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Pumpkin, Cucurbita maxima / Cucurbitaceae
Postharvest Atmosphere Management
Curing is a technique whose objective is to provide a suitable environment to heal the wounds caused to pumpkins during the harvesting, before the product is stored. They are left to sundry in the plot. Moreover, this process enables pumpkins to reach complete maturation. In regard to the effectiveness of curing, the varieties have different behaviours. The conditions recommended for curing are temperatures around 20 or 25ºC, for 2-4 weeks, up to 25-30ºC for approximately 20 days.

This variability indicates the need to adapt the temperature handling according to the variety.

The pre-cooling technique is not used in this species, since it has a low metabolic activity during harvesting, enabling the natural reduction of the temperature. This species is quite sensitive to chilling injuries, so if it is subject to pre-cooling, the temperature will not be lower than 10-13ºC. Air pre-cooling is the most appropriate system.

The fungicide treatments are very efficacious to control post-harvest rottings. Dicloran is useful to treat Sclerotinia sclerotiorum, Botrytis sp. and Rhyzopus. The latter is also controlled with sorbic acid and its potassium salt. Waxen fruits are also subject to a considerable reduction of alterations.

The conservation depends on the genetic characteristics of the pumpkins. This denomination refers to two species, C.maxima and C.moschata, with differences between both of them, as well as among the varieties. Thus, the optimal storage conditions will depend on each one of them.

They are usually stored in environments without temperature control, well-ventilated, with low relative moisture (70%) to avoid the development of rottings. For this purpose they are placed in places with good ventilation, on platforms or gratings to enable good air circulation.

The conditions to maintain the quality for a long period of time are temperatures between 6 and 12ºC and low relative moisture, between 50 and 70%. The fruits must be harvested in a state of complete maturation.
Postharvest Problems
Unlike other Cucurbitaceous, as courgettes and cucumbers, pumpkins are harvested when they are completely ripe. For that reason, their metabolic activity is quite low and therefore they have a longer post-harvest life.

The factors that affect the loss of quality in pumpkins are: loss of weight, yellowness, fibrousness, chilling injuries, diseases, compositional changes and behaviour in relation to ethylene.

Loss of weight
This process is not so appreciable as in other products, since the thickness of the rind prevents the wrinkling. However, in pumpkins with neck the reduction must not exceed 15%, in order to avoid the occurrence of cavities in this part of the fruit.

Storage at low temperatures (7-10ºC) reduces the speed of this phenomenon.

Yellowness
Pumpkins of green rind tend to discolorate during storage, acquiring undesired yellow tones. This process is accelerated at temperatures between 13 and 21ºC. In order to slow down it is recommended to store them at 10ºC. If they are kept at low temperatures (0-4ºC) there is no yellowness, but on the contrary they suffer from chilling injuries.

Fibrousness
In some pumpkins fibrousness increases with time, reducing their organoleptic qualities. The optimal temperature for storage must be 10ºC.

Chilling injuries
Storage in cold rooms is not a usual practice for pumpkins due to economic reasons and sensitivity to low temperatures. The fruits affected by chilling injuries are easily attacked by the fungus Alternaria when they leave the store room.

Diseases
The main diseases that affect pumpkins are: Alternaria, rot caused by Fusarium Antractonis, wet rot, rot caused by Phoma, Xanthomonas campestris pv. cucurbitae and other pathogens.

- Alternaria: Caused by the fungus Alternaria alternata, affecting weak tissues. It usually affects already harvested pumpkins and sometimes in the field. The pathogen attacks through the wounds and the infection may spread all over the fruit. The affected tissues become dry and spongy. The mycellium fructifies, giving rise to a dark efflorescence that is shown in the affected areas. The fruit that has been harvested before maturing or too late are more sensitive to this disease.

- Rot caused by Fusarium: Several species of this genus may cause this rotting during storage; affecting in some cases the whole fruit; the tissues disintegrate. On the surface there is a white or pink- white mycellium. In order to reduce its incidence it is recommended to carry out a correct handling in order to avoid damaging the rind of the fruit, as well as to remove the fruit with alterations in the area that was in contact with the floor. It is recommended to apply preventive pulverizations with benomyl before harvesting, to reduce the inoculum reaching the warehouse. Treatments by immersion or pulverization during post-harvesting, previous to storage, with tiabendazol or potassium sorbate along with sorbic acid, help to control the disease.

- Antracnosis: The fungi that cause this disease are Colletrotichum gloesosporioides, C.lagenarium and C.dematium. All of them infect the pumpkins during cultivation attacking the injured leaves, although the attacks are often observed during storage. High temperatures favour the development of this pathogen. The symptoms are round spots, of approximately 1 cm of diameter, sometimes resulting in a wide affected area. The fruitful bodies of the fungus have small spots with a concentric shape. The internal tissues show a wet rot. The application of a solution of sodium hypochlorite (1500 ppm) on the damaged fruit prevents the transmission of the disease to the healthy fruit.

- Wet rot: This disease may represent a serious problem in post-harvest if the storage atmosphere is humid. Caused by the fungus Rhyzopus stolonifer causing a watery, depressed spot of a brown colour, softening the tissues and sometimes spreading to the rest of the fruit. The fructifications are black. The pathogen has a good preservation both in the ground as in vegetal remains and penetrates through the wounds. For that reason it is important to keep the warehouses clean and to handle the fruit with care in order to diminish the attack. The application of the dicloran fungicide and the immersion of the fruit in a solution of 2% of potassium sorbate and sorbic acid reduces the inoculum. The optimal temperature for its development is high (25-30ºC), whereas it practically stops at 5ºC.

- Rot caused by Phoma: This disease begins in the field but it is evident during storage. The attacks occur in the wounds; at the beginning they are small spots of dark colour; they grow while the infection spreads all over the pulp. This fungus is favoured by high relative moisture and moderate temperatures.

- Xanthomonas campestris pv. cucurbitae: The small cankers caused by this bacterium in post-harvest grow into the tissues, causing a wet rot.

- Other pathogens: Other pathogenic micro-organisms affecting pumpkins during storage are the "bacterial spot’, caused by Pseudomonas lachrymans; " black rot’ caused by the fungus Mycosphaerella citrullina, with a high incidence in fruit with frost damage, as it happens with Alternaria.

Compositional changes
During storage, the starch is converted in sugars, being this a complete conversion in long periods of storage. At the same time, there is a decrease of the sugar consumed by respiration.

The percentage of sugar is subject to changes too. The glucose percentage increases during storage, although the cultivar Butternut has high levels of sucrose.

Behaviour in relation to ethylene
The rate of ethylene production in pumpkins is low, although some varieties have shown damages caused by this gas, originating in exogenous sources.
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