Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
Watermelons and cantaloupes are setting fruit, pickle harvest is underway, summer squash is in full production and pumpkin planting is finishing up. With the many rainy days as well as high winds and storm damage this season, fruit loads in some early planted vine crops has been affected and higher numbers of quality defects are evident.
Lack of fruit set can result from a lack of pollination due to reduced bee activity, reduced pollen viability, or reduced pollen germination in high heat. As an example, a female watermelon flower will need around 500-1000 pollen grains to be fertilized effectively. This will require a minimum of 8 visits by a honey bee. Research has shown that over 20 visits may be required to achieve full set and full size in some cucurbits.
This year (2018), early fruit set in watermelon and cantaloupe may be 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. Bees also do not fly much below 55°F, so on cold mornings, as we often had in June, bee activity will not pick up until later in the morning. Unfortunately, female watermelon flowers open early in the morning, are most receptive before 10 am, and then close in the afternoon.
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 numbers of 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 2-4 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 2 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.
Another factor to consider is where fruit set is occurring. Crown sets are desired in watermelons, especially in early plantings. Crown sets are those that occur on nodes closest to the base of the plant, within the first 8 nodes. Having good crown sets requires that plants have good early growth so that adequate leaf area is produced that can support early set fruit as well as proper pollination (sufficient bees). Lack of crown set is a sign of poor early growth, early plant stress, or of problems with pollination.
Growers with early-planted watermelons this year (those planted the last week in April or first 2 weeks in May) are likely to see reduced crown set and may see 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 this year 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 1-2 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 5 lb average will carry 4-7 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.
In gynoecious cucumbers grown for once over pickle harvesting, there will be two fruits set on adjacent nodes that are ready for harvest at any one time. These will be set on nodes 2-6 commonly. The pollinizers that make up a small percentage of the population will set pickles every fifth node generally and therefore only one fruit will be ready for harvest. Yield reductions in gynoecious pickling cucumbers occur when there is a loss of set so that fruits are not on adjacent nodes. Parthenocarpic pickle varieties that set fruit without pollination will commonly have 3-5 pickles on adjacent nodes ready for harvest at any one time. This allows them to be planted at much lower densities.