Fruit set, fruit carry in vine crops (The Vegetable Grower)
Many vine crops on Delmarva such as watermelons and pumpkins have lower fruit set than desired in 2018.
Lack of fruit set can result from a lack of pollination due to reduced bee activity, reduced pollen production, reduced pollen viability, or reduced pollen germination in high heat.
As an example, a female watermelon flower will need around 500-1,000 pollen grains to be fertilized effectively.
This will require a minimum of eight visits by a honey bee.
Research has shown that more than 20 visits may be required to achieve full set and full size in some cucurbits.
This year, early fruit set in watermelon in some fields was off due to poor weather during early flowering.
Bees flights are reduced significantly in rain and when winds are 15 mph or greater.
Cloudy weather also reduces bee activity.
In addition, in early mornings and during poor weather, bees usually visit plants closest to the hives.
As the temperature rises or the weather improves, the bees will forage further from the hive.
This means that in bad weather vine crops closest to the hives will have the best set and furthest from the hives will have the worst.
This year another problem is that some watermelon fields have lost significant pollenizer plants due to poor weather conditions during or after planting.
This means that pollen will be limiting. Research has shown that were pollen is limiting, fruit numbers will be reduced with distance from a pollen source.
In fields with limited pollen, expect reduced fruit set or reduced fruit size in areas where pollenizers are missing.
Another common question from growers and crop consultants is how many fruit should a plant carry and what will affect fruit “carry” in vine crops.
For watermelons, a healthy, vigorous plant may set 3-7 fruits initially.
However, for mid-size and larger watermelons, the plant will only carry two to fruit fruit at any time.
Smaller fruited varieties will more fruits per plant but essentially the same total pounds as larger types.
This is the carrying capacity of the plant and is directly related to the quantity of photosynthates being produced by the plant, mostly in the leaves.
Any additional fruits, even if initially set, will be aborted. Once the first fruit ripens and is harvested, additional sets can be carried.
To carry the maximum amount of fruit, it is necessary to maintain high plant vigor and good foliage health.
This requires paying close attention to irrigation and fertility programs; having excellent disease, insect, and mite control; and having good pollinator activity during pollination and fruit set.
In watermelons, if average fruit carry is less than two per plant, this is a sign that the plants have reduced vigor and are under stress.
Repeated fruit set depends on maintaining vine health through the season.
Growers with early-planted watermelons this year (those planted the last week in April or first two weeks in May) saw reduced crown set and increased numbers of seedless melons with defects such as distinct lobes (noticeably triangular) or hollow heart and standard seeded pollenizers with pinched ends.
These are signs that pollination was lacking during early fruit set.
This can occur when there is a lack of pollen — pollenizers have not produced enough male flowers or are delayed in producing male flowers.
In some years, fields have had losses of pollenizers, due to the poor weather during transplanting, requiring replanting.
This may reduce pollen for the first set female flowers in triploids, reducing crown sets. Reduction in bee activity during the stormy weather this June may also reduce early sets.
Early plant stress such as the wind damage, flooding, and hail damage we have seen in can also cause abortion of flowers leading to reduced crown set.
With pumpkins harvest is limited to those fruits set initially, because pumpkins are not repeat harvested as are watermelons.
Medium-sized Jack-o-lantern types will carry one to two fruits, larger types closer to 1.
All others will be aborted. Smaller types will carry more depending upon their size in pounds (for example a variety with a five-pound average will carry four to seven fruits).
Maximum carrying capacity in pumpkins is largely affected by variety (varieties with some heat tolerance will carry more fruits in our climate) and foliage health.
Excess nitrogen fertilization will often delay fruit set in pumpkins.
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