Poor Fruit Set in Pumpkin

Gordon Johnson, Extension Ag Agent, Kent Co.; gcjohn@udel.edu

Each year we come across pumpkin fields with poor fruit set and this year is no exception. Poor fruit set can be a major problem in pumpkin production, especially with large jack-o-lantern types, and may have a number of causes. Pumpkins produce both male and female flowers and require insect pollinators, primarily bees (honey bees, bumble bees, squash bees, and other native bee pollinators). The first nodes will produce all male flowers and then female flowers will be produced some time later on (commonly after the eighth node). This early male flower production attracts bees, initiates bee flights to the field prior to female flowers opening, and insures that an adequate supply of pollen will be available for pollination to occur. Jack-o-lantern types will carry only 1-2 pumpkin per plant so anything that affects fruit set will reduce the total crop yield dramatically. Poor crops occur when 1) female flowers or small fruits are aborted or 2) when production of female flowers is delayed and late sets do not have time to develop before shorter days and colder weather set in.

In the first case, female flowers can be aborted due to stresses before pollination occurs, can abort due to lack of pollination or incomplete pollination, or small fruits can abort after pollination due to stress or injury. High temperature is the most common problem causing these abortions. Day temperatures in the 90s and night temperatures in the mid to high 70s (F) can lead to loss of these female flowers or small fruits. High respiratory demand will limit photosynthates so the plant cannot support the production of both fruits and new growth (leaves and stems) – fruit set is sacrificed until growing conditions improve. This can be very variety dependent; however, recommended varieties for this region have proven to be well adapted even at relatively high temperatures. High temperatures also have an effect on the seed set due to reduced pollen viability and poor pollen germination leading to early fruit abortions or deformed and unmarketable pumpkins. In 2009, we had only a few days with temperatures that high in the last week of July and again in the second week of August so only late planted pumpkins would have been affected by high temperature losses.

Drought stress can also cause problems with fruit set and cause abortions. Dry weather during early growth will cause plants to develop a high male to female flower ratio. Severe drought and wilt will reduce photosynthesis and limit the number of fruits that are carried. Some areas of Delaware had 3-4 weeks of dry weather in late June through late July that may have caused some reduced set; however most pumpkins are irrigated and much of the state had adequate moisture. In fact, some areas had excessive water at times. Flooded soils or soils that stay saturated for long periods will cause pumpkin roots to shut down and can lead to temporary wilting that will also cause some flower or fruit abortion.

Planting at too high of a density (closer spacings), especially with high nitrogen, can cause excessive foliage and increased shading that will limit early fruit sets. This can also occur when fields are seeded heavily (more than one seed per hole) and then are not properly thinned. As a guideline for jack-o-lantern sizes (15-25 lbs), semi-vining varieties need 15-30 square feet per plant, full vining types 20-35 square feet per plant. Some varieties will handle higher densities better than others (check with your seed company for recommendations). Excessive foliage and high densities can also limit the ability of bees to effectively move between flowers and complete pollination.

As stated, pollination depends on bees. Even though native pollinators are present, we have reduced numbers due to loss of habitat and use of insecticides. We therefore recommend 1-2 strong colonies (hives) of honeybees per acre of pumpkin field, the higher the planting density, the higher the number. Inadequate number of hives or weak hives can limit fruit set. Colonies should be placed as first male flowers are produced. Delays in hive placement can delay fruit set. Pumpkin flowers are open for about 6 hours starting at daybreak and pollination must be completed during that 6 hour period for fruit to set. Bees must move pollen from male to female flowers and multiple visits to the female flower are needed to complete pollination (one visit every 15 minutes). Bee flights are reduced in cold conditions (below 60°F) and are most active above 70°F. Windy weather (more than 12 mph) will also reduce bee flights. Windy, stormy, weather will reduce fruits set during those periods. Hive placement and management, length of rows, alternative flower sources, and improper insecticide use can also impact bee pollination effectiveness. We had a cool July in 2009 and some significant stormy periods during flowering this year that could have affected fruit set by reducing bee activity.

Insect feeding on flowers or very young fruit can cause abortions directly. Certain insects can cause stress by feeding on plants or can stunt plants so much that flowers are aborted. We had heavy squash bug and cucumber beetle infestations in pumpkin fields at times this year that might have reduced fruit set (squash bug in particular).

Poor crops can also be a result of delayed female flower production. This occurs in two opposite conditions. As previously stated, drought during early growth will favor male flower production and delay female flower production (not usually an issue for DE growers). In contrast, heavy nitrogen application and ample water will often lead to vines remaining vegetative for longer periods of time, producing female flowers only later in the season (too late to mature in time). This is likely to occur on heavier ground, high organic matter soils, fields with heavy manure application (more than 3 tons of poultry manure for example) and where more than 100 lbs of nitrogen are applied with fertilizers.

Due to the many factors mentioned above, planting date can also be important in achieving good pumpkin crops. As planting is delayed into early June, the risks associated with poor early fruit sets become greater. If first sets are lost, later sets may not have enough time to make a crop or may mature out of the main marketing window. To reduce these risks, plant at least a portion of the crop before mid June. In addition, consider using multiple varieties in case one is more sensitive to a particular stress. Consider spitting N applications and assess whether or not the second N application is needed according to vine growth and tissue tests.

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