Posts Tagged ‘sweet corn’

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.

Leaf Scald in Sweet Corn Again in 2012

Thursday, July 12th, 2012

Gordon Johnson, Extension Vegetable & Fruit Specialist;

Several sweet corn varieties in our fresh market bicolor variety trial are showing leaf scald symptoms in 2012. We saw similar leaf scald last year in processing varieties. Leaf scald is a physiological disorder similar to necrotic sunburn in fruits and vegetables. It occurs when leaf temperatures rise above a critical level, cells die rapidly, leaving a bleached white appearance. While newly emerged leaves in the upper canopy of susceptible varieties that are the most exposed are the most likely to scald, some of the leaf scald we are seeing this year has progressed deeper into the canopy, even showing up on some of the corn husks. Leaf scald occurs most commonly when temperatures are in the high 90s or over 100, skies are clear (high solar radiation), and humidity is low. While effect on yield is usually minimal, leaf scorch at the ear leaf level can affect kernel fill.

Leaf scald symptoms on fresh market sweet corn in a 2012 trial of bicolor varieties.

Poor Vigor in Later Plantings of Sweet Corn

Friday, June 15th, 2012

Gordon Johnson, Extension Vegetable & Fruit Specialist;

Each year we see sweet corn field fields with stand and plant vigor issues even in corn planted later in the season when soils were warm. There can be many causes for stand loss and weak seedlings: surface compaction and crusting, soil insects, soil diseases affecting seeds or seedlings, wet soils, fertilizer injury, deep planting, and herbicide injury are just a few examples.

Corn seedlings depend on the seed for food to grow for several weeks after emergence until sufficient leaf area has been produced and nodal roots have become established. Sweet corn is more susceptible stand loss and poor vigor problems than field corn because the seed has less food reserves. If you dig up low vigor seedlings and kernels are disintegrated and there is darkening at the mesocotyl attachment this means that the seeds deteriorated prematurely and the full content of the food reserves in the seed were not available for seedling development leading to the stand and vigor issues.

Seed deterioration and/or poor vigor seedlings can be due to diseases that cause seed rots, seedling blights and/or root rots. Fungal disease organisms such as Pythium, Fusarium, Rhizoctonia, Aspergillus, and Penicillium are common in soils and many can even be carried on seeds.

While most of these seed diseases are problems in cold and wet soils, Penicillium is a common problem in warmer soils. Penicillium can survive in the soil and can also be seed borne. Plants infected with Penicillium will be stunted and off-color and seeding roots and mesocotyls will show discoloration below ground. Blue-green mold may evident on or in the seed remnant.

Fungicide seed treatments are critical to control seedling diseases and a systemic fungicide such as difenoconazole (a component of Dividend Extreme) will be necessary for diseases such as Penicillium that can be seed borne.

Poor vigor can also result from poorer quality seed. Work with seed suppliers to obtain their best seed lots and the largest seed sizes. Avoid old seeds and obtain varieties that known for good seedling vigor.

Problems With Corn Seedlings This Year

Friday, June 1st, 2012

Nancy Gregory, Plant Diagnostician;

Uneven stands have been seen this spring in corn fields. Corn seedling issues have been attributed mostly to environmental conditions, including dry soil at planting, resulting in variable depth of seeds and uneven emergence. Our early May weather resulted in wet, cool soil conditions conducive for fungi that invade young seedling roots and mesocotyls. Stress brought on by recent hot weather has exacerbated stress on seedlings. A recent article from Purdue University highlighted some issues also seen in the Midwest that apply to corn in Delaware:

Options for Postemergence Weed Control in Sweet Corn

Friday, June 1st, 2012

Mark VanGessel, Extension Weed Specialist;

Two broad-spectrum herbicides that have exhibited good crop safety to sweet corn are Impact and Laudis. Both products perform better with 0.25 to 0.5 lbs of atrazine. Both will control a broad range of weeds and grasses. It is important to consider crop rotation prior to treating sweet corn fields with a postemergence herbicide. Double-cropped vegetables are very problematic since few products allow such short rotations. Products that will allow double-cropping include Aim, Basagran, and Cadet. But these products only control small (less than 2 to 3 inches tall) plants. Most other herbicides are either not labeled for short rotations, or they have precautions about potential crop injury. Be sure to read and follow herbicide labels.

Risk of Stewart’s Wilt in Sweet Corn is High in 2012

Thursday, March 29th, 2012

Bob Mulrooney, Extension Plant Pathologist;

The Stewart’s wilt prediction for 2012, as you can imagine with a very mild winter, is severe for all three counties in Delaware. We have not had such a mild December, January and February since the winter of 2001-2002 when the Index was 123.1 for Georgetown and 123.0 for Newark. The following table tells it all.

Winter Temperature Index For Predicting Stewart’s Wilt in Delaware Sweet Corn: 2002-2012

Average monthly temperatures in °F at Georgetown, DE, REC: 2002-2012

  2011-2012 2010-2011 2009-2010 2008-2009 2007-2008 2006-2007 2005-2006 2004-2005 2003-2004 2002-2003
December 43.2 31.3 37.9 41.8 39.7 43.5 36.2 38.9 38.6 36.7
January 39.5 31.0 32.7 31.0 36.8 39.7 43.0 34.9 29.5 28.9
February 40.4 39.6 31.1 39.2 39.9 30.1 37.4 36.7 35.2 33.8
INDEX 123.1 101.9 101 .7 112.0 116.4 113.3 116.6 110.5 103.3 99.4


Average monthly temperatures in °F at Newark, DE, Experiment Station: 2002-2012.

  2011-2012 2010-2011 2009-2010 2008-2009 2007-2008 2006-2007 2005-2006 2004-2005 2003-2004 2002-2003
December 41.3 30.8 34.9 37.1 37.5 42.5 34.0 35.5 34.0 33.5
January 37.0 28.7 31.6 28.0 35.5 37.3 39.5 31.0 26.4 27.1
February 39.5 35.2 31.0 35.8 36.5 27.8 34.5 34.2 33.1 29.5
INDEX 117.8 94.7 97. 5 100.9 109.5 107.6 108.0 100.7 93.5 90.1

Severity Index: < 90, usually absent; 90-100, intermediate; >100, usually severe.

The index is used to predict overwintering flea beetle populations that vector the Stewart’s wilt bacterium, Pantoea stewartii.

Prediction for 2012

      Average Temperature for Dec, Jan & Feb
Location Prediction   2011-2012 2010-2011
Georgetown 123.1=Severe   41.0°F 34.0°F
Dover 121.0=Severe   40.3°F 32.5°F
Newark 117.8=Severe   39.3°F 31.6°F


Control Strategies
For processing and fresh market growers the high risk of Swetart’s wilt means that if you are planting susceptible or moderately susceptible hybrids, flea beetle control is very important. A number of strategies are available including seed treatments, granular insecticides at planting and/or foliar applied insecticides after emergence. For foliar applied insecticides treat susceptible cultivars at spike stage when 5% of the plants are infested. See the 2012 Delaware Commercial Vegetable Production Recommendations for control suggestions.

Note: Weather records from University of Delaware Carvel REC, Georgetown, DE, DE State Fire School, Dover, DE and University of Delaware Ag Experiment Station Farm, Newark, DE. Data records found online at

Vegetable Crops are Off to an Early Start

Thursday, March 29th, 2012

Gordon Johnson, Extension Vegetable & Fruit Specialist; & Emmalea Ernest, Extension Associate-Vegetable Crops;

Asparagus harvest began last week which is four weeks early. Pea planting is ahead of schedule with heat units accumulating much more rapidly than the thirty year average making it likely that harvest will not extend past June. Peas have emerged rapidly and stands look very good. Average soil temperatures have already been in the mid 60s. Some fresh market sweet corn growers have their first and second plantings in already and sweet corn under plastic has emerged and is growing fast. Plastic mulch laying for summer crops is already underway. As we progress into April, a concern will be the potential for frosts and freezes with advanced crops.

The earliest peas will start flowering at less than 675 heat units and will be at full flower at 775 heat units. Currently, peas planted on February 25 have accumulated 306 heat units. In 2011 during the same period only 181 heat units accumulated. If accumulations continue at an accelerated pace and peas flower early, there is increased risk for frost damage to flowers in early plantings, causing reduced yields, and split sets.

Early planted sweet corn can also be at higher risk this year of frost damage. If the sweet corn growing point is not out of the ground and a freeze occurs, the emerged leaves will be injured but the plants can continue to grow. The growing point of sweet corn is still below or at ground level until the 5-6 leaf stage and is therefore protected against frost injury. If a frost or freeze event does occur with sweet corn, wait 3-4 days to evaluate. If you seed signs of regrowth, the sweet corn can recover. If regrowth is not evident, start splitting the stems of a sample of plants and look at the growing point. If it is firm and white or cream colored it is still alive. If it is soft, water soaked, or gray in color, it has died and you will lose some stand.

Processing sweet corn planting will begin in the next 2 weeks. It is interesting to note that the industry is moving to supersweet varieties for processing. In the past, our recommendations were to avoid early planting of supersweets and wait until soil temperatures were above 65° F, because these varieties tended to have less seed vigor. In 2010 and 2011 we planted supersweet variety trials on April 12. Surprisingly, a number of the supersweet varieties we tested had emergence rates over 90%. This suggests that high quality seed of supersweet varieties with demonstrated cold tolerance can probably be planted earlier, before soil temperatures reach 65°F. Detailed reports on the 2010 and 2011 trials are available online at This year, early April supersweet plantings may be justified, given that soils have already warmed substantially.

Some rye windbreaks used for watermelon and other vegetable protection have already reached full height and are heading out. Growers may have to kill windbreaks much earlier than normal or realize that seed will likely set which is a problem if rotating into wheat or barley (seed mixing with rye next year). Forage radishes and spring oats did not fully winter kill this year.

As we go into next week, highs are forecast in the 60s and 70s. However the extended forecast for both 6-10 and 8-14 days from NOAA is for “enhanced odds for below average temperature along the Mid-Atlantic”

Processing Acres Up This Year

Friday, March 2nd, 2012

Gordon Johnson, Extension Vegetable & Fruit Specialist;

Processing vegetable acres will be up this year in Delaware and on Delmarva including peas, lima beans, and sweet corn. Pickle acreage will remain steady. This increase in acres is largely due to reduction in stocks. For example, for lima beans, cold storage stocks in January 2011 were 58 million pounds and in January 2012 these stocks were down to 44 million pounds. Peas were down from 234 to 210 million pounds and sweet corn was down from 521 to 436 million pounds. This drop in sweet corn stocks is a major driver in picking up acres. It is interesting to note that in 2011, Delmarva lima bean acres were close to 19,000. This represented over 60% of the US acres and 40% of the production. California acres are down due to competition for land with more profitable crops.

Sweet Corn Fungicide Label Updates

Friday, March 2nd, 2012

Bob Mulrooney, Extension Plant Pathologist;

A few new fungicide combinations are now labeled for sweet corn as well as field corn. Stratego YLD is a new combination of trifloxystrobin plus Proline (prothioconazole). Headline AMP is a combination of pyraclostrobin and Caramba (metconazole) and Quilt Xcel is a higher concentration of Quadris (azoxystrobin) plus Tilt (propiconazole). They will provide excellent control of Northern corn leaf blight, gray leaf spot, and rust if used as directed on the label.

Vegetable Disease Updates – August 12, 2011

Friday, August 12th, 2011

Bob Mulrooney, Extension Plant Pathologist;

Sweet Corn
With a return to some cooler weather be on the lookout for corn leaf rust on sweet corn. Scout the plantings and if you see rust on plants at the whorl stage or younger, rust could become an issue if the hybrid is not resistant to rust. Rust, when heavy, can affect plant health and reduce ear size. The best control is to plant resistant hybrids, but the strobilurin or triazole fungicide work well. On fresh market corn rust on the husks makes ears unsightly to consumers.

Cucurbit downy mildew
was recently observed on cantaloupe in the sentinel plot in Newark. These lesions resemble the same symptoms as seen on cucumber but spore production appears to be very sparse. It has not moved to any pumpkin, winter squash or watermelon so far. The susceptible cucumber in the plot is almost totally defoliated.

Lima Beans
So far weather conditions have not been favorable for downy mildew. It looks like the weather may be changing and getting a bit cooler with more dew and possibly fog in the early morning hours. If it should start raining soon growers and crop consultants should be scouting for downy mildew. Race F of Phythophthora phaseoli was the only race identified in 2006, 2008, 2009, and 2010. Preventative applications of 2 lbs fixed copper, 2 lbs Ridomil Gold/Copper, or 3- 4 pts Prophyte have provided control of downy mildew in the past. The newest formulation of fixed copper from DuPont is Kocide 3000 and it performs as well as the other formulations of copper at the rate of 1.3 lbs/A. The best controls continue to be Ridomil/Gold Copper, Prophyte, or other labeled phosphonate fungicides, and Omega, especially when disease pressure is high. Application at flowering or when pods are first forming is recommended if weather is favorable for disease. If disease is present Ridomil/Gold Copper and phosphonate fungicides have shown to provide some curative activity if applied when downy mildew is first seen. If downy is present in the field do not use copper fungicides alone for curative control, they will not provide control. Another product that is labeled on lima beans for white mold control is Omega but not downy mildew, but in DE this would be a 2ee use that someone like me can recommend since the fungicide is labeled on lima beans. I have three years data that show excellent control of downy mildew at 5.5 fl oz and 8.0 fl oz/A as a preventative application (before disease is found in the field). Omega is not labeled for aerial application, however. Headline from BASF is also labeled for downy mildew. I have tested it and it has provided good control of downy when applied on a 10-day schedule at 6.0 fl oz /A. It does not give as good disease control as Ridomil Gold/Copper or the phosphonates preventatively but the yields have been comparable. It is also labeled for anthracnose which the other products do not control.

Downy mildew caused by Phytophthora phaseoli

Downy mildew on raceme and petiole

Phytophthora capsici on lima bean pod.

Phytophthora capsici will infect lima bean pods as well and can look very similar to downy mildew. P. capsici or lima bean pod rot is usually found in wet low spots in the field. The fungus growth looks more granulated or “pebbly” than downy mildew, microscopic confirmation is encouraged.

Downy mildew on the upper pod and lima bean pod rot on the lower pod. Note the granular appearance of the fungus on the lower pod and the lack of a reddish brown border on the pod infected with lima bean pod rot or Phytophthora capsici.