FOR weeks, NAREI in Focus has looked at the principal postharvest diseases that affect lemon crops. This week’s focus is being placed on postharvest disorders.

Chilling Injury : Lemon fruit are susceptible to low-temperature injury, commonly known as chilling injury (CI) if held at temperatures below 10″C (50’F). CI is a physiological disorder which adversely affects the appearance and quality of the fruit. Fruit symptoms include pitting and sunken lesions on the peel surface, skin discolouration, staining of the membranes separating the segments, decay, and off-flavour of the pulp.

Damage is a function of temperature and time, with more CI incurred at lower temperatures and longer exposure durations. The use of pre-harvest applications of the growth regulator gibberellic acid reduces susceptibility to CI, as does maintenance of a very high RH (>95%) storage environment. Preconditioning lemons by holding the fruit at ambient temperature for several days before storing at chilling temperatures will also reduce injury. Waxing the fruit and a postharvest fungicide application will lower the amount of damage and decay from CI.

OLEOCELLOSIS
Oleocellosis, or oil spotting, is a result of mechanical damage to the peel. The outer portion of the lemon fruit contains oil glands which are rich in oil and terpenes. When the oil glands are ruptured, the oil and terpenes leak out and are toxic to the surrounding cells. This results in death of the adjacent epidermis and the formation of irregularly shaped yellow, green, or brown spots in which the oil glands of the skin stand out prominently because of slight sinking of the tissues between them. Turgid fruits are most likely to have oleocellosis because their oil glands are more easily ruptured. Fruit turgidity is greatest in the early morning and under very humid, wet conditions. Harvesting under such conditions or while dew is on the fruit should be avoided.

Fruit should be picked when the fruit surface is dry and should be handled carefully so that oil glands are not punctured or ruptured. Oleocellosis can be minimised by picking fruit in the afternoon of sunny days, waiting to harvest 2 or 3 days after a rain or irrigation, using padded harvest containers, and having pickers wear cotton gloves. Lemons may also be left in the field overnight in their harvest container to allow the turgor pressure to decline. The lower fruit turgor pressure permits safer transport to the packing area.

PETECA
Lemons may develop large sunken areas of the rind or pits soon after packing. Peteca is a type of rind pitting in which the edges of the depressions are gently rounded. The outer layer of the rind sinks, at first without losing its normal colour, but eventually, the oil glands begin to darken. Heavy brushing of the peel increases the incidence of peteca. Also, storage of lemons at l00Yo RH will significantly increase the amount of peteca. Although the cause of peteca is not known with certainty, it is thought that heavy oil sprays prior to harvest can increase the severity of this disorder. Lemons should not be subject to excessive brushing and should be waxed with a camauba based wax.