Disorders of the skin
A. Disorders of the skin
Occurring naturally during storage:
- Patches of skin turn bronze or brown and become slightly sunken. In the early stages of development un-degraded green skin is interspersed with affected skin.
- The incidence and severity of the disorder increases with duration of storage and may develop rapidly when fruit is brought to ambient temperatures.
- Of the apple cultivars currently grown in the UK Bramley’s Seedling is the most scald susceptible.
- The ethylene antagonist SmartFreshSM provides effective control of superficial scald.
- Alternative methods of control include scrubbed low oxygen or low ethylene storage.
- In AHDB Horticulture Project TF 191, the use of SmartFreshSM or ethylene scrubbing were both found to reduce the incidence of scald in long-term CA stored Bramley’s Seedling.
- Where scald is detected during store monitoring the affected consignments should be marketed immediately.
- Scald susceptibility is influenced markedly by climatic conditions during fruit development and is aggravated by early harvesting.
- As the name implies there are certain cultivars of apple that develop a browning of the skin when the fruit is over-stored.
- Generally this type of scald should not be a problem where storage recommendations are adhered to.
- Gala is susceptible to senescent scald late in the storage period.
- The problem is aggravated by late harvesting, poor control of storage conditions, over-storage and delayed marketing.
- DPA is unlikely to provide control of the problem.
- Susceptible cultivars (Fiesta (Red Pippin), Gala and Jonagold) required for long storage should be stored in CA conditions with a relatively high carbon dioxide content.
Lenticel blotch pit(Cox)
Lenticel blotch pit
- Lenticel blotch pit is a disorder that is related closely to bitter pit.
- Brown lesions form in each ‘pit’ beginning at a lenticel.
- Bramley, Cox and Egremont Russet apples are particularly prone to the disorder that occurs in fruit with abnormally low levels of calcium.
- Consignments of Cox apples containing more than 3.8 mg 100g-1 fresh weight of calcium are unlikely to develop lenticel blotch pit.
- Fruit analysis should be used to determine risk.
- Control measures are the same as those described for bitter pit.
Skin necrosis on Gala
Skin necrosis in Gala apples
- A skin disorder has been observed on Gala apples stored under commercial conditions in the UK.
- The problem occurs rarely and has only been observed on fruit that has been harvested very late and kept in air storage.
- The problem has not been apparent on CA-stored fruit or in air-stored fruit picked at the correct stage of maturity and held at the recommended storage temperature for the prescribed period.
- Mineral composition of affected and non-affected apples was similar.
- Provided that growers adhere to guidelines provided on harvest dates and storage recommendations this disorder should not be a problem in commercial consignments of Gala.
External carbon dioxide injury (Bramley)
Induced by CA conditions:
External carbon dioxide injury
- This form of damage never occurs in air-stored apples and is not common in apples stored under CA conditions in the UK.
- Generally carbon dioxide injury occurs where fruit is kept in atmospheres containing carbon dioxide at higher than recommended concentrations.
- In the early stages of development damage symptoms may be confused with those of superficial scald.
- However, lesions caused by excessive carbon dioxide are more sharply defined than those described for scald.
- Moreover, external carbon dioxide injury occurs within the first few weeks of storage and does not progress thereafter.
- In contrast, superficial scald usually develops after several months and becomes progressively worse with time in store.
- Bramley’s Seedling apples are susceptible to external carbon dioxide injury under recommended CA conditions.
- Fortunately DPA applied for scald control also prevents carbon dioxide injury.
- Where DPA treatment is omitted it is important that the establishment of CA conditions is delayed in order to prevent injury to the fruit.
- Where SmartFreshTM is applied to Bramley not treated with DPA the delay before establishing CA conditions must be extended further.
- Empire, a cultivar not widely grown in the UK, is highly susceptible to external carbon dioxide injury particularly when treated with SmartFreshTM.
Low oxygen injury (McIntosh)
Low oxygen injury
- Injuries to the skin of apples due to low-oxygen storage are generally not recognised on apples grown in the UK.
- In other growing regions low-oxygen injury is expressed in the form of darkening of the red and green regions of the skin.
- Ribbon scald may also be induced in some varieties by low-oxygen atmospheres. McIntosh is considered the most sensitive to this type of injury.
- The risk of low-oxygen injury is minimised by picking at the correct stage of maturity, establishing storage conditions promptly and maintaining CA conditions within prescribed limits.
Diphenylamine (DPA) damage
Induced by chemicals:
Post-harvest DPA application
- In the UK diphenylamine (DPA) used to be applied as a dip or drench treatment to control superficial scald on Bramley’s Seedling apples.
- In other apple growing regions of the world DPA is used widely on a number of dessert cultivars for scald prevention.
- Damaging concentrations of DPA used to occur where liquid was allowed to accumulate or where there were slow drying conditions in bulk containers.
- Symptoms appeared as small black spots or as a grey/black stain of the skin.
- In the most severe cases of injury extensive areas of the skin became blackened and sunken. The latter symptoms were likely to occur on fruit at the base of treated bins that remained in contact with the solution.
- DPA could also cause injury to the core area in Bramley apples that had an open calyx tube.
- In wet harvests internal damage caused by DPA could provide a site for infection by fungal pathogens such as Mucor spp.
- In such years it was particularly important to maintain hygienic conditions by changing the solution in accordance with manufacturer’s instructions. This practice also ensured the most efficacious treatment for scald control.
Post-harvest calcium chloride damage(Cox)
Post-harvest calcium chloride damage (Bramley)
Post-harvest calcium application
- The concentration of calcium in apple fruits can be increased by post-harvest application of calcium chloride and proprietary formulations containing calcium.
- Such treatment reduces susceptibility of apples to calcium-dependent disorders such as bitter pit and senescent breakdown.
- Higher application rates result in greater uptake of calcium but there is a corresponding increase in the risk of injury to the fruit.
- The injury is usually centred on lenticels although in severe cases entire areas of skin may be affected.
- The areas around the lenticels become brown or black and may become sunken.
- There may be localised greening of the skin related to the localised uptake of calcium.
- There is marked varietal variation in susceptibility to calcium injury.
- It is important therefore to treat only those cultivars advised by the manufacturers and to apply materials in strict accordance with label instructions.
- Pre-harvest sprays containing calcium may also cause lenticel injury to the fruit.
- Damage symptoms from calcium sprays are generally similar to those described for post-harvest treatments although early sprays can result in distortion of the fruit of sensitive cultivars