Posts Tagged ‘strawberry’

High Tight Beds for Plasticulture Strawberries

Friday, August 31st, 2012

Gordon Johnson, Extension Vegetable & Fruit Specialist;

Highest yields in plasticulture strawberries require proper bed formation. While bed makers and plastic layers used for vegetables can be used, it is preferred to use bed makers specifically designed for plasticulture strawberry systems (such as a Kennco or Reddick machines). These produce beds that are 32 inches wide at the base, 30 inches wide at the top and 10 inches tall with a crowned top for good drainage. With this type of bed, you can have bed centers at 5 ft between beds. It is also important to lay the plastic so that it is tight against the soil. This allows for effective heat transfer that will promote good strawberry growth in the fall.

When to Plant Plasticulture Strawberries

Friday, August 10th, 2012

Gordon Johnson, Extension Vegetable & Fruit Specialist;

In 2011, the common observation was that later planted Chandler strawberries out-yielded earlier plantings. This illustrates the dramatic effects that fall and winter temperatures can have on plasticulture production.

Chandler has been our main plasticulture berry and has shown consistently high yields. For most of Delaware, the recommendation has been to plant Chandler the second week in September. In conversations with Dr. Barclay Poling, who is the recognized expert in plasticulture production in North Carolina, he stated that Chandler is more sensitive to fall and winter temperatures than other varieties and in warmer conditions Chandler will put on too much growth, leading to small berries the following spring; therefore, knowing when to plant is difficult. If you could accurately predict fall and winter temperatures, you could adjust planting dates, but of course this is not possible.

One strategy has been to make multiple plantings of Chandler one week apart starting the second week in September. This will insure that a part of the crop will come out of winter with the proper number of crowns (not too many, not too little). Unfortunately, this means that part of the crop will be low yield and part will have small berries.

Another strategy is to switch to varieties that are less susceptible to putting on too much growth. This is where the variety Camarosa may have a fit, as it is less temperature sensitive than Chandler in the fall and is not prone to putting on excessive growth. Camarosa is however sensitive to high April temperatures which can halt flowering in the spring, but in normal years will extend the season better than Chandler.

Sweet Charlie, the early berry that also can put on a second late crop, is normally planted 7-10 days ahead of Chandler. It is not an option to replace Chandler. For other varieties being tried, such as Festival and Bish, we still do not have enough research in our region to know if they can be replacements for Chandler.

Another strawberry that should be considered by growers is Albion, a day-neutral variety. It too is not sensitive to when it is planted in the fall. While much less productive in the main Chandler season, it has some unique properties that make it valuable to growers. First, it will give some early production, ahead of Chandler. Second, even though production is lower, it produces evenly over an extended period of time from April through early July. In general it will give 5-6 weeks more production than Chandler. It is a large, firm berry, that, while not as sweet early in the season, has good quality in May and June. Because plants are smaller and there are fewer berries per plant, it should be planted at a higher density than Chandler.

Sunburn in Fruits and Fruiting Vegetables

Friday, July 6th, 2012

Gordon Johnson, Extension Vegetable & Fruit Specialist;

High temperatures, clear skies and high light radiation, and long daylengths are a recipe for developing sunburn in fruits and fruiting vegetables. We commonly see sunburn in watermelons, tomatoes, peppers, eggplants, cucumbers, apples, strawberries, and brambles (raspberries and blackberries).

There are three types of sunburn which may have effects on the fruit. The first, sunburn necrosis, is where skin, peel, or fruit tissue dies on the sun exposed side of the fruit. Cell membrane integrity is lost in this type of sunburn and cells start leaking their contents. The critical fruit tissue temperature for sunburn necrosis varies with type of fruit. For cucumbers research has shown that the fruit skin temperature threshold for sunburn necrosis is 100 to 104°F; for peppers, the threshold is 105 to 108°F, and for apples the critical fruit skin temperature is 125-127 °F. Fruits with sunburn necrosis are not marketable.

The second type of sunburn injury is sunburn browning. This sunburn does not cause tissue death but does cause loss of pigmentation resulting in a yellow, bronze, or brown spot on the sun exposed side of the fruit. Cells remain alive, cell membranes retain their integrity, cells do not leak, but pigments such as chlorophyll, carotenes, and xanthophylls are denatured or destroyed. This type of sunburn browning occurs at a temperature about 5°F lower than sunburn necrosis (115 to 120° F in apples). Light is required for sunburn browning. Fruits may be marketable but will be a lower grade.

The third type of sunburn is photooxidative sunburn. This is where shaded fruit are suddenly exposed to sunlight as might occur with late pruning, after storms where leaf cover is suddenly lost, or when vines are turned in drive rows. In this type of sunburn, the fruits will become photobleached by the excess light because the fruit is not acclimatized to high light levels, and fruit tissue will die. This bleaching will occur at much lower fruit temperatures than the other types of sunburn.

Genetics also play a role in sunburn and some varieties are more susceptible to sunburn. Varieties with darker colored fruit, those with more open canopies, and those with more open fruit clusters have higher risk of sunburn. Some varieties have other genetic properties that predispose them to sunburn, for example, some blackberries are more susceptible to fruit damage from UV light.

Control of sunburn in fruits starts with developing good leaf cover in the canopy to shade the fruit. Fruits most susceptible to sunburn will be those that are most exposed, especially those that are not shaded in the afternoon. Anything that reduces canopy cover will increase sunburn, such as foliar diseases, wilting due to inadequate irrigation, and excessive or late pruning. Physiological leaf roll, common in some solanaceous crops such as tomato, can also increase sunburn.

In crops with large percentages of exposed fruits at risk of sunburn, fruits can be protected by artificial shading using shade cloth (10-30% shade). However, this is not practical for large acreages. For sunburn protection at a field scale, use of film spray-on materials can reduce or eliminate sunburn. Many of these materials are Kaolin clay based and leave a white particle film on the fruit (such as Surround, Screen Duo, and many others). There are also film products that protect fruits from sunburn but do not leave a white residue, such as Raynox. Apply these materials at the manufacturer’s rates for sunburn protection. They may have to be reapplied after heavy rains or multiple overhead irrigation events.

Day Neutral Strawberry Production

Friday, June 1st, 2012

Gordon Johnson, Extension Vegetable & Fruit Specialist;

Strawberry season started early this year and is now winding down. The heat over the holiday weekend has essentially shut down additional flowering and fruiting in our spring bearing varieties so expect limited production into June. The critical high temperature for strawberries varies but any temperatures in the high 80s or 90s will reduce or stop flowering altogether.

There has been significant interest in the use of repeat blooming strawberries for extended production. These “day neutral” varieties can provide good spring production with continued production into the summer and fall, depending on the planting date. Day neutral strawberries are different from spring bearing types because they are not triggered to flower by daylength and can flower and fruit repeatedly as long as temperatures are suitable. Day neutrals vary in their ability to flower during the summer, and have been classified as either weak or strong. Strong day neutrals produce flowers and runner sparsely during the summer, flowers form on runners, and plants tend to be small with a moderate number of crowns. Intermediate and weak day neutrals, have more of the spring bearing characteristics, such as a stronger tendency to runner in summer. We recommend strong day neutrals for summer and fall production in our area.

In the past, the day neutral varieties Tribute and Tristar were used in this region; however, berry size is small. Certain day neutral varieties bred for use in other regions (California and Northern Europe) have been successfully used in this region and have larger berry size and higher yields, Seascape, Evie 2 and Evie 3 for example. Unfortunately these varieties have soft berries. Albion has shown good promise as a day neutral with large firm berries, small plants, and long term, even production and low disease pressure. It is lower yielding than others but works well in our region. Other California bred day neutral varieties such as Monterey and Portola have shown promise for our area. The USDA breeding program in Beltsville, MD is currently selecting day neutral varieties that would be better adapted to our region.

Day neutrals can be planted in the spring for summer, fall and carryover spring production; planted in summer for fall and carry over spring production; and planted in fall for spring, summer and fall production the next year.

For summer production with day neutral varieties, the use of aluminized reflective plastic mulch with drip irrigation is recommended. Additional provision for heat abatement will be necessary. This may include low volume misters for evaporative cooling during hot daytime temperatures or the use of white or reflective shade cloth. Drip irrigation should be run during the day to further limit bed heating.

Remove runners from all plants throughout the season. Runnering decreases markedly after fruiting begins, so while this task is somewhat intensive early in the season, it becomes insignificant later.

Flowers should be removed for 6 weeks following planting to allow the plants to achieve sufficient size for fruiting. Failure to remove flowers will result in small plants and low yields. Extending the period of flower removal beyond 6 weeks will result in larger plants, berries and second-year yield, but less production in the first year. Varying the flower removal period will not affect the timing of production peaks.

Day neutrals benefit from a continuous supply of nitrogen and potassium. Additional phosphorus is not necessary provided an adequate supply has been incorporated before planting. Apply 5 to 6 lbs/A of nitrogen through the drip irrigation system every week. Calcium nitrate is the preferred source of nitrogen early in the season, UAN solution can be substituted when temperatures warm. Supplement preplant potassium with 10 lb/A of K2O at monthly intervals, or 2 lb/A at weekly intervals through the drip irrigation system during the growing season. Day neutrals tend to be heavy consumers of boron because of their large commitment to reproduction. Monitor leaves occasionally to ensure that boron levels do not fall below 30 ppm. An application of 2 lb/A Solubor may be required in midsummer if boron levels are too low.

Gray mold is the biggest disease problem of day neutral strawberries. Because berries are continuously present, mold inoculum tends to increase during the season. Remove moldy berries from the planting, and protect flowers every 10 days to 2 weeks with an application of fungicide, especially after rainy periods.

To extend production in the fall floating row covers, clear row covers, or a combination can be used to conserve heat. If fall production is to be targeted, mid-summer plantings on black plastic mulch would be recommended but overhead irrigation is essential for establishment.

Day neutral plantings can be carried over to a second year. Plants should be cut back and crown thinning may be necessary in some varieties.

Some information in this article was taken from this factsheet:

Monitoring Nutrient Levels for Plasticulture Strawberry Production

Thursday, April 5th, 2012

Gordon Johnson, Extension Vegetable & Fruit Specialist;

In last week’s Weekly Crop Update, I gave some recommendations for petiole sap nitrate and potassium levels in plasticulture strawberries. This is based on sampling leaf petioles from the most recently expanded leaves from plants in the field, extracting the sap, and using portable nitrate and potassium meters. The procedure can be found at this website, along with recommended levels for different growth stages.

While this is a quick way to monitor nutrient levels, growers are also encouraged to take petiole and leaf samples for laboratory analysis. The recommended levels for petiole nitrate and leaf tissue contents from laboratory analyses can be found at this publication from North Carolina: A description of how to sample is also given. We do now have a lab on Delmarva that can run tissue samples. Leaf tissue nitrogen levels should be maintained as follows: N (%) 3–4, P (%) 0.2–0.4, K (%) 1.1–2.5, Ca (%) 0.5–1.5, Mg (%) 0.25–0.45. Petiole nitrate content until the end of the month should be around 4000 ppm (3000 ppm is low and 5000 ppm is high).

Strawberry Angular Leafspot

Thursday, March 29th, 2012

Bob Mulrooney, Extension Plant Pathologist;

There have been several samples from DE and MD with angular leafspot that have been diagnosed this spring. As you can see from the picture this bacterial leafspot produces angular watersoaked spots initially (Figure 1) that turn dark and eventually brown with time (Figure 2). The bacteria are limited by the vein pattern in the leaf which gives it the diagnostic angular pattern. This disease can cause leaf loss, and when conditions are very favorable during fruit set, the calyx can become infected and that can reduce the marketability of the fruit. Wet conditions favor the disease, especially if irrigation is needed for frost protection. The bacteria that cause the spring symptoms come from systemically infected overwintered plants and dead leaves, and from infected transplants. Copper sprays can be effective in limiting spread once it is identified but over-application can be phytotoxic, so be careful. Prevention of angular leafspot in the plant nursery and its dissemination in transplants is crucial to controlling this disease.

Figure 1. Watersoaking symptoms

Figure 2. Angular leafspot symptoms

Plasticulture Strawberry Fertilization

Thursday, March 29th, 2012

Gordon Johnson, Extension Vegetable & Fruit Specialist;

Plasticulture strawberries should have had nitrogen applications already, prior to bloom. Addition of 3-5 lbs of nitrogen per acre per week may be warranted. Nitrogen is critical prior to and during early bloom. Including potassium at a 1:1 or 1:2 ratio with nitrogen will often improve fruit quality (sugars). Use of tissue testing throughout the strawberry season is recommended to monitor nitrogen and potassium levels. Targets are 500 ppm petiole sap nitrate and 2500 ppm petiole sap potassium. Additional calcium may be needed as the season progresses.

Weather Worries for Fruit Growers

Thursday, March 29th, 2012

Gordon Johnson, Extension Vegetable & Fruit Specialist;

Peaches, plums, and apricots have bloomed, several weeks ahead of normal. Strawberries have been blooming for weeks in plasticulture systems. These fruit crops are at great risk of losses due to freeze events. Other fruit may also flower early and be at risk. For example, pears are in bloom now and cherries and blueberries are starting to bloom.

Normally, the average date of the last frost in Delaware is somewhere between April 20-25. We still have four weeks of worry ahead for our fruiting crops.

For all these fruit crops the most susceptible stage of injury is when flowers have just opened. Closed buds have higher cold tolerance as do small fruit. For most fruits, critical temperature for losses after fruits have formed is 28° F.

 Plasticulture strawberries blooming 3- 29-2012.

Frost and freeze protection methods vary with fruits and the type of freeze expected. Advective freezes occur with freezing temperatures and high winds. This is the most difficult to protect against. For strawberries, two layers of floating row covers may be the most effective strategy for advective freezes. Double covers have been shown to be more effective than single heavy covers in this case. Irrigation along with double covers can provide even more protection if done properly.

Radiational freezes occur on cold, still nights. In this case cold air is near the ground and warmer air is above. Wind machines and helicopters have been successfully used to stir the air and raise the temperatures in orchards in this case. Row covers in strawberries will protect against radiational freezes too.

Irrigation has also been successfully used for frost protection but it has to be done properly. How irrigation works is that as ice forms on plants heat is released. The key is to keep ice formation occurring through the night and continue through melt in the morning. Remember that initially, until ice starts forming, there will actually be evaporative cooling of the plant. The latent heat of fusion (water freezing) will release heat (approximately 144 BTUs/lb of water), whereas evaporative cooling will absorb heat from the plant (absorbing approximately 1,044 BTUs/lb of water) and lower plant temperatures. Therefore, irrigation must start well above critical temperatures. Also, the volume of water needed needs to be matched with the expected temperature drop and wind speed. In addition, uniformity of water application is critical. This is difficult to do in high wind situations.

This past week temperatures dropped below freezing in parts of Delaware on three nights, with some areas in the mid-twenties. NOAA has predicted an increased risk for lower than normal temperatures in the Mid-Atlantic region for the next 2 weeks.

Strawberries, Row Covers & Freeze Protection

Friday, March 2nd, 2012

Gordon Johnson, Extension Vegetable & Fruit Specialist;

Row cover management in plasticulture strawberries has been difficult this year due to the mild winter. In normal winters, row covers applied in December serve as winter protection to limit stand losses, dessication damage, and low temperature damage to buds. While plants are in a dormant state or when buds are not yet active in strawberries, the buds can tolerate temperatures down to 10°F.

Removing row covers during warmer winter periods can help to delay bud activity and reduce susceptibility to later freezes. Replace row covers in times when freezes are expected. Highest yield potentials are usually obtained by uncovering and covering in the late winter and spring based on expected temperatures when compared to the practice of keeping row covers on continuously into the flowering stage.

Once buds have begun to emerge, even when tight, they can only tolerate temperatures down to 22°F. As they begin to open, the critical temperature for damage increases (popcorn stage 26°F, open blossom 30°F).

For growers that have not been taking row covers on and off and will be leaving them on until bloom, the potential for losses due to freeze events will be greater during March due to the increased bud activity. Prior to forecasted freeze events, check the plant bud stage, and apply additional freeze production to limit losses. This may include double covering with row covers (2 layers), or the use of low volume sprinklers through the night and into the morning as a frost protection over the row covers. Loss of buds or flowers due to freeze events will reduce yields and profits substantially. A single 1.2 ounce floating row cover will give about 4 degrees of protection.

Be Sure to Monitor for Spotted Wing Drosophila in Strawberry and Brambles This Year

Friday, March 2nd, 2012

Jerry Brust, IPM Vegetable Specialist, University of Maryland;

By now everyone should know that the newest invasive pest, the spotted wing drosophila (SWD), is here in the mid-Atlantic. It was found heavily infesting blackberries and raspberries in central Maryland this past summer and fall. Just about everywhere we trapped for it (I am still trapping adults in February in brambles, SWD overwinter as adults) we have found it on the western shore. We know it is on the eastern shore through trapping efforts by the University of Delaware. What we do not know about the eastern shore is how bad SWD infestations might be this coming season. The first crop that may get hit is strawberries. Information from Oregon and Michigan shows that their strawberries are not attacked to any great extent, but we DO NOT know what the fly may do to our strawberry crop. That is why it would be prudent to put SWD traps out and monitor for the adult flies. Males have a spot at the end of their wings (Photo 1), females do not (Photo 2), but the females do have a strong ovipositor they use to saw into non ripe fruit and lay their eggs—which is why they are such a devastating pest.

Most growers we visited did not think they had SWD on their farm and yet we found it everywhere we looked. The damage is often mistaken for early rotting berries or fruit (Photo 3). Early control is essential, if this fly is allowed to build its population through the summer into the early fall it will be very difficult to control and will be present on your farm basically forever. There are several web sites you can use to build your own traps (just Google spotted wing drosophila traps), or you could ask for help from me or your Extension educator about trapping. The key is to use a very common, inexpensive product as bait in the traps – apple cider vinegar. Traps should be placed in the field within the plant canopy, out of the sun if possible, and checked once a week for flies. Some traps should be located near the edge of the strawberry field and others along a woods edge. There will be many fly species in the trap, if you are not sure you have SWD take it to your local Extension educator for identification.

Photo 1. SWD adult male

Photo 2. SWD adult female

Photo 3. SWD damage to blackberries