Posts Tagged ‘greenhouse’

Don’t Mix PVC Pipe and Polyethylene Greenhouse Film

Friday, September 14th, 2012

Emmalea Ernest, Extension Associate-Vegetable Crops;

Here is another article you can file under “Learn from My Mistakes/Mishaps” (previous example here). When my inflated poly greenhouse at the Georgetown research farm went flat about a month ago I thought the blower had gone bad. It turned out that there were actually giant holes in my less-than-one-year-old five-year greenhouse film. The holes corresponded to areas of wear in places where the plastic touched the PVC electrical conduit. There was no sign of wear around the metal support structure. Could the conduit be reacting with the plastic film?

Greenhouse film around the PVC conduit with cloudy discoloration and signs of wear.

One of numerous large holes where the conduit touches the greenhouse film.

Yes, it turns out, that PVC electrical conduit can out-gas chlorine which destroys the UV light stabilizers in the polyethylene greenhouse film and causes the film to degrade. In talking with various greenhouse suppliers and plastic manufacturers I learned that there is a sizable list of materials that can react with greenhouse film and cause wear including any type of PVC pipe or PVC tape, oil based paints or wood preservatives, chlorine based disinfectants, and certain pesticides, especially those containing sulfur or copper.

To prevent premature wear to greenhouse film, manufacturers recommend that surfaces that the film touches should be covered with white acrylic latex paint (but not the mildew resistant kinds that contain fungicides) or non-PVC tape. The film should not be in direct contact with any of the materials listed above that can cause breakdown or wear. In fact failure to follow these recommendations can cause the warranty on the film to be void. There seem to be some differences between manufacturers in tolerance of films to chemical interactions and installation recommendations and requirements, so it is a good idea to check with your greenhouse film supplier and the film manufacturer if you have a concern.

I have been told that some growers wrap PVC pipe that contacts the greenhouse film with old poly film. That would be an inexpensive means of dealing with the problem, but in my greenhouse I was concerned about that creating habitat for pests and diseases. After talking with the plastic manufacturer of the new film I am purchasing, I have decided to try covering the conduit with pipe insulation made out of polyethylene, which is readily available and inexpensive.

Some of the problematic conduit, soon to be covered with pipe insulation.

Managing Diseases of High Tunnel Tomatoes

Friday, June 1st, 2012

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

I have received several questions about timber rot caused by Sclerotinia sclerotiorum, leaf mold caused by Fulvia fulva, and gray mold caused by Botrytis cinerea over the past week for greenhouse and high tunnel tomatoes in Maryland and Delaware.

Timber rot is common where tomatoes (or another susceptible host) have been planted in ground beds in the past. The fungus Sclerotinia sclerotiorum causes disease on hundreds of plant species. Therefore rotation is difficult. Even when a high tunnel is moved between seasons, the disease can be severe because the fungus overwinters both in and around the tunnels. Usually the primary source of inoculum is outside of a high tunnel. In the spring when the soil is moist, the fungal fruiting bodies emerge and spores (ascospores) are released. These ascospores will be released continually throughout the spring and are carried on wind into the doors or raised sides of nearby high tunnels. Ascospores are usually carried or dispersed less than 330 feet. Therefore it is important to use sanitation within 330 feet of a high tunnel. No plants, leaf clippings, potting mix, or soil from the tunnels should be discarded within this area.

There are some practices that will help reduce timber rot pressure, such as minimizing the length of time that the soil stays wet. The biocontrol, Contans has been effective in managing Sclerotinia diseases in the field. Contans, which is a formulation of the fungus Coniothyrium minitans, parasitizes the survival structures of S. sclerotiorum. If it is sprayed on the area around the high tunnel and watered into the soil, it may help reduce ascospore formation in future years. Because the product is a live organism, it must be handled carefully to preserve its effectiveness. Contans would be a good choice for fields or areas around high tunnels, which are used repeatedly for a susceptible crop. See the Contans label for additional information. Other products labeled for Sclerotinia timber rot are Endura, which is labeled for field use, and Botran, which is labeled for greenhouse use.

Leaf mold and gray mold are both favored by high humidity and therefore improving air flow can reduce the extent of disease spread. There are several fungicides that are labeled for greenhouse use that will help reduce disease. These include Scala for leaf mold, Mycostop and Decree for suppressing gray mold, mancozeb products such as Dithane F-45, and copper. In addition to timber rot, Botran has activity on gray mold.

Air Pollution Damage to Transplants in the Greenhouse

Thursday, May 3rd, 2012

Gordon Johnson, Extension Vegetable & Fruit Specialist;

We have recently seen air pollution damage to greenhouse transplants. This can occur where coal or wood burning stoves are used and exhaust escapes when loading, where temporary unvented heaters are used in greenhouses, where heat exchangers in vented heaters have cracks, where exhaust pipes are leaking, or where fumes from burn piles or other sources are drawn into houses.

In fuel combustion, noxious gases can be produced if combustion is not complete. This can include: ethylene, sulfur dioxide, nitrous oxide and carbon monoxide. Even a clean burning furnace can have problems in airtight plastic greenhouses in cold periods where heaters are in constant operation. The level of oxygen can be depleted over several hours of continuous heating thus starving the combustion process of adequate oxygen and contributing harmful gasses.

During the combustion process, sulfur in fuel sources is converted to sulfur dioxide. If this leaks into the greenhouse and combines with the moisture there, sulfuric acid is formed. Low levels of sulfur dioxide may result in flecking and premature leaf drop. Higher levels can cause severe leaf burn, especially on young leaves.

Ethylene is a clear, odorless gas is a byproduct of the combustion of fuels. Ethylene can be damaging at levels as low as 0.05 ppm and even short exposures can cause leaf distortion, abortion of flower buds, defoliation and chlorosis. For more information on ethylene in greenhouses see the past article by Jerry Brust titled ‘Greenhouse Air Pollution Caused by Ethylene’ in WCU 17:6.

Tomato Spotted Wilt Problems in High Tunnel/Greenhouse Tomatoes

Friday, May 6th, 2011

Jerry Brust, IPM Vegetable Specialist, University of Maryland;

I recently visited a grower that grows both tomatoes and bedding plants. The plants are grown in a high tunnel-like setting, i.e., with heat. The grower was having problems in his tomato plants, but not in his bedding plants. The tomato plants looked like they had tomato spotted wilt virus (TSWV). The symptoms were found on most of his tomato plants, which would be unusual, as most of the time only an occassional plant here and there would be infected with the virus. The grower thought that is what he had as did an alert County Educator—and they were right. The grower unfortunaetly had a perfect storm in his operations that produced high percentages of TSWV infection in his tomato crop, but not his bedding plants.

Tomato spotted wilt virus is an obligate parasite; it must have a living host and must be moved from one plant to another by thrips or through cuttings or possibly seed. TSWV is transmitted most efficiently by Western flower thrips (WFT) (Frankliniella occidentalis), and less so by Onion thrips (Thrips tabaci), Tobacco thrips (Frankliniella fusca) and a few other thrips species. It is not transmitted by Eastern flower thrips (Frankliniella tritici).

WFT completes its life cycle in about 10-18 days. Eggs are laid in the leaf or tomato fruit. When WFT oviposit into tomato fruit they often cause a deeper dimple (black arrows Fig. 1) than other thrips species and very often the dimple is surrounded by a halo of white tomato tissue (white arrow Fig 1). This is how I could tell the grower had WFT present at one time (when I visited I could find no live thrips) at fairly high levels by the ovipositioning marks on his tomatoes. Larvae hatch in about three days and immediately begin to feed and in so doing pick up the virus. After four days, they pupate in the soil, and in a little over three days, the pupae become adults. Only immature thrips can acquire the virus, which they can acquire within 15 minutes of feeding, but adults are just about the only stage able to transmit the virus. Adults can transmit the virus for weeks.

TSWV infected leaves may show small, dark-brown spots (Fig. 2) or streaks on stems and leaf petioles (we found one prickly lettuce weed with such a symptom). Growing tips are usually affected with systemic necrosis and potentially stunted growth. Tomato fruit will have mottled, light green or yellow rings usually with raised centers (Fig 3).

Weed hosts function as important virus reservoirs for TSWV and can survive in and around greenhouses or even high tunnels through the winter. Some of these weeds include prickly lettuce, chickweed, (Fig. 4) spiny amaranth, lambquarters, black nightshade, shepherd’s purse, galinsoga and burdock. This grower unfortunately had a good crop of prickly lettuce at one end of his high tunnel.

The grower had been able to control his thrips populations with spinosad, but western flower thrips are notorious for developing resistance and sure enough have developed resistance to this insecticide in many greenhouses. So populations of WFT increased and with the weeds that were around and in the high tunnel some of which tested positive for TSWV, but negative for INSV, it was a perfect scenario for an outbreak of TSWV. I should note here that we tested for both INSV and TSWV on the tomatoes, weeds and impatiens. Only TSWV was found in the tomato and the weeds. No INSV was found in any sample. Although both viruses are transmitted by the same thrips species these viruses tend to infect either bedding plants (INSV) or tomato/pepper plants (TSWV). The grower threw out all his infected tomato plants and is in the process of killing his weeds in and around his high tunnel. He was able to get control of his thrips in his bedding plants using combinations of pylon and pyrethroids. One variety of tomato the grower was growing that did not show any symptoms of TSWV, even though it was right next to the other infected varieties was Mountain Glory.

Fig 1 Tomato fruit with WFT ovipostion marks

Fig. 2 Tomato leaves with TSWV symptoms and positive immunostrip (two black arrows; Agdia, Inc)

Fig. 3 Tomato fruit with TSWV symptoms


Fig. 4 Two common weed hosts of TSWV; prickly lettuce and chickweed


Producing and Sourcing Quality Transplants

Friday, March 11th, 2011

Gordon Johnson, Extension Vegetable & Fruit Specialist;

Transplant production is underway throughout the region. Cabbage, tomatoes, peppers, watermelons, cantaloupes, cucumbers, squash, lettuce, and even pole lima beans are commonly transplanted along with many other vegetables.

Producing quality transplants starts with disease free seed, a clean greenhouse and clean planting trays. Many of our vegetable disease problems including bacterial spot, bacterial speck, bacterial canker, gummy stem blight, bacterial fruit blotch, tomato spotted wilt virus, impatiens necrotic spot virus, and Alternaria blight can start in the greenhouse and be carried to the field. A number of virus diseases are transmitted by greenhouse insects.

Buy disease indexed seeds when available. To reduce bacterial seed borne diseases in some crops such as tomatoes, peppers, and cabbages, seeds can be hot water treated. Chlorine treatment can also be useful on some seeds as a surface treatment but will not kill pathogens inside the seed. Go to this factsheet for more details:

If possible, do not grow ornamental plants in the same greenhouse as your vegetable transplants and do not overwinter any plants in areas where transplants are to be grown.

For greenhouse growing areas, remove any weeds and dead plant materials and clean floors and benches thoroughly of any organic residue. Use a disinfectant applied to surfaces to kill pathogens. Choices are: quaternary ammonium products (Qam), chlorine bleach in a 1 part bleach to 9 parts water ratio, or hydrogen dioxide products. If possible, use new planting trays. If trays are reused, then one of these products should be used to disinfest trays. Bleach and Qam products require 10 or more minutes of contact to be effective.

One of the most important considerations is managing stretch or height of transplants. The goal is to have a transplant of a size that it can be handled by mechanical transplanters without damage and that have reduced susceptibility to wind.

Managing transplant height can be a challenge. Most growth regulators that are used for bedding plants are not registered for vegetable transplants. One exception is Sumagic which is registered for use as a foliar spray on tomato, pepper, eggplant, groundcherry, pepino and tomatillo transplants. See this past WCU article for more information Research is being conducted on ABA products for transplant management and other products may be registered in the future.

For other crops alternative methods for height control must be used. One method that is successful is the use of temperature differential or DIF, the difference between day and night temperatures in the greenhouse. In most heating programs, a greenhouse will be much warmer in the daytime than nighttime. The greater this difference, the more potential for stretch. By reducing the day-night temperature difference, or reversing it, you can greatly reduce stem elongation. The critical period during a day for height control is the first 2 to 3 hours following sunrise. By lowering the temperature during this 3-hour period plant height in many vegetables can be controlled. Drop air temperature to 50° – 55°F for 2-3 hours starting just before dawn, and then go back to 60° – 70°F. Vegetables vary in their response to DIF. For example, tomatoes are very responsive, squash is much less responsive.

Mechanical movements over transplants can also reduce size. You accomplish this by brushing over the tops twice daily with a pipe or wand made of soft or smooth material. Crops responding to mechanical height control include tomatoes, eggplant, and cucumbers. Peppers are damaged with this method.

Managing water can be a tool to control stretch in some vegetables. After plants have sufficient size, allow plants to go through some stress cycles, allowing plants to approach wilting before watering again. Be careful not to stress plants so much that they are damaged.

Managing greenhouse fertilizer programs is another tool for controlling height. Most greenhouse media comes with a starter nutrient charge, good for about 3-4 weeks. After that, you need to apply fertilizers, commonly done with a liquid feed program. Greenhouse fertilizers that are high in ammonium forms of nitrogen will cause more stretch than those with high amounts of nitrate nitrogen sources. Fertilizers that are high in phosphorus will also tend to lead to stretch.

Exposing plants to outside conditions is used for the hardening off process prior to transplanting. You can also use this for height control during the production period. Roll out benches that can be moved outside of the greenhouse for a portion of the day or wagons that can be moved into and out of the greenhouse can be used for this.

Seedless watermelons have specific requirements: germination at high temperatures for 24 hours (to achieve even germination) then move immediately into a cooler greenhouse to grow out. See this past WCU article for more information

Many growers choose not to produce their own transplants but contract with greenhouse growers locally or in the South. Majorities of these transplants are of high quality and perform well in the field. However, each year, there are some shipments that have problems. The most common problem is transplants shipped before they are ready – without adequate root systems. These transplants will not perform well in the field, especially in earlier plantings. If possible, they should be placed in a greenhouse to finish growing before use.

Another issue is diseases. Bacterial diseases (such as bacterial spot), fungal blights (such as Alternaria or Gummy Stem), and viruses (such as Tomato Spotted Wilt Virus and INSV) have all been found in transplants at times. If a disease is suspected, have it quickly diagnosed and inform the Plant Industries section of the Delaware Department of Agriculture. Do not plant diseased plants in the field. Southern grown transplants are most often the source so make sure that you are dealing with a grower with a good reputation for producing disease free plants.

Plants that are shipped without trays (already pulled) or that are bare rooted that are packed tightly in boxes must be planted quickly. Delays will lead to plant deterioration, leaf loss, and potential disease buildup.

Timber Rot, White Mold, Sclerotinia Rot in Spring Greenhouses and High Tunnels

Thursday, May 6th, 2010

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

The fungus Sclerotinia sclerotiorum along with other Sclerotinia spp. cause disease on hundreds of plant species, including most vegetables. Diseases caused by Sclerotinia, such as timber rot or Sclerotinia rot are becoming very serious problems in vegetables grown in greenhouses and high tunnels. The diseases overwinter in the soil as large resistant sclerotia, which multiply during years of susceptible crop production. However, even when a high tunnel is moved between seasons, the disease can be severe because the fungus overwinters both in and around the greenhouses and tunnels. The primary source of inoculum may originate inside as well as outside these structures. In the spring when the soil is moist, the fungal fruiting bodies emerge and spores (ascospores) are released. These ascospores will be released continually throughout the spring and are carried throughout the house if originating inside the structure. If the ascospores originate outside the house, they are carried on wind through the doors, vents or raised sides of nearby structures. Ascospores are usually carried or dispersed less than 330 feet. Therefore, it is especially important to use sanitation within 330 feet of a greenhouse or high tunnel. No plants, leaf clippings, potting mix, or soil from the greenhouse or high tunnel should be discarded within this area.

Inside the greenhouse or high tunnel, improve air flow in and around the plants by increasing in-row spacing and trellising plants to reduce disease incidence. Conversely, the proliferation of leaves near the soil will increases disease. The biocontrol Contans has been effective in managing Sclerotinia diseases in the field. The active ingredient of Contans, Coniothyrium minitans, parasitizes the overwintering (or surviving) sclerotia. If Contans is sprayed on the area around the high tunnel and watered into the soil, it may help reduce ascospore formation in future years. Contans must sprayed long before disease development occurs (2 months) to be effective within a crop year. Because the product is living, handle it carefully prior to use. Contans would be a good choice to try in fields or areas around greenhouses and high tunnels that are used repeatedly for a susceptible crop. See the Contans label for additional information.

Other fungicide products labeled for Sclerotinia in the greenhouse are Botran and Terraclor. Maximizing fungicide coverage to plants during application is important. Apply fungicides prior to disease development for greatest efficacy. Keep in mind that the fungus becomes established on senescing tissue first and then colonizes the plant. See the Botran and Terraclor labels for information on individual vegetable crops.

The black sclerotina on the small tomato fruit will overwinter and result in ascospore formation in future years. The fruit should be either buried or discarded more that 330 feet from the high tunnel.

Seed Treatment and New Selected Fungicides and Bactericides Labeled for Greenhouse Use Tables in 2010 Recommendations Guide

Thursday, April 1st, 2010

Andy Wyenandt, Assistant Extension Specialist in Vegetable Pathology, Rutgers University;

All seed used in transplant production as well as any transplants brought into the greenhouse should be certified “clean” or disease-free. Important diseases such as bacterial leaf spot of pepper and tomato can cause major problems in transplant production if introduced into the greenhouse. Bacterial leaf spot of pepper and tomato can be seed-borne and infested seed can be a major source of inoculums in the greenhouse and cause problems in the field later in the growing season. As a rule for any crop, any non-certified or untreated seed should be treated, if applicable, with a Clorox treatment, or hot water seed treatment, or dusted to help minimize bacterial or damping-off diseases. For more information on seed treatments for specific crops please see Table E-13 on page E46 in Section E of the 2010 Delaware Commercial Vegetable Production Guide.

An updated table for selected fungicides and bactericides labeled for greenhouse use is available in Section E of the 2010 recommendations guide. The table includes a comprehensive list of fungicides and biological agents approved for greenhouse use. Table E-14 can be found on pages E47-48 of the 2010 Delaware Commercial Vegetable Production Guide.

Greenhouse Sanitation and Inspection is Important for Disease Management in Vegetable Transplant Production

Thursday, April 1st, 2010

Andy Wyenandt, Assistant Extension Specialist in Vegetable Pathology, Rutgers University;

Proper greenhouse sanitation is important for healthy disease free vegetable transplant production. Efforts need to be made to keep transplant production greenhouses free of unnecessary plant debris and weeds which may harbor insect pests and diseases. All equipment, benches, flats, plug trays and floors should be properly disinfested prior to use and efforts need to be taken throughout the transplant production season to minimize potential problems. Any weeds in or around the greenhouse structure should be removed prior to any production. Any transplant brought into the greenhouse from an outside source needs to be certified ‘clean’, as well as, visually inspected for potential insects and diseases once it reaches your location. Remember, disinfectants, such as Clorox or hydrogen dioxide products (Zerotrol-for commercial greenhouses, garden centers and Oxidate-commercial greenhouse and field), kill only what they come into direct contact with so thorough coverage and/or soaking is necessary. The labels do not specify time intervals for specific uses, only to state that surfaces be “thoroughly wetted”. Therefore labels need to be followed precisely for different use patterns (i.e. disinfesting flats vs. floors or benches) to ensure proper dilution ratios. Hydrogen dioxide products work best when diluted with water containing little or no organic matter and in water with a neutral pH.

Producing Quality Seedless Watermelon Transplants

Thursday, March 25th, 2010

Gordon Johnson, Extension Fruit & Vegetable Specialist;

Watermelon transplants for early production are being started this week. Those destined for later field plantings will be started throughout the month of April. Growing quality seedless watermelon transplants requires attention to detail and conditions required are different than for seeded types. Common problems include incomplete germination, uneven growth, weak plants, and stretching (leggy plants).

Seedless watermelon production can be broken into 6 phases: seeding, initial germination, emergence, seed leaf stage to first true leaf, first true leaf to second true leaf, and hardening off.

Trays with square cells at least 2 inches deep and 1 inch square should be evenly filled with a general greenhouse growing medium like Pro-Mix BX , Fafard #2, or Sunshine #1 (these all have a starter fertilizer charge). Do not use fine seed starter or plug mix types. Do not compress the media. Trays should be watered to capacity and then allowed to drain off excess for 24 hours. During this 24 hour period, trays should be placed in a heated area so that the media reaches a temperature of 85°F. Make planting holes 1” deep with a dibber and plant seeds with the pointed side up. Cover with a small amount of warm moist media just enough to fill over seeds in the holes. Do not water after seeding. Seeding should be done in a way that trays stay at 85° F (do not allow trays to get cold).

Initial Germination
Germination should be done in a room or chamber where temperatures can be maintained at 85-90°F and where there is high humidity. Uniform tray temperature is critical. This phase will last 2 day – trays should be kept in this high temperature growth area for 48 hours and no more. To insure even germination, it may be necessary to move trays around after 24 hours (trays on bottom shelves moved to top shelves and vice versa). In this phase the seed root will emerge but the crook that will carry the seed leaves above the surface should not be visible. If you see crooks, you have left trays in the germination area too long and you may experience plant stretch during emergence (if plants have emerged you are too late – stretch has already occurred).

After initial germination, it is critical to move plants immediately from germination areas to the greenhouse for emergence. If you are having another grower germinate your seeds, it is important to schedule pickup or delivery so that there are no delays. Greenhouses should be set for 72-75°F day temperatures and 65°F night temperatures. Do not water until after you see emergence and even then water sparingly as needed to keep trays and emerging seedlings from drying out. Excess water and high greenhouse temperatures during the emergence phase will lead to stretch.

Seed Leaf Stage to First True Leaf
Maintain greenhouse temperatures in the 72-75°F day and 65°F night range during this period. Water moderately to keep plants from drying out but do not fertilize during this period (this assumes that the media you are growing in has a starter fertilizer charge). You want the plants to grow slowly for highest quality.

First True Leaf to Second True Leaf
Continue maintaining greenhouse temperatures in the 72-75°F day and 65°F night range during this period. You can fertilize once the first true leaf emerges. Generally 2 fertilizations of 100 ppm nitrogen concentration one at first true leaf and one at second true leaf will be all that is needed. If a constant feed system is used, set for 50 ppm nitrogen each watering once the first true leaf has emerged. These fertilization rates are for the media listed in the seeding section that contain a starter fertilizer charge. Avoid using fertilizers with high amounts of ammonium N as the nitrogen source as this can lead to stretch (use fertilizers with calcium nitrate and potassium nitrate as the main nitrogen sources). Avoid over-watering. Again, you want plants to grow slowly for highest quality.

(Some growers will use a media with no starter fertilizer charge. If that is the case, a different fertilizer program will be needed. Use fertilizers with calcium nitrate and potassium nitrate as nitrogen sources. Use 50 ppm N from emergence to first true leaf every 3 days, 200 ppm N every other day from first true leaf to second true leaf)

Hardening Off
It will take 4-6 weeks to finish transplants. Prior to transplanting, harden off plants starting one week before setting in the field. This is accomplished by lowering day time temperatures in the greenhouse (if greenhouses have side curtains roll them up during days if temperatures are not too cool). Reduce watering and stop fertilization. Some growers have the ability to place plants on wagons or move benches outside during the day, bringing them in at night. This is advised where possible but make sure the area is sheltered from high winds and avoid days where the temperature is below 60°F.

The above information is for growing the seedless watermelons. Seeded pollenizers do not need special germinating conditions and can be grown directly in the greenhouse. The key is to time the production so that plants are produced and hardened off at the same time as the seedless types. They also should be grown slowly and attention should be paid to avoid stretch. Follow the same recommendations from seed leaf stage through hardening off.

Greenhouse “Air Pollution” Caused by Ethylene

Friday, April 24th, 2009

Jerry Brust, IPM Vegetable Specialist, University of Maryland;

Ethylene (C2H4) occurs in trace amounts in gasoline and natural gas and is produced when these substances are burned. It also is present in wood and tobacco smoke. Ethylene is a plant hormone produced by plants during their growth and development. However, ethyl­ene produced through defective heating equipment can be detrimental to greenhouse crops, because it is pro­duced in greater quantities. Ethylene pollution influences the activities of plant hormones and growth regulators, which affect developing tissues and normal organ development, many times without causing leaf-tissue damage. Injury to broad-leaf plants occurs as a downward curling of the leaves and shoots (epinasty), followed by a stunting of growth. Other symptoms of excess ethylene exposure include the abscission of flower buds (figure below), petals or leaves; water-soaking of older leaves; chlorosis; and wilting of flowers. Crops vary in their sensitivity and response to ethylene toxicity. The degree to which a crop is affected depends on the variety, temperature, ethylene concentration, and the duration of exposure. High temperatures and high light levels will increase the severity of ethylene damage. In high tunnels that burn propane, kerosene or use motors that burn gasoline and have poor or no ventilation, even minute amounts of this pollutant can cause severe damage to tomatoes. Unvented unit heaters in greenhouses can at times also cause problems. These problems tend to increase in very tight greenhouse structures, i.e., those that have little exchange with the outside air. Symptoms of ethylene damage can be very subtle, especially if there are no plants grown in clean air avail­able for comparison.

Proper heating system installation and maintenance are the best ways to prevent problems. A maintenance plan should include cleaning the unit heater and fuel orifice twice a year. Propane flames should have a small yellow tip when properly adjusted and natural gas flames should be a soft blue with a well-defined inner cone. To ensure proper combustion, heater units should have a clean air intake and should be vented to the outside with a stack, which keeps exhaust gas from being drawn back into the greenhouse through the ventilation system.