Heat and Moisture Effects on Cole Crops

Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

September-maturing cole crops have been negatively affected by the high August and September temperatures and uneven moisture (dry to wet). While cabbage, kale, and collards can tolerate high temperatures; Brussels sprouts, broccoli, and cauliflower are more sensitive to excess heat. These three crops do best under moderate and even temperatures and even water supplies. They do not develop properly when temperatures are in the 90s.

In broccoli, high temperatures can lead to uneven development of the crown leading to a bumpy appearance and looser head. This reduces the grade and price potential. In Brussels sprouts high temperatures can caused sprouts to be very loose, elongated and unmarketable. In cauliflower high heat can cause loose curd.

The following are some other disorders that can be prevalent when cole crops are exposed to uneven moisture and excessive heat.

Tipburn of Cauliflower, Cabbage, and Brussels Sprouts
This problem can cause severe economic losses. Tipburn is a breakdown of plant tissue inside the head of cabbage, individual sprouts in Brussels sprouts, and on the inner wrapper leaves of cauliflower. It is a physiological disorder which is associated with an inadequate supply of calcium in the affected leaves, causing a collapse of the tissue and death of the cells. Calcium deficiency may occur where the soil calcium is low or where there is an imbalance of nutrients in the soil along with certain weather conditions. (High humidity, low soil moisture, high potash and high nitrogen aggravate calcium availability). Secondary rot caused by bacteria can follow tipburn and heads of cauliflower can be severely affected. Some cabbage and cauliflower cultivars are relatively free of tipburn problems.

Cabbage Splitting
Cabbage splitting can develop when moisture stress is followed by heavy rain. The rapid growth rate associated with rain, high temperatures and high fertility cause the splitting. Proper irrigation may help prevent splitting and there are significant differences between cultivars in their susceptibility to this problem.

Lack of Heads in Broccoli and Cauliflower
During periods of extremely warm weather (days over 86°F and nights over 77°F) broccoli and cauliflower can remain vegetative since they do not receive enough cold for head formation. This can cause a problem in scheduling the marketing of even volumes of crop.

Cauliflower Purple Coloring and Yellowing
The market demands cauliflower which is pure white or pale cream in color. Heads exposed to sunlight develop a yellow and/or red to purple pigment. Certain varieties are more susceptible to purple off-colors, especially in hot weather. Self-blanching varieties have been developed to reduce problems with curd yellowing. For open headed varieties, the usual method to exclude light is to tie the outer leaves when the curd is 8 cm in diameter. Leaves may also be broken over the curd to prevent yellowing. In hot weather blanching may take 3 to 4 days, but in cool weather, 8 to 12 days or more may be required. Cauliflower fields scheduled to mature in cool weather (September and October) that are well supplied with water and planted with “self-blanching” cultivars will not need tying. Newer orange cauliflower and green broccoflower varieties are being planted. They are less susceptible to off-colors but still can develop purpling under warm conditions.

Cauliflower Ricing
“Riciness” and “fuzziness” in cauliflower heads is caused by high temperatures, exposure to direct sun, too rapid growth after the head is formed, high humidity, or high nitrogen. “Ricing” is where the flower buds develop, elongate and separate, making the curd unmarketable.

Development of Curd Bracts in Cauliflower
Curd bracts or small green leaves between the segments of the curd in cauliflower is caused by too high of temperature or drought. High temperatures cause a reversion to vegetative growth with production of bracts on the head. In a marketable cauliflower head, the individual flower buds are undeveloped and undifferentiated.

Scouting and Estimating Corn Yields

Jarrod O. Miller, Extension Agronomist, jarrod@udel.edu

Although corn is still undergoing grain fill in many Delaware fields, estimates of yield can be performed right now. The University of Kentucky has several methods for estimating yield, depending on the amount of information you have on hand: http://www2.ca.uky.edu/agcomm/pubs/agr/agr187/agr187.pdf. Since these methods involve counting kernels and rows on a few ears, you will need sharp eyes and the ability to keep your place.

Our variety trials in Georgetown were planted on May 2nd, so they have gone through the deluge of rainfall, as well as the droughty conditions during pollination. The earlier maturing varieties (110 days) were tasseling a little ahead of the later maturity (115 days) in the trials, during higher night and daytime temperatures. There could be many variables besides temperature that explain yield loss, including fertility, compaction and flooding. Actual ear size is determined during the vegetative stages, and also needs to be considered. However, by scouting fields right now, you can at least determine if you had some issues during the reproductive states. Tipback will be common (see the image below), where kernels were either not pollinated or aborted. Pollination occurs at the bottom of the ear first, so the most likely loss of kernels is at the tip.

Based off counting rows and kernels, we did observe a difference in potential yield between the earl, mid and late maturing varieties. The early and mid-maturities (110-112) both had yield estimates of 150-240 bu/acre, while the later maturity (115) was statistically higher at 185-290 bu/acre. Again, temperature alone does not explain the difference; it could also just be the hybrids. If weather is the issue, it is hard to control, but if it turns out fertility, disease or hybrid selection was the issue, we have a little more control when planning for next year.

Variable Growth in Plasticulture Vegetables

Each year we see fields were vegetable growth in drip irrigated plastic mulched beds is irregular. While root diseases, nematodes, or soil insect feeding can cause variable growth, the following are other potential causes due to water quality problems, cultural practices, or irrigation system issues.

Plugged Emitters
Drip emitters can become plugged with fine particles, mineral deposits, or biofilms. When emitters become clogged, the plants nearest the clogs will receive less water and have more water stress and grow less or be stunted. This is seen most commonly in higher density planted crops such as peppers.

A common cause of plugged emitters is water containing high levels of dissolved iron. This often causes a proliferation of iron utilizing bacteria. These bacteria can form heavy biofilms on the inside of the drip tube. They also oxidize the iron in the water (as part of their metabolism) and leave behind iron precipitates that can plug emitters. Chlorination of drip lines is needed to control iron bacteria.

Another common problem in some aquifers, is well water with high levels of calcium and magnesium (“hard water”). In high water pH conditions, these can precipitate out as calcium or magnesium carbonates that will clog emitters. If you look inside the drip tubing you will see a white or chalky film. In addition, if soluble phosphorus fertilizers are put into water with high levels of dissolved calcium or magnesium salts, they can precipitate out as calcium or magnesium phosphates, also plugging emitters. Acidification of water can reduce or eliminate this problem. Also, avoid running phosphorus through the drip if you have hard water.

Inadequate filtering is another possible cause of plugged emitters. While this is most common when using surface water from ponds, ditches or streams it can also occur in wells that have fine particles in the water.

Pinched Drip Tape
Drip tape pinching will reduce water past the pinched area and result in poor growth. Pinched or folded tape often occurs near connections with lay flat hose, where tomato stakes have been installed directly over the drip tape, and in rocky or cloddy soils.

Improperly Designed or Maintained Drip Systems
Improperly designed drip systems can lead to over-watering or under-watering portions of the bed and cause variable crop growth. This most commonly occurs when systems are in too large of zones or have too small of supply lines, where pressure and volume is too low, or where length of run is too long. In these cases, the ends of the drip line will have much less water than the beginning of the run and will lead to a gradient of plant growth. Leaks in drip lines will also cause lower water delivery past the leak, leading to reduced plant growth.

Variable Depth of Planting and Transplant Handling
Many transplanted crops will show variability due to depth of planting. This is most common when the root ball is left partially exposed and dries out. If these plants survive they often will be stunted or will have reduced growth compared to plants around them. Planting too deep can also lead to variability in some plants. Rough handling or root ball disturbance can slow establishment of sensitive transplants leading to variability.

Variable Bed Formation
Variability in bed density and plastic laying can cause differences in plant growth. This is most common when plastic is laid in wet or cloddy soils. This results in variable bed densities affecting root growth and water movement and variability in plastic contact with the soil surface leading to warm and cool spots thus slowing or speeding plant growth.

Periods of heavy rain in June and now July causes local flooding in plasticulture fields where water covered over the top of beds and filled the bed through the planting holes. These areas will have poor growth due to lack of oxygen to the root systems and generally do not recover.

Blossom End Rot Revisited

Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

Blossom end rot (BER) is showing up again this year in peppers and we expect it to be prevalent in tomatoes because of the recent hot weather. BER is a disorder where developing fruits do not have enough calcium for cell walls, cells do not form properly, and the fruit tissue at the blossom end collapses, turning dark in color. Calcium moves through cation exchange with water movement in the fruit, so the end of the fruit will be the last to accumulate calcium. Larger fruits and longer fruits are most susceptible. With fruits, the rapid cell division phase occurs early in the development of the fruit, the two weeks after pollination, and if calcium accumulation in the fruit is inadequate during this period, BER may occur. Over 90% of the calcium taken up by the fruit will occur by the time the fruit is the size of a nickel. While it may not be noticed until the fruit expands, the deficiency has already occurred and cells have already been negatively affected. We most commonly see signs of blossom end rot on fruits several weeks after the calcium deficiency has occurred.

Understanding blossom end rot also requires an understanding of how calcium moves from the soil into and through the plant. Calcium moves from the soil exchange sites into soil water and to plant roots by diffusion and mass flow. At plant roots, the calcium moves into the xylem (water conducting vessels), mostly from the area right behind root tips. In the xylem, calcium moves with the transpirational flow, the movement of water from roots, up the xylem, and out the leave through stomata. Calcium is taken up by the plant as a divalent cation, which means it has a charge of +2. It is attracted to negatively charged areas on the wall of the xylem, and for calcium to move, it must be exchanged off the xylem wall by other positively charged cations such as magnesium (Mg++), potassium (K+), ammonium (NH4+), or other calcium cations (Ca++). This cation exchange of calcium in the xylem requires continuous movement of water into and up through the plant. It also requires a continuous supply of calcium from the soil

In general, most soils have sufficient calcium to support proper plant growth. While proper liming will insure there is adequate calcium, it is not the lack of calcium in the soil that causes blossom end rot in most cases. It is the inadequate movement of calcium into plants that is the common culprit. Anything that impacts root activity or effectiveness will limit calcium uptake. This would include dry soils, saturated soils (low oxygen limits root function), compaction, root pathogens, or root insect damage. In hot weather on black plastic mulch, roots can also be affected by high bed temperatures. Low pH can also be a contributing factor. Calcium availability decreases as pH drops and below a pH of 5.2, free aluminum is released, directly interfering with calcium uptake. Again, proper liming will insure that this does not occur. Applying additional calcium as a soil amendment, above what is needed by normal liming, will not reduce blossom end rot.

In the plant, there is a “competition” for calcium by various plant parts that require calcium such as newly forming leaves and newly forming fruits. Those areas that transpire the most will receive more calcium. In general, fruits have much lower transpiration than leaves. In hot weather, transpiration increases through the leaves and fruits receive lower amounts of calcium. High humidity will reduce calcium movement into the fruit even more. Excess nitrogen that causes excess foliage will increase blossom end rot. Tissue tests will often show adequate levels of calcium in leaf samples; however, fruits may not be receiving adequate calcium. In addition, in hot weather, there is an increased risk of interruptions in water uptake, evidenced by plant wilting, when transpirational demand exceeds water uptake. When plants wilt, calcium uptake will be severely restricted. Therefore, excess heat and interruptions in the supply of water (inadequate irrigation and/or rainfall) will have a large impact on the potential for blossom end rot to occur. Proper irrigation is therefore critical to manage blossom end rot. This means a steady, even, uninterrupted supply of water in the soil surrounding the plant roots.

In high tunnels, lack of air movement can also be a factor, as transpiration is reduced, thus limiting calcium movement in the plant. In periods where tunnels are closed tight due to adverse weather, this may also increase the potential for blossom end rot. First fruits formed in early planted tomatoes and peppers are the most susceptible to blossom end rot, especially in high tunnels.

As a positive cation, there is “competition” for uptake of calcium with other positive cations. Therefore, if potassium, ammonium, or magnesium levels are too high in relation to calcium, they can reduce calcium uptake. To manage this, do not over-fertilize with potassium or magnesium and replace ammonium or urea sources of nitrogen with nitrate sources.

Applying additional soluble calcium through irrigation, especially drip systems, can reduce blossom end rot to some degree if applied prior to and through heat events and if irrigation is applied evenly in adequate amounts. Foliar applications are much less effective because fruits do not absorb much calcium, especially once a waxy layer has developed, and calcium will not move from leaves into the fruit (there is little or no phloem transport).

In conclusion, the keys to controlling blossom end rot are making sure roots are actively growing and root systems are not compromised, soil pH is in the proper range, and irrigation is supplied in an even manner so that calcium uptake is not interrupted. Supplemental calcium fertilization will only marginally reduce blossom end rot if water is not managed properly.

Corn Reproduction and High Temperatures

Jarrod O. Miller, Extension Agronomist, jarrod@udel.edu, Cory Whaley, Extension Agent – Agriculture, Sussex County; whaley@udel.edu, and Phillip Sylvester, Extension Agent – Agriculture, Kent County; philllip@udel.edu

Droughty conditions aren’t our only worry right now. Corn that survived the deluge of rain is tasseling in our earliest planted fields, and our current heat index can be an issue with pollination. While corn enjoys warmer weather, anything above 86°F will actually slow plant growth. A corn plant also prefers cooler nights, with temperatures in the 60s.

Pollination occurs during tasseling and silking stages, and high temperatures can adversely affect kernel formation either during pollination or grain fill. First of all, warmer weather this week may accelerate corn maturity earlier than we would like. High daytime temperatures (> 86°F) may limit photosynthesis which provides sugars for ear formation. A survey of worldwide research on corn growth and temperatures observed that the ideal temperature for growth as well as flowering was 86-87°F. Temperatures over 99°F severely affect pollen production during silking, but consecutive days in the 90s will at least reduce pollination. For grain fill, optimum temperatures are 80°F, with a maximum of 97°F. In the mid-west, high night-time temperatures (>72°F) have been observed to reduce grain fill, possibly due to the use sugars for respiration (energy) rather than kernel production.

This photo from last year shows a worst-case scenario when temperatures are high, with many kernels failing to pollinate. In most fields, it may just be a few kernels per ear that fail, but over several acres that can add up.

Over the week of June 28-July 4, when tasseling started in some fields, we have had temperatures at least 86°F, with four to six days in the 90s (Table 1). New Castle has seen two nights above 72°F, while both Kent and Sussex have had three. As of writing this, cooler days and nights are in the forecast, as more of our earlier planted fields will start pollination. That is good news for most fields that are just getting started, but for all the late-planted corn and replants, watch the temperatures in mid and late July. Temperature may be an additional factor to consider if this year’s yields are lower than expected.

Table 1: Number of days above threshold temperatures over the week of June 28-July 4

New Castle Kent Sussex
1 Week = 7 days possible
Daytime > 86°F 7 7 7
Daytime in the 90s 6 4 6
Nightime > 72°F 2 3 3


Sunburn in Fruiting Vegetables and Fruit Crops

Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

Recent weather has produced conditions where there is high potential for sunburn in fruits and fruiting vegetables. Growers may need to consider ways to protect against sunburn. Sunburn is most prevalent on days with high temperatures, clear skies and high light radiation. 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 fruits. 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. Research has shown that the fruit skin temperature threshold for sunburn necrosis is 100 to 104°F for cucumbers; 105 to 108°F for peppers, and 125 to 127°F for apples. Fruits with sunburn necrosis are not marketable. Injury may be white to brown in color.

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. 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. Damaged tissue is often white in color.

Storms that cause canopies in vine crops to be more open will expose fruits to a high risk of both sunburn necrosis and photooxidative 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.

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 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. These materials are kaolin clay based, calcium carbonate (lime) based, or talc based and leave a white particle film on the fruit (such as Surround, Screen Duo, Purshade 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.

While particle films have gained use in tree fruits, their usefulness in vegetables is still unclear. Research in a number of states has shown reduced fruit disorders such as sunburn in peppers and white tissue in tomatoes when applied over those crops. Watermelon growers have used clay and lime based products for many years to reduce sunburn in that crop in southern states.

There are some drawbacks to the use of particle films. If used for sunburn protection on fruits, there is added cost to wash or brush the material off at harvest. Where overhead irrigation is used, or during rainy weather, the material can be partially washed off of plants, reducing effectiveness and requiring additional applications. Produce buyers can also have standards relating to the use or particle films and may not accept products with visible residues.

Guess the Pest! Week #13 Answer: Ozone Damage

Bill Cissel, Extension Agent – Integrated Pest Management; bcissel@udel.edu

Congratulations to Keith McGowan for correctly identifying the damage in the photo as ozone damage and for being selected to be entered into the end of season raffle for $100 not once but five times. Everyone else who guessed correctly will also have their name entered into the raffle. Click on the Guess the Pest logo to participate in this week’s Guess the Pest challenge!

Guess the Pest Week #13 Answer: Ozone Damage

Photo and article by Nancy Gregory

Ozone damage on vegetables is observed during hot and humid weather in our region. The most susceptible crops include potato, watermelon, cantaloupe, pumpkin, squash, snap bean, and lima beans. Symptoms occur on the top, younger leaves, and show up as dark spots on potato. On cucurbits, ozone damage shows up as yellowing with spots having a white crusty center. Watermelon is the most susceptible in this group. On beans, spots are small and bleached looking, and leaves may have a bronzed appearance on the top surface. Affected leaves may yellow and drop. Ozone is easily misdiagnosed and hard to accurately confirm, since an episode of air pollution cannot be chemically tested for. Inversion layers in hot humid weather result in trapped air at the field level that has a high level of air pollutants. Air remains at field level until winds pick up and disperse it, and four or five hours at levels of 80 ppb can result in damage. Some cultivars have some tolerance, and keeping plant stress low can help to avoid damage.

Vegetable Recovery from Temporary Flooding

Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

On June 9, at our Georgetown research station, we received 4.2 inches of rainfall in a one-hour period. Many of our vegetable research plots were temporarily flooded and most of our plasticulture research area had water that topped the beds.

If this flooding would have lasted for more than 24 hours, most of our crops would have been lost. However, there was enough drainage early on so that the majority the area has come through, albeit with some complications. The following are some pictures that show the recovery and losses:

Pepper plants with leaf drop. Ethylene buildup in saturated soil conditions can cause leaf drop, flower drop, fruit drop, or early plant decline in many vegetable crops. The bed area around this plant was saturated for two weeks. For the plant to recover, the bed must dry out to allow the plant to regenerate roots. The pepper plant has additional sets of buds to allow for new leaves to form but the recovery, if it happens, will put the plant several weeks behind other non-affected plants. Another concern is potential for root rots. In severe cases, cutting away the plastic can save a crop by allowing for better drying out and aeration.

Watermelon plant in saturated bed conditions after flooding showing little growth. Oxygen starvation to vegetable roots will cause roots to cease to function resulting in plant stunting, or collapse, with limited recovery potential. For this plant to recover the bed must dry out and aerate enough for the plant to fully regenerate roots.

Flooded pickling cucumber planting. Soils in in this area stayed saturated for 3 days while seeds were germinating. Lack of oxygen caused seeds to deteriorate and plants did no emerger in much of the area.

Tomatoes fully recovered from temporary flooding. In this case the crop did not drop leaves and was able to regenerate new roots quickly.

Our watermelon variety trial stopped growing and aborted fruits after the flooding but has now fully recovered and plants are starting to fill in. The beds were allowed to dry out for 9 days after flooding and then the crop was fertigated with nitrogen and sulfur. New fruits are now being set but the trial will have few early watermelons.

Revisiting Rainshelters for Vegetable and Fruit Production

Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

I had an interesting conversation with a Delmava-based crop consultant on the use of rainshelters for vegetable and fruit production. As the climate is expected to become wetter with more extreme rainfall events, losses of vegetable and fruits to excess rain will become more of a concern.

A rainshelter is a high tunnel structure that is used to cover plants during fruit formation and development. Multi-bay “European” style tunnels are most commonly used as rain shelters covering from ½ acre to several acres. Rain shelters are used extensively in high rainfall areas such as England to protect rainfall sensitive crops such as strawberries. Covers may be used for the whole season or just for the fruiting period.

Rainshelters are also used for tomatoes and other fruits such as cherries which are susceptible to cracking. Some fruits crack from absorbing water through the skin of the fruit when they are ripe or near ripe, others crack with excess water in the root zone, and there can be a combination of the two processes. Rainshelters control both types of fruit cracking. Rain shelters also reduce foliar wetting and rain splash and therefore can reduce fungal and bacterial diseases considerably if left on for the whole growing cycle.

While a high tunnel will serve as a rain shelter, less expensive low-tunnel structures can also be used that have a plastic cover over hoops from 2 to 6 feet tall.

Rain shelters have been shown to improve the quality of tomatoes throughout the growing season and can be a valuable tool to increased marketable fruits of high quality. They are also useful for fruit crops such as cherries, strawberries, brambles, grapes, and blueberries. Specialty melons prone to cracking will also benefit from the use of rainshelters.

Dwarf cherries being grown under a rain shelter.

Dealing with Salt Affected Soils

Jarrod O. Miller, Extension Agronomist, jarrod@udel.edu

If you have had issues with tidal inundation of soils in the past, or had some flooding this spring, you may be thinking about remediation methods to get the salt content down. While gypsum is a great method of ameliorating these soils, it works best if your salt issues come from sodium (Na). The calcium (Ca) in gypsum can replace Na on your CEC, which will let rainfall or irrigation leach it below the root zone. Fields where Na contents is greater than 15% of the CEC are the most likely to show toxicity issues and should be taken care of first.

However, if your total salt content is high due to Ca, Mg and Na, gypsum will not solve this issue. In these fields irrigation is your best option with recommendations of up to 6 inches of water to reduce salinity by 50% and up to 24 inches to reduce it by 90%.