Pumpkins and grammas: commercial production

Eric Coleman, Department of Primary Industries and Fisheries.

Traditionally Cucurbita maxima and Cucurbita moschata have been placed into two groups - pumpkins and grammas respectively. In recent years in Australia, popular description of these species has grouped both of them as pumpkins. Other members of the cucurbit family include cucumber, choko, rockmelons, watermelons, zucchini and gourds. The Kabocha pumpkin is a cross between Cucurbita maxima and Cucurbita moschata.

Climate and soils

Pumpkins and grammas are very frost susceptible, but are tolerant of high temperatures provided the soil moisture is adequate. Ideally, they need 4-5 months of warm weather (daily maximum temperatures higher than 22^oC) to grow and mature. High temperatures and long days result in optimum growth, fruit set and yields of quality produce at harvest. Seeds need a minimum soil temperature of 16^oC to germinate.

Well-drained, fertile, sandy to clay loams with a high level of organic matter are preferred. The pH should be near neutral (6.5-7.5). The yield will decrease if the soil is too acid or alkaline. Saline soils should be avoided.

Establishment and maintenance

A deep, early ploughing is desirable and should be followed by a shallower working to produce a fine, firm seedbed free of plant residues. Raised beds are used on lighter coastal soils only. Planting times in the Moreton, Lockyer and South East Queensland regions are August to September for the spring crop and December to late January for the summer planting.

The milder winters of the northern, central and coastal regions of Queensland (frost-free areas) allow spring crop planting from August to September and planting for the autumn crop from April to June. Bundaberg's mild winters allow for planting from July to early August. Seed may be pre-germinated, but then must be hand-sown to avoid damage. A crop rotation cycle of 3 to 5 years with no cucurbit crops immediately following a pumpkin or gramma planting will help to minimise the risk of pest and disease problems. Use of transplants is becoming more popular as it decreases establishment time and assists in weed control.

Table pumpkins are planted at 100-180 cm intervals on rows 3-4 m apart. The main varieties of pumpkin grown in Queensland include Jarrahdale, Sweet Grey (hybrid), Queensland Blue and Kabocha types such as Delica.

Grammas may be planted at 60 cm intervals on rows 2-3 m apart. The main gramma varieties grown include Butternut, Butternut Large, *Late Potkin and Japanese (Jap) such as Ken's Special. Plant intervals may be adjusted according to row spacings. Allow approximately 3 m² per plant for pumpkins and 1 m² per plant for grammas.

*(The Late Potkin cultivar is relatively male sterile and requires a companion cultivar to provide adequate male pollen).

Pollination

Both male and female flowers occur on the vines. For efficient pollination to occur (and ultimately fruit set) there must be:

• Flowers on the vines
• Both male and female flowers and
• Bees to move pollen

There are a number of factors that can influence this pollination process. Rapid growth promotes earlier flowering. However high temperatures,long days,and high rates of nitrogen can result in large amounts of vegetative growth and not many flowers and or a higher proportion of male to female flowers (only female flowers produce fruit). It is important to check the sex of the flowers, normally the ration of 1 females to 7 males is considered adequate. Cross-pollination between pumpkin and gramma varieties is common, but will not occur between pumpkins and other cucurbits.

Figure 1. Typical male (a) and female (b) cucurbit flowers.

Bees are necessary for pollination. Bees need to be present and active in the crop. If bees are not plentiful, then import at least 2 hives/ha, spreading them around the field perimeter and check to ensure that male flowers are producing pollen. Flowers are most receptive to pollination during the morning hours when bee activity is usually the highest. Bee activity is related to climate and is lower in cooler weather. Spraying and irrigation should be coordinated to occur when bees are least active. If possible, use agrichemicals with minimal bee toxicity.

Nutrition and irrigation

The residual fertiliser from the previous crop is usually sufficient on fertile alluvial soils, particularly where an intensive vegetable has been grown. Bundaberg growers planting after sugar cane need to apply fertiliser. Side dressings of nitrogen may be necessary when the vines begin to run. On lighter soils, apply a general NPK mix and side-dress with the same when the vines begin to run.

As mentioned in the pollination section, over application of nitrogen can result in rapid production of vegetative growth with little or no reproduction. Availability of potassium is essential for reproductive growth. Molybdenum (Mo) deficiency may occur on sandy soils, acidic soils and the alluvial soils of the Lockyer and Fassifern Valleys. Symptoms include stunting of plant growth and mottling, yellowing and rolling of older leaves. The symptoms typically look similar to nitrogen deficiency with the addition of a necrosis (scorching) on the margin of affected leaves. Molybdenum deficiency can be corrected by applying a foliar spray of sodium molybdate at the four-leaf stage and repeating prior to vining.

Zinc, copper and boron deficiencies may also occur, but are most likely in leached, strongly acid, sandy soils or where large quantities of liming materials have been applied. For this reason the highly alkaline soils of the Lockyer Valley may respond to zinc and boron application.

Fruit abortion is common in a number of circumstances and is a result of some form of stress. Moisture stress due to extremely hot dry conditions, nutritional stress due to trace element deficiencies can also cause fruit loss. This may be corrected by applications of boron and/or calcium. Please note that abortion of young fruit can also occur if stung by cucumber fly. Cucumber fly will sting fruit and not always deposit eggs so larvae or maggots may not be obvious. Cucumber fly are worse during periods of frequent rainfall as rain stimulates adults to hatch out in the ground.

Calcium deficiency can cause blossom end rot, abortion of fruit and some plant stunting in pumpkins and grammas. Deficiencies tend to occur in leached acid soils, but factors such as high temperature, water stress and excessive use of nitrogenous or potassium fertiliser can also inhibit plant uptake of calcium from the soil.

During vine growth, pumpkins and grammas have reasonably high water requirements. Strategic irrigation may also be necessary at planting, flowering and fruit development. Irrigation can be supplied by trickle, sprinkler or furrow systems. Furrow is the least expensive option if the soil is suitable. Trickle irrigation allows the most efficient use of available water and is compatible with fertigation (application of soluble fertilisers in the irrigation water). Trickle may not be suitable for vining types as they require moisture and nutrient where peg roots form next to developing fruit. This is particularly the case in dry areas where there may not be enough water to sustain fruit without some supplementary overhead irrigation or rainfall. Regular watering is preferred to infrequent heavy irrigation. Approximately 3 ML of water should be available for each hectare of crop to be grown.

The use of permanent or temporary wind breaks can help to reduce water loss and wind damage to the flowers, young fruit and stems. Reduced wind also encourages bees to forage.

Pests and diseases

Weeds

Weeds should be controlled prior to the vines running. Cultivation is usually carried out by disc harrowing between the rows. Some chipping between plants may be necessary. Some herbicides are available, but should be used with caution, as cucurbits are very susceptible to herbicide damage.

Insects

Pumpkins and grammas are prone to damage by a number of insect and mite pests, the most common of which are listed in Table 1.

Diseases

Diseases are common in cucurbit crops - Table 2 lists the major disease problems. As the crop gets close to harvest, control of some diseases (e.g. powdery mildew) may be uneconomic. Nematodes are difficult to detect and expensive to treat. To reduce nematode occurrence in areas prone to this pest, plant pumpkins after another crop that was treated for nematodes. There are now a number of pumpkin varieties available that have resistance to disease. For example, Jap hybrids like Ken's special and Dulong QHI (a Jarrahdale type) have some tolerance to common cucurbit viruses like cucumber mosaic virus.

Table 1: Insect and mite pests of pumpkins and grammas

Pest	Damage	Management
Aphids	Vector for viruses.	Reflective mulch; #registered aphicides; crop hygiene.
Melon thrips	Kill surface cells by piercing and sucking, this causes silvering, yellowing and bronzing of affected areas. Thrips attack leaves, growing points, flowers and fruit.	Reflective mulch, hygiene, and overhead irrigation (to wash thrips off leaves). Note: insecticide use increases problem.
Cucumber fly	Targets damaged fruit in packing shed causing fruit breakdown. Can sting young fruit through the stem causing them to yellow and drop off.	Hygiene measures in field and shed. +Use of protein extract lure in field
Cucurbit ladybird	Windowing damage to leaves.	Registered insecticides.
Cutworms	Damage young seedlings.	Hygiene, #registered insecticides.
Mites	Stippling of leaves; defoliation.	#Registered miticides or predatory mites.
Pumpkin beetle	Shot-hole appearance to leaves; fruit drop.	#Registered insecticides.
Rutherglen bug	Stunted growth, deformed fruit.	#Registered insecticides.

Table 2: Diseases of pumpkins and grammas

Disease	Symptoms	Management
Bacterial spot	Small water-soaked areas on leaves.	Crop rotation, crop hygiene and seed treatment.
Brown etch	Brown markings on butternuts.	Reduce irrigation near maturity, plant *resistant varieties.
Downy mildew	Small yellow patches, which enlarge into irregular, brown spots.	#Registered protectant and systemic fungicides, Queensland Blue variety has resistance.
Gummy stem blight	Circular leaf spots and lesions on stems oozing reddish-brown gum.	#Registered fungicides.
Nematodes	Stunting, yellowing, prone to wilting.	Cultural practices, soil fumigation registered nematicide.
Powdery mildew	Greyish areas on older leaves, leaf drop may cause sunburn.	#Registered protectant and systemic fungicides.
Mosaic viruses	Mosaic pattern, distorted leaves and fruit.	Aphid management, plant *resistant varieties.

*There have been recent achievements in R&D with the development of genetic tolerance to virus in Jarrahdale (see further information section). Jap type pumpkins also display higher levels of virus tolerance than other varieties.
#The Infopest CD-Rom provides an up to date database of registered pesticides of all vegetable crops including pumpkins(see information section)
+Protein extract can be purchased from Bugs for Bugs (see further information section).

Harvesting and marketing

Pumpkins/grammas mature in 15-20 weeks depending on variety and season. They are usually harvested after the vines die off. If harvested when immature, a section of the stem (30 to 100 mm) should be left on the fruit. Some Bundaberg growers harvest fruit as it ripens this allows them to supplythe early market.

Winter and spring crops are sold immediately, but the autumn crops of some varieties may be stacked and stored preferably in a cool area away from wind and rain. Pumpkins will store for a longer period than grammas. Some varieties are susceptible to sunburn and must be harvested as they mature.

The best indication of maturity is when the stalks become cracked and corky. The 'thumbnail test' can be used as an indicator for maturity; the skin should resist entry of thumb or fingernail and make a 'crisp' sound when it breaks the skin. If the skin is soft the fruit is immature.

Pumpkins and grammas are sold through local merchants or market agents. Pumpkins are sold in bulk bins or pumpkin bags, while grammas may be packed in onion bags, half-tonne bins or cartons. Careful handling to prevent damage to the skin will help prevent many post-harvest rots.

Competition pumpkins

Exhibitions and fairs often have competitions for the largest pumpkins. To achieve pumpkins this size, choose a variety with extra-large fruit, such as Atlantic Giant, and prepare a well-fertilised (organic and inorganic) seedbed. Seedlings should be thinned to allow at least 5-6 square metres of ground per vine. Side-dress the vines every 2-3 weeks. Keep the vines well watered and only allow one fruit to develop on each plant.

Further information

For further general information and specialist referral services Queensland residents can contact the DPI&F Business Information Centre on 132523 for the cost of a local call. The Business Information Centre is open 8am to 6pm Monday to Friday (excluding public holidays); E-mail callweb@dpi.qld.gov.au .

Current national information on agricultural chemicals registered on pumpkins is available on INFOPEST CD-ROM. Write to DPI&F GPO Box 46, Brisbane Qld 4001, or email infopest@dpi.qld.gov.au , or phone DPI&F Business Information Centre 13 25 23.

A virus tolerant Jarrahdale pumpkin variety called Dulong QHI is available by contacting your local seed representative or South Pacific Seeds 48-50 Willandra Avenue Griffith NSW 2680, phone: 02 6962 7333.

Protein extract lure for cucumber fly is available from Bugs for Bugs, Bowen St. Munduberra Qld 4626, phone: 07 4165 4663.

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Information contained in this publication is provided as general advice only. For application to specific circumstances, professional advice should be sought. The Department of Primary Industries and Fisheries, Queensland has taken all reasonable steps to ensure the information in this publication is accurate at the time of publication. Readers should ensure that they make appropriate inquiries to determine whether new information is available on the particular subject matter.

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Last updated 04 February 2004