Vigorous plants such as courgettes, marrows, pumpkins and squashes can make huge amounts of leafy growth but this doesn’t always equate to good flowering or fruiting. There may be a number of reasons for this:

Lack of female flowers

Note – female flowers can be identified by the swelling of the immature fruit at the base of the flower.

Immature plants tend to produce only male flowers early in the growing season. As the season progresses the production of male and female flowers is more balanced.

Low temperatures, lack of sunshine and adverse growing conditions can also be accountable for lack of female flowers.

• Do not plant out too early; wait until the risk of frost has passed
• Protect plants during cold nights in early summer with fleece or cloches
• Water regularly to prevent drought stress
• Apply a balanced fertilizer to promote stronger growth. Feed vigorous plants with a high-potassium feed such as a tomato feed
• Avoid planting in a shady position and overcrowding the plants

Lack of male flowers

High temperatures during periods of hot weather or in a poorly-ventilated greenhouse can result in a lack of male flowers. This problem is most common in late summer and autumn.

• Avoid planting out transplants too late in the season (after mid-summer)
• Ensure good ventilation if plants are grown in the greenhouse or under cloches

Plants will naturally produce fewer male flowers as the autumn approaches.

Fruit set and development problems

Cucurbits, with the exception of glasshouse ‘all female’ cucumber cultivars, produce separate male and female flowers that require pollination to set fruit (note: older glasshouse cucumber cultivars produce male and female flowers but if pollination occurs the resulting fruits are bitter so this must be prevented by removing the male flowers). Pollination is the transfer of pollen from the male flower to the female flower and is usually carried out by insects.

Environmental factors and unsuitable growing conditions can be the reason why plants fail to set fruit despite the plant producing both male and female flowers. The young fruitlets are aborted and will drop off.  Sometimes, the fruit will begin to develop, swelling mostly at the stalk end. The flower end of the fruit will often shrivel up and start rotting. The reason may be:

• The plants may be too young and small to sustain fruits.
• Lack of pollination due to low activity of pollinating insects, mainly honeybees, during periods of poor and cold weather. Hand pollination will ensure transfer of the pollen:
  1. First identify a male flower. Male flowers differ from female flowers by not having a fruitlet at the base of flower
  2. Pick off a male flower from the plant, then carefully remove the flower petals to expose the pollen-bearing anthers
  3. Press the male flower into the centre of the female flowers. One male flower will pollinate several female flowers
• When the crop is grown under cover such as a greenhouse or cloche where the insect access is limited. Leave doors and vents open on hot days to encourage insects, but hand pollination may be necessary.
• Plants that are stressed and weak through unsuitable growing conditions. Feed and water regularly especially container-grown plants.
• Failure to pick the mature fruit regularly. This is the plant’s way of regulating the number of developing fruits that it is able to support. Ensure regular harvests of mature fruit or, in the case of courgettes and cucumbers, immature fruits.

As the autumn approaches growth rate and fruit set will be naturally lower.

Bitter tasting fruit

The bitter taste of some fruit is caused by an over-production of plant defence chemicals called ‘cucurbitacins’. This is mainly a problem in courgettes and summer squash and is caused primarily by a mutation within the plant. The problem is more likely when plants are grown from saved seeds, where inadvertent cross-pollination may have occurred.

Affected fruit should not be eaten as it causes stomach upsets and affected plants should be removed.

Fungal diseases

Cucurbits are prone to powdery mildew. Typically, this causes a white dusty coating on the leaves, stems and flowers, especially later in the season as the weather gets warmer and drier.

• Drought stressed plants are more prone to the attack
• Improve moisture retention by incorporating bulky organic matter such as garden compost or well-rotted manure to the planting area
• Apply mulch to the area around the base of the plants to reduce evaporation from the soil
• Make sure to leave a mulch-free area close to the base of stem
• Alternatively, grow the plants through black plastic sheeting
• Water regularly during dry spells but try to avoid overhead irrigation
• Improving air flow around the plants will also be beneficial
• Some cultivars, particularly courgettes, claim some resistance
• Chemical control: tebuconazole with trifloxystrobin (Provanto Fungus Fighter Plus) can be used for powdery mildew control on glasshouse crops of cucumber, courgette and summer squash
• Sulphur (Mildew Clear for Edibles) can be used on cucumbers (full details on label)
• SB Plant Invigorator, Growing Success Fungus Stop, RHS Bug and Mildew Control, Vitax Rose Guard, Growing Success Rose Guard, Ecofective Bug & Mildew Control and the Ecofective ‘Defender’ range contain a blend of surfactants and nutrients and can be used on any edible or ornamental plants, with no harvest interval. They have a physical mode of action and may be used against powdery mildews, as well as a range of damaging invertebrates such as whiteflies, aphids, spider mites, mealybugs, scale insects and psyllids

Inclusion of a product does not indicate a recommendation or endorsement by RHS Gardening Advice. It is a list of products currently available to the home gardener.

Various fungal foot and root rots can cause darkening and rotting of the base of the stem. The root system may also be affected. If the upper parts of the plant start wilting and dying back, Verticillium wilt may be to blame.

• Unfortunately there is no cure. Remove and dispose of the collapsed plant including the roots as soon as possible with the soil or compost from around the roots
• When planting in heavy soils, plant onto a slight mound (5cm/2in) to keep the base of the plant proud of the surrounding soil
• Use sterilized compost for raising seed and water with mains water to reduce the likelihood of an infection

Virus problems

Courgettes, marrows, pumpkins and squashes can be affected by a number of viruses including Cucumber mosaic virus causing yellow mottling, distorted and stunted growth. There is no remedy but to discard the affected plant. Resistant cultivars are available.

Slugs and snails are the animals that most commonly damage outdoor plants. Young plants can be badly affected and need protecting but mature plants will tolerate the damage.

Aphids are occasionally troublesome, primarily on the soft growth at the tips. They can potentially spread virus diseases. Take action early before they become established.

Red spider mite, which causes mottling of the leaves and glasshouse whitefly can be a problem not only on crops grown under cover but also on outdoor plants in hot summers, but damaging infestations generally do not build up soon enough to affect cropping.

Accidental exposure to weedkillers will cause stunted and distorted growth and yellowing of the foliage. In the last few years there has been a number of cases of weedkiller damage to plants traced to hormonal weedkiller residues in contaminated manure.