Posts Tagged ‘pollination’

Poor Fruit Set in Pumpkins

Friday, August 17th, 2012

Gordon Johnson, Extension Vegetable & Fruit Specialist;

Each year we see pumpkin fields with poor fruit set or fruit carry. Remember that in larger pumpkin sizes, each plant will only 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 carry because of this competition.

Too much available nitrogen can also delay pumpkin fruit set so that many of 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. Anything above 100 lbs/acre N will cause the pumpkins to put on excessive vine growth and limit fruiting.

A major reason for poor fruit set in some fields this year 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 carry. 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. In particular, bumblebees and squash bees are 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 solely 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.

Sweet Corn Pollination Problems

Friday, July 27th, 2012

Gordon Johnson, Extension Vegetable & Fruit Specialist;

Growers are experiencing quality problems in sweet corn this year related to poor pollination as a result of high heat. This problem is more severe in less stress tolerant varieties and where irrigation is inadequate.

In corn silk elongation begins 7 to 10 days prior to silk emergence from the husk. Every potential kernel (ovule) on an ear develops its own silk that must be pollinated in order for the ovary to be fertilized and develop into a kernel. The silks from near the base of the ear emerge first and those from the tip appear last. Under good conditions, all silks for an ear will emerge and be ready for pollination within a span of 3 to 5 days and this usually provides adequate time for all silks to be pollinated before pollen shed ceases.

Pollen grains are borne in anthers, each of which contains a large number of pollen grains. The anthers open and the pollen grains pour out after dew has dried off the tassels. Pollen is light and can be carried considerable distances (up to 600 feet) by the wind. However, most of it settles within 20 to 50 feet. Pollen shed is not a continuous process. It stops when the tassel is too wet or too dry and begins again when temperature conditions are favorable.

Under favorable conditions, a pollen grain upon landing on a receptive silk will develop a pollen tube containing the male genetic material, develop and grow inside the silk, and fertilize the female ovary within 24 hours. The amount of pollen is rarely a cause of poor kernel set. Each tassel contains from 2 to 5 million pollen grains, which translates to 2,000 to 5,000 pollen grains produced for each silk of the ear shoot.

Poor seed set is often associated with poor timing of pollen shed with silk emergence (silks emerging after pollen shed). Shortages of pollen are usually only a problem under conditions of extreme heat and drought. Extreme heat and desiccating winds can affect pollen germination on silks or pollen tube development leading to poor seed set. Insects that clip silks during pollination can cause similar problems.

Tomato Pollination and Excessive Heat

Thursday, July 12th, 2012

Jerry Brust, IPM Vegetable Specialist, University of Maryland;

The extreme heat we had will play havoc on tomato fruit that was just flowering or ripening, causing problems in fruit development due to poor pollination. Constant exposure of a tomato plant to high temperatures (day/night temperatures of 95/80°F) significantly reduces the number of pollen grains produced and released per flower and decreases the pollen’s viability. Most pollen is shed between 10:00 a.m. and 4:00 p.m. and 3-hours or more at 103oF on two consecutive days can cause fruit set failure. Temperatures at night may play a more important role in determining whether or not pollination takes place than day time temperatures. This is because ideal fruit set occurs within a very narrow range of night temperatures (60°-70° F). If tomato plants experience night temperatures above 75°F, interference with the growth of pollen tubes can occur preventing normal fertilization and causing blossom drop (Fig. 1). Prolonged high humidity (>80%) also will hinder good fruit set as the pollen either will not shed freely or the pollen grains may bind together, resulting in poor pollination. Poor pollination may result in under-size fruit that looks ‘normal’ but is just a great deal smaller. Other problems include poor development of the gel inside the fruit. This causes the fruit to appear angular and soft when squeezed (Fig. 2). When this type of fruit is cut in half, open cavities can be seen between the seed gel and the outer wall (Fig. 2). High temperatures during the ripening period additionally can cause ‘internal whitening’ in tomato fruit (Fig. 2). This white tissue only is noticeable when the fruit is cut. The hard, white areas tend to be in the vascular tissues in the outer and center walls of the fruit. Low potassium levels are also associated with ‘internal whitening’. There is not a great deal that can be done about any of the environmental problems other than to be sure to water enough and do not over fertilize during these extreme conditions. Although growth regulating chemicals can be used sometimes to help fruit set under cooler than ideal conditions there is no growth regulator that will induce normal fruit development under high temperature conditions.


Figure 1. Blossom drop (arrows) in tomato due to high night temperatures

Figure 2. Angular sides of fruit due to poor pollination. When cut open you can see the lack of gel resulting in pockets inside the fruit as well as ‘internal whitening’–spots in the outer wall.

Pollination Disorders in Cucurbits

Thursday, July 12th, 2012

Gordon Johnson, Extension Vegetable & Fruit Specialist;

Watermelon harvest is underway on Delmarva; cantaloupe harvest started early this year, squash and cucumbers have been producing for over a month; and pumpkins and winter squash are setting fruit in earlier plantings. Each year, we see pollination problems with vine crop fruits, especially when weather conditions are unfavorable.

Signs of incomplete pollination in cucurbits include bottlenecked fruit or fruit with a pinched end, crooked or lopsided fruit, fruit small in size or nub-like; and fruits with prominent lobes or that are triangular in shape. Causes of incomplete pollination may be inadequate pollen transfer by pollinating insects; inadequate pollen sources (pollenizers); or hot, dry weather that reduces pollen viability or that desiccates flower parts during pollination. Research has shown that a minimum of 1,000 grains of pollen are required to be distributed over the three lobes of the stigma of the female flower of a watermelon to produce a uniformly shaped fruit.

Hollow cavities in fruit and vacant seed cavities are related to lack of seed formation, again traced back to poor pollination. Fruit tissue separation, such as hollow heart in watermelon, may also be due to inadequate pollination and may be worsened by rapid fluctuation in environmental conditions affecting fruit development.

Fruit Loads in Vine Crops

Friday, June 29th, 2012

Gordon Johnson, Extension Vegetable & Fruit Specialist;

Early watermelons are sizing now, later plantings are setting fruit, pickle harvest is underway, and pumpkin planting is finishing up. A common question from growers and crop consultants is how many fruit should a plant carry and what will affect fruit set and 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 amount of 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. If average fruit carry is less than 2 per plant in watermelons, that 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 watemelons, 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.

With pumpkins the carrying capacity is similar; however, because pumpkins are not repeat harvested as are watermelons, harvest is limited to those fruits set initially. 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 4-6 pickles on 3-5 adjacent nodes ready for harvest at any one time. This allows them to be planted at much lower densities.

Poor Fruit Set in Pumpkin

Friday, September 11th, 2009

Gordon Johnson, Extension Ag Agent, Kent Co.;

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.