Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
A number of fruit and fruiting disorders have been observed in summer squash and cucumbers recently including lack of fruit set, bottlenecking, pinched blossom ends, crooks, nubs, hollow centers or cavities, fruit zippering and scarring.
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. Water stress will compound this problem. When day temperatures are in the 90s and night temperatures are in the high 70s, plants will commonly abort fruits or produce misshapen fruits. To reduce losses due to heat, apply irrigation so that plants are never under water stress.
Growers should note that some squash (mostly zucchini) varieties will still set fruit without pollination. Steve Reiners at Cornell did a trial in 2013 with 21 varieties of summer squash to determine which were capable of setting fruit without pollination. Female flowers were bagged prior to opening to exclude pollinating insects. After 1 week, bags were removed and fruit rated as to whether it was marketable or not. The results can be found at this web site: http://www.hort.cornell.edu/expo/proceedings/2014/Vine%20crops/Seedless%20squash%20Reiners.pdf. For example, ‘Golden Glory’ and ‘Dunja’ zucchini both were able to set a high percentage of fruit without pollination. Selecting varieties with this ability can reduce losses due to poor pollination.
Golden Glory Yellow Zucchini can set fruit without pollination.
Parthenocarpic varieties of cucumbers and zucchini that set fruit without pollination are also available and can be less susceptible to environmental extremes or conditions that limit bee activity in monoecious or gynoecious varieties. We currently are evaluating 16 parthenocarpic pickle varieties for adaptation to our region.
Corinto cucumber, a parthenocarpic slicing type well suited for high tunnel production.
Lack of fruit set can also be due to harvest management. When summer squash or cucumbers are allowed to progress to an overly mature stage, plants will “shut down” and not reflower for a period of time. To manage this problem, frequent picking is necessary. This requires picking every 2 days in the summer.
Misshapen fruits commonly are found in high numbers with high temperatures and water stress in the summer or low night temperatures in the fall. This includes bottle necking, pinched blossom ends, crooked fruits or fruits with “narrow waists”. These defects are most commonly due to effects on pollination. Other stresses such as herbicide injury, root pruning in cultivation, or wind damage can increase the number of misshapen and unmarketable fruit. Potassium deficiency can also cause pinching at the stem end.
Hollowness or open cavities in cucumber and summer squash fruit can be caused by inadequate pollination and reduced seed set. Boron deficiency or the combination of boron and calcium deficiency can also result in increased hollowness.
Progression from marketable to unmarketable pickle fruits that are crooked, waist pinched, tip pinched or tip pinched with crook.
Small cavities in cucumber fruit. In a more severe form hollowness and cavities can render the fruits unmarketable or reduce processing (pickling) quality.
Pumpkin Fruit Set
Pumpkins have become an important income source for many Delmarva vegetable growers including u-pick, local sales and regional wholesale.
Each year we see pumpkin fields with poor fruit set or fruit retention. In larger pumpkin sizes, each plant will normally carry 1-2 fruits. The large vining plants also need considerable space – 25 to 50 square feet per plant. While planting jack-o’-lantern types at higher densities might at first seem to be a way to achieve higher yields, interplant competition will increase and you can decrease fruit retention because of this competition. Matching pumpkin types with space requirements is very important to optimize fruit set.
As with summer squash, a major reason for poor fruit set in pumpkins is high temperatures during flowering in July. Day temperatures in the 90s or night temperatures in the high 70s will cause flower and small fruit abortion. For pumpkin growers that do wholesale and start shipping right after Labor Day, this will limit early pumpkin availability. Varieties vary considerably in their ability to tolerate heat and to set under hot conditions. Inadequate irrigation and excessive water stress can also reduce fruit set, increase abortions, and reduce fruit retention. High temperatures and water stress reduce photosynthesis and the ability of the plant to carry fruits. Drought can also cause a higher than normal male/female flower ratio, thus affecting the amount of fruit per plant.
Another major factor that will reduce fruit set is poor pollination. Misshapen fruit can also result from inadequate pollination. A pumpkin plant has both male and female flowers and the first female flower opens one week after the first male opens. The flowers only last a few hours, blooming at dawn and closing later in the morning but well before noon. Pollinators need to be active during this short period.
Native pollinators can be very effective in pollinating pumpkins and some research has shown that most of the fruit set is occurring because of these native pollinators. Bumblebees and squash bees are native bees active in pumpkins. The squash bee is of particular interest because it has evolved along with pumpkins and squash in the Americas and is dependent on pollen from pumpkin and squash plants.
Other research has shown that honeybees do provide additional pollination benefits above what native pollinators are providing. In research from Illinois, Walters and Taylor found that while pumpkin fruit number was not increased with the addition of honeybees, pumpkin weights and size were increased significantly. Research has shown that 10-15 visits by honeybees transferring 1200 pollen grains will result in full fruit set.
Other reasons for poor fruit retention include foliar diseases and storm damage that reduce effective leaf area and photosynthesis.
Too much available nitrogen can also delay pumpkin fruit set so that many of the pumpkins that are produced do not reach maturity in time. Pumpkins do not normally need more than 80 lbs/acre N to grow a crop. Fertilizing above 100 lbs/acre N may cause the pumpkins to put on excessive vine growth and delay fruiting.