Posts Tagged ‘20:3’

Watermelon Seedling Diseases in the Greenhouse

Thursday, April 5th, 2012

Kate Everts, Vegetable Pathologist, University of Delaware and University of Maryland;

Bacterial fruit blotch (BFB)
of watermelon, which is caused by the bacterium Acidovorax avenae subsp. citrulli, produces large olive green to brown water-soaked lesions on fruit, making them unmarketable. Symptoms of BFB on seedlings are water-soaked areas of the lower surface of the cotyledons and inconspicuous lesions on leaves. BFB lesions will become necrotic often with yellow halos. Lesions are frequently delimited by veins. Infected seedlings collapse and die.

If the bacterium is present, conditions in greenhouse transplant houses are highly favorable for the development of BFB symptoms and the spread of disease. Good practices for greenhouse transplant production are to disinfect surfaces before planting (benches, walls, walkways, etc.). The seed source should have tested negative for the pathogen with a minimum assay number of 10,000 seeds. Clean transplant trays (disinfect trays if they will be reused) and new soil must be used. Destroy any volunteer seedlings and keep the area in and around the greenhouse weed free. Avoid overhead watering if at all possible, or water in the middle of the day so that the plants dry thoroughly before evening. The bacterium can spread on mist and aerosols, so keep relative humidity as low as possible through proper watering and good air circulation in the greenhouse. Separate different seedlots, to reduce lot-to-lot spread. If BFB is suspected, collect a sample and submit it to your Extension educator, or specialist. Destroy all trays with symptomatic plants. Remove adjoining trays to a separate – isolated – area for observation. Monitor these isolated seedlings daily and destroy trays where symptoms develop. The remaining trays should be sprayed with a labeled fungicide and the applications continued until the plants are transplanted to the field.

Olive green water-soaked lesion on watermelon fruit. (Image courtesy David B. Langston, University of Georgia,

An inconspicuous lesion of bacterial fruit blotch on a watermelon transplant.

Other Greenhouse Diseases:

Angular leaf spot, which also is a bacterial disease, looks similar to BFB. This “look-alike” disease occurred in Delmarva’s greenhouses several years ago. Symptoms are small dark brown irregular lesions on cotyledons or leaves. Angular leaf spot is favored by cool wet weather. Usually conditions after transplanting to the field do not favor angular leaf spot disease development.

The fungal diseases gummy stem blight, Alternaria leaf blight, anthracnose, and Fusarium wilt can also be introduced into the greenhouse on watermelon seed or through inoculum from a previous crop. Diseases that are transmitted on seed often are randomly located throughout the greenhouse. Initial infections will occur as ‘foci’ or clusters of diseased plants.

Gummy stem blight infected transplants occur as clusters in an area around the initial infected seedling (foci).

Although I have not seen Fusarium wilt infected transplants in local commercial greenhouses, it has occurred in other states. Symptoms are wilted seedlings that may remain green or become chlorotic (yellow). This disease is of special concern because new strains or races can be introduced into an area on seedlings grown from infested seed.

Bottom line: If the seedlings appear diseased, identification of the problem is critical. Do not ship any trays containing plants with disease symptoms.

Tomato Pinworms Can be a Problem for Greenhouse Growers

Thursday, April 5th, 2012

Cory Whaley, Sussex Co. Extension Ag Agent; and Joanne Whalen, Extension IPM Specialist;

Tomato pinworms have recently been found infesting tomatoes in a greenhouse in Delaware. According to Jerry Ghidiu, an entomologist with Rutgers University, once pinworms become established they can move quickly through the house and threaten the entire crop. He stated that tomato pinworm is difficult to control and it is best to catch it early; multiple applications of insecticides will be needed for control. First instars are foliar feeders and mine into the leaf. Older larvae may fold the leaf over itself, or stick leaves together. In severe infestations all leaves are infested and the crop may have a burnt appearance. Larvae can then move into the fruit, making control much more difficult. Larvae will leave small pinholes at entry points. Pinworm can complete its life cycle in 28 days and there may be more than 7-8 generations per year. Adults are small gray moths and once adults are seen, infestations are severe. Pheromone traps can be used to detect and monitor the moths.

Entomologists in the region recommended Pylon as the best control labeled for greenhouse tomato use to control this pest. They also noted Entrust as another material to consider especially for organic production; however, it carries the following restriction: “Do not apply to seedling fruiting vegetables (which includes tomatoes) and okra grown for transplant within a greenhouse, shade house, or field plot.” As with all materials, you need to read the label for rates and restrictions before applying. Please refer to the Commercial Vegetable Production Recommendations for additional products to use in a rotation.

 Tomato pinworm larva

 Tomato pinworm larva on a damaged leaf

Tomato pinworm frass and leaf damage

Burnt appearance and leaf folding caused by tomato pinworm feeding

Can Antitranspirants and Antidesiccants Improve Vegetable Transplant Survival?

Thursday, April 5th, 2012

Gordon Johnson, Extension Vegetable & Fruit Specialist;

Antitranspirants and antidesiccants are materials applied to plants to limit water loss. They have been used with some success in horticulture, especially in the ornamental industry. I was recently asked if they could be used to improve transplant survivability for vegetable crops. Research has shown some benefits, but results have been inconsistent.

Transplants most commonly desiccate when water loss exceeds water uptake. This commonly occurs in transplants because root systems are small, limiting uptake, and water loss in windy conditions or on hot days is high.

Remember that most water moves out of plants through regulated pores in the leaves called stomates. This is called transpiration. These openings have specialized cells that can open and close, depending on environmental conditions and plant internal signals. Controlling stomates and transpiration can reduce water loss and plant wilting.

Leaves also have waxy cuticles that limit non-stomatal water losses. However, in plants that have thin cuticles, in new leaves where cuticles are still forming, and under extreme drying conditions, such as high winds, water loss through the cuticle can be significant.

Antitranspirants/antidesiccants work in one of 5 ways:

1. Chemicals such as hexaoctadecanol, cetyl-alcohol and steryl-alcohol reduce transpiration by entering the leaf and forming a barrier from within to reduce transpiration loss of water.

2. Chemicals that are metabolic inhibitors such as PMA and DSA prevent stomatal opening.

3. The plant hormone abscisic acid (ABA) causes stomatal closure.

4. Wax and oil emulsions or chemical film materials such as di-1-p-Menthene prevent water loss by completely covering the leaf surface with a film. This limits losses through the epidermis and by covering part of the stomatal opening.

5. Reflective antitranspirants, most commoly clay based, reflect light energy thus reducing leaf heating and water losses. This is most useful for later plantings under heat load. It does not have much impact on wind desiccation.

Not all of these materials are registered for food crops so you need to read the label before using them on vegetables. The most commonly used with vegetables have been film coverings and reflective materials.

In addition to these materials, gels have been used to protect transplant roots. Transplants are dipped in the gels prior to planting and this can reduce root loss due to drying and thus improve the ability of transplant roots to survive, grow, and take up water.

In a 4 year trial with cantaloupes, researchers in Nebraska evaluated the effects of an antitranspirant (Folicote) sprayed on plants and a polyacrylamide gel root dip (SuperSorb) on early growth of transplanted muskmelon with or without windbreak protection. They found that “overall transplanting success and early growth were enhanced the most by wind protection, followed by the polyacrylamide gel root dip, and least by the antitranspirant foliar spray”.

Currently research is underway looking at the plant growth regulator ABA for height control of transplants and to improve early season survivability. It is being trialed in crops such as tomato and watermelon in several states. ABA is the plant hormone that controls stomates. Results have been encouraging and it may be labeled for this use in the future.

My base recommendation is that for maximum transplant survival you should produce compact, well hardened off transplants and provide wind protection at transplanting. Antitranspirants and antidesiccants are additional tools that can marginally improve transplant success.

Avoiding Failures with Early Planted Vegetables

Thursday, April 5th, 2012

Gordon Johnson, Extension Vegetable & Fruit Specialist;

The mild weather has many growers eager to get an early start with summer vegetables. Early markets are often the most profitable with higher prices. However, growers should proceed with caution and realize that failures can occur if cold sensitive vegetables are planted when temperatures are sub-optimal. As we get back to more seasonable weather in April, there will be many nights ahead with temperatures in the 30s and frosts and freezes are still a concern.

Each vegetable crop has a minimal temperature at which growth will occur. Our summer vegetables such as tomatoes, peppers, cucumbers, watermelons, and squash simply do not grow if temperatures are in the 40s or 50s. Squash and cucumbers do not put on growth with temperatures below 60°F, cantaloupes, watermelons, tomatoes, peppers, and eggplants will not put on growth with temperatures below 65°F. If temperatures are below these minimums, plants will just “sit still” and will be at risk of cold injury, wind injury, and damage from early season insects and diseases. Cold soils will limit root growth, further placing plants at risk due to inadequate water uptake and the risk of desiccation. Excess cold can also stunt some summer vegetables so that they do not fully recover. This is especially true of cantaloupes.

When planting summer vegetables early, growers need to consider all the tools available to maximize heat accumulation and minimize heat loss. The following is a list of these tools:

  • · Use raised beds or ridges. Ridges that are oriented east-west with crops planted on the south side, will benefit from the additional heat accumulation from the increased solar radiation on that side. Sandy soils heat up quicker due to lower water content.
  • · Use planted windbreaks, most commonly rye, between beds or rows. Windbreaks reduce heat loss from cold winds and help to accumulate heat. Rye reaches full height by the end of April on most of Delmarva. Cold winds are the most damaging to summer crops. Sand blasting during dry wind storms can actually cut plants off at the soil level. Growers doing field plantings for early crops in unprotected areas should always use windbreaks.
  • · For direct seeded crops, choose cold tolerant varieties, plant shallower and into well drained soils, and choose protected fields for earliest plantings. Also till soils well ahead of plantings to allow for them to heat up. Plant as soon as soil temperatures are adequate for germination. Also choose seed that has high quality and performs well in a cold germination test.
  • · To warm the soil more quickly, use plastic mulches. Plastic mulches increase soil temperature and help hold heat during night periods. They can increase soil temperatures 5-20 F° depending on mulch color. In order of lowest to highest heat accumulation Black < Red < Blue < Olive/Brown < Clear in selecting mulches. Mulches should be laid tight on a firm moist bed that is clod free. This will allow for more effective heat transfer and accumulation. Loose plastic and cloddy soils will reduce plastic mulch benefits.
  • · Use clear poly plastic covers. Most commonly, these come with slits or perforations to vent excess heat. They can be placed over direct seeded or transplanted crops with wire hoop supports (low tunnels) or they can be placed over ridges with transplants or seeds planted in the depression between the ridges. Zip tunnels and vented systems, where clear plastic can be easily closed and opened, have also been used. High tunnels also use poly plastic for protection and heat accumulation. I will discuss high tunnel management further in additional articles.
  • · Use spun bond poly or woven poly floating row covers to insulate, frost protect, reduce wind, reduce heat loss from soils and beds, and accumulate some heat. They can be placed directly over low growing crops such as strawberries or can be used with wire supports for other crops. The insulation they provide can protect 2-8 F° depending on thickness. Usually a 0.9-1.2 oz. cover is used to provide protection but not limit light too much.
  • · For smaller plantings, use of additional heat sinks to absorb heat during the day and then release it at night can promote earliness. Heat collection devices are usually filled with water and may be clear or black plastic containers or tubes.

Combinations of these practices will provide greater cold protection, heat accumulation, and earliness. This could include plastic mulch + row cover, plastic mulch + clear row cover + floating row cover, plastic mulch + row cover + heat sink, plastic mulch + clear row cover + floating row cover + heat sink. Use of these combinations in a high tunnel will further enhance success with early planted summer vegetables.

Vegetable Crop Insects – April 6, 2012

Thursday, April 5th, 2012

Joanne Whalen, Extension IPM Specialist;

The first asparagus beetles are now active. Since harvest has started, be sure to check for asparagus beetles laying eggs on asparagus spears. As a general guideline, a treatment is recommended if 2% of the spears are infested with eggs. Since adults will also feed on the spears, a treatment is recommended if 5% of the plants are infested with adults.

Imported cabbage worm (ICW) and diamondback moths (DBM) can be found laying eggs in recently planted fields. Be sure to begin scouting fields within a week of transplanting for recently hatched larvae. As a general guideline, a treatment is recommended if you find 5% of the plants infested with larvae. If DBM is the predominant species, be sure to select an insecticide for this insect pest since it can be difficult to control. Materials labeled on DBM include Avaunt, Coragen, the Bt insecticides, Proclaim, Rimon, Radiant, Synapse, Vetica, and Voliam Xpress.

This week’s weather has been favorable for aphid development so be sure to sample for pea aphids as soon as small seedlings emerge. On small plants, you should sample for aphids by counting the number of aphids on 10 plants in 10 locations throughout a field. On larger plants, take 10 sweeps in 10 locations. As a general guideline, a treatment is recommended if you find 5-10 aphids per plant or 50 or more aphids per sweep. In general, aphid development is favored by cool, dry weather which slows beneficial activity but is favorable for the development of aphids.

Season Extension Workshop & Tour

Thursday, March 29th, 2012

Friday, April 20, 2012     1:00-4:00 p.m.
Delaware State University
Outreach & Research Center
Smyrna-Leipsic Road, Smyrna, DE

This workshop for farmers and Ag service providers is presented by DSU Cooperative Extension.

Penn State University Professor of Vegetable Crops, Bill Lamont,
is a renowned expert in plasticulture and high tunnels.  He will teach participants how to prepare their tunnels for the growing season including choosing the right locations, soil preparations, and even trellising options.

DSU’s Dr. Rose Ogutu will share her experience of last season’s high tunnel vegetable production, including growing organically and extending the season.

DSU’s Mike Wasylkowski will show off some of his successes using various season extension technologies including transplants, row covers, and more.

Topics Include:
· Soil Preparations
· High Tunnel Options
· Tunnel Tomatoes
· Trellising Choices
· Greens and Other Veggies
· Vegetable Transplants
· Long-Storage Products
· Staggered Plantings
· Farmer Perspective

Participants can make hands-on comparisons of materials and techniques while touring the farm!

To register for the free workshop or for more information, call Jason Challandes at 302-388-2241 or by emailing

RSVP by Friday April 13, 2012