected blocks. However, there is no scientific literature indicating that magnesium causes this disorder on fruit. Observations from one grower affected showed that monthly applications of magnesium also had no impact in reducing the disorder.

In a small study conducted in February 2015, peel nutrient levels of affected and unaffected fruit were analysed. The findings showed that no major nutrient differences were observed between the two fruit types, although unaffected fruit did have slightly higher levels of calcium, magnesium, manganese and iron. The ratio of calcium to magnesium however was the same in both the unaffected and affected fruit types.

Leaf and soil samples collected from R2E2 trees on farms with or without the disorder in January 2016/17 showed no obvious differences in nutrient levels. All the farms had high leaf boron and manganese levels (typical of the area for all cultivar types). Trees from both farm types also had good leaf calcium and acceptable leaf magnesium levels.

Soil nutrients on all farms were typical of the area and soil type; in this case being relatively low in most nutrients. Interestingly, calcium soil levels were slightly lower on farms where symptoms of the disorder were present. There were however no apparent differences in soil magnesium, which overall was on the low side. Farm blocks with low magnesium had no issues when fruit were picked early.

The disorder has been reduced when best farm management practices have been followed. This includes the use of an appropriate fertiliser program following a leaf and soil analysis, sufficient tree watering and regular monitoring of tree health. These practices may not eliminate the disorder in a bad year but it will certainly help to reduce the incidence.

Fruit maturity

A number of reports indicate that the severity of the disorder is higher on more mature fruit, for example fruit left longer on the trees to capture the more lucrative Chinese New Year market. Growers with farms in the same area or with similar soil types reported no issues with the disorder when fruit were harvested early, only when fruit were harvested late.

Temperature management

The practice of harvesting fruit under cooler conditions, such as during early morning, can reduce the incidence or severity of the disorder. Additionally, gradual removal of field heat before placement in a cold room environment (<16°C) has also shown promising results in terms of reducing the disorder.

Recommendations

There is unfortunately no treatment that is a ‘silver bullet’ for eliminating this disorder; however damage can be reduced by following a number of best practices, including:

  • Implementing nutrition and irrigation best practices and ensuring:
    • Leaf and soil tests are undertaken at harvest and then another leaf test pre-flowering
    • Ideal leaf and soil nutrient levels used are those communicated at AMIA field days and in AMIA articles
    • Appropriate nutrients are applied, in particular calcium. Most soils need both spread gypsum (end of wet season) and a liquid or powdered form applied during flowering and early fruit development. The first six to ten weeks is the most critical period for calcium application
    • Importantly, boron is needed as well for calcium to work effectively
    • Nutrients are applied at the right time in the crop cycle. Different nutrients are required by the tree at different times of the year
    •  Adequate soil moisture is maintained and monitored during fruit development.
  • Harvesting fruit early (after reaching the recommended dry matter minimum standard)
  • Harvesting during the cooler part of the day (for example early morning)
  • Cooling harvested fruit down gradually to remove field heat, and not transferring immediately into a cold temperature room
  • Holding fruit for 24 hours before packing
  • Holding packed fruit for two days before dispatch to allow any disorder to become apparent. This provides an important opportunity where fruit should be sorted again to remove affected fruit – do not send them to market without re-checking.

We must accept that under some conditions the Leathery Skin disorder will occur, and until further information on the causes are known, we will have to manage it as much as possible by following the recommendations suggested above.

Article submitted by Ted Winston, Tropical Horticultural Consulting Pty Ltd.