Vermont Vegetable and Berry News - April 1, 2005
Compiled by Vern Grubinger
University of Vermont Extension
(802) 257-7967 ext.13
(adapted from Cornell’s NY Berry News)
Research at Cornell and elsewhere has found spring row covers to positively impact plant carbohydrate reserves and productivity in strawberry fields. To protect plants from winter injury, straw mulch is applied over the strawberry planting in late fall. However, plants left under winter straw mulch the following April show greater than 50% decline starch content, a loss of root biomass and subsequently lower yields. Creating a more favorable plant micro-climate in late March and April by removing straw mulch and covering plants with synthetic row covers improves photosynthetic rates of leaves, enhances starch accumulation, accelerates plant development (including earlier fruiting) and increases total fruit yield.
As a rule, March is an appropriate time to remove protective winter straw mulch and apply row cover (early or mid-April in most of Vermont). Then the row covers should be removed soon after flowers are observed. Without wind or bee activity, pollination will be reduced and fruit will be deformed. If cold temperatures (below 30F) occur when row covers are still in place and flower trusses have emerged, water can be applied directly over the row covers for frost protection. The economics of row cover use is favorable if the material is reused for several springs.
Details on spring microclimate studies can be found in the following article: Lori J. Bushway and Marvin P. Pritts. 2002. Enhancing Early Spring Microclimate to Increase Carbon Resources and Productivity in June-bearing Strawberry. Journal of the American Society for Horticultural Science 127(3): 415-422.
(adapted from an article by Don Elliot, Applied Bionomics Ltd.)
Starting with low pest levels is critical to success using biological control, and it is also helpful to getting good control with insecticides. Monitoring crops visually on a weekly basis and using sticky cards is relatively easy and inexpensive. Use 3" by 5" yellow sticky cards placed at frequent intervals in the greenhouse.
Hang cards just above the plants to catch insects such as fungus gnats, whitefly, thrips, aphids, lygus bugs, and moths. Replace or add new cards every 5 weeks as the glue dries and will no longer trap small insects such as whitefly. Use a felt pen mark to cover the insect once counted, to prevent repeat counts. To help identify pests use a 5 power headband magnifier or hand lens. Walk the rows at least once a week checking cards, scanning the crop and leaves for damage and recording results.
If you do not have the time to monitor, assign and train a staff member to do it. Some growers have excellent success by giving pest monitoring and biocontrol responsibility to one staff member. You or your staff will have to identify the various stages of the major pests and their damage. If in doubt contact or send samples to your extension entomologist or biocontrol supplier. Many pictorial resources are now more available to help with identification.
Record trap counts on a weekly map or report. A greenhouse map is a valuable monitoring tool for tracking progress of pests and beneficials. As a visual aid it is a good idea to mark the initial infestation sites as well as pest hot spots by hanging different colored survey flagging tapes above the plants and at row ends. Row numbers should be clearly marked on support posts for easy reference. Some intensive IPM greenhouses are now numbering all support posts to use as map grid reference points.
Act without delay when a pest is identified. Pests can build very rapidly under greenhouse conditions so IPM strategies based on past season’s pest levels often involve "preventative" introductions of biological control agents even before the pest has been detected. Many biological control agents (e.g. whitefly and aphid parasites, Hypoaspis predatory mites) are very good at finding their host at low densities but will take too long to provide control if their introduction is delayed until host populations are high. Other pests such as spider mite descend onto the crop over a period of time and must be treated with "curative releases" of biological control agents as soon as the pests are detected. Releases must continue until biocontrol agents are well established on all infested plants.
Use recommended rates, methods and release frequencies. When using a biocontrol make sure temperature and hours of daylight are not limiting. Whitefly parasite activity is greatly reduced at temperatures below 18C and introduction numbers must be increased. Predators such as Aphidoletes will stop laying eggs and enter diapause when daylight is less than 16 hours in March and September. When using pesticides, spot treatments, reduced rates and short residue periods are important to minimize harm to the biological control agents. It is critical to use recommended rates and introduction schedules. Know what to look for so biocontrol agents are not removed from the crop when pruning leaves. Poorly timed or excessive deleafing can upset an entire program for the season as most of the biocontrol reproduction occurs on the crop.
Live predators and parasites are very perishable and must be checked for viability on delivery and handled carefully under the appropriate conditions. Release instructions should include life history and handling information. Once released into the greenhouse the biological control should establish and be found on the pest. This may take 2 to 6 weeks and can be difficult to detect at low pest densities. Contact your biocontrol supplier at the first sign of any problem for help and to determine the best solution.
(adapted from an article by John Bartok, UConn Extension, retired)
Air pollutants from a faulty heating system can affect greenhouse plant production by damaging leaves or flowers and reducing plant growth. Although the visible effect on the plants may be quite obvious, the cause may be difficult to find. The most common pollutants are sulfur dioxide, ethylene and fumes from escaping fuel.
Sulfur Dioxide. All fuels contain sulfur, some more than others. During the combustion process, sulfur is converted to sulfur dioxide. If it leaks into the greenhouse and combines with moisture, sulfuric acid is formed. In excess, this can be toxic. At high levels, which might occur when using unvented heaters during a power outage, severe leaf burn can occur. Young leaves seem to be more susceptible. Long term, low levels, which may occur from a cracked firebox or leaky exhaust pipe, may result in flecking and premature leaf drop. Sulfur dioxide concentrations as low as 0.5 ppm can cause injury. When purchasing fuel, specify sulfur content of less than 0.02% by weight to help avoid this.
Ethylene, a clear, odorless gas, is a byproduct of the combustion of fossil fuels. Ethylene can be damaging at levels as low as 0.05 ppm. Even a few hours of exposure can cause devastating effects on the growth and flowering of plants. Injury includes leaf distortion, abortion of flower buds, defoliation and chlorosis.
Fuel fumes from leaks of raw fuel can also affect plants. Propane or natural gas at levels of 50 ppm can have damaging effects. Also, a fuel oil, if it volatilizes on a hot surface, can put harmful vapors into the air. Check the piping frequently for leaks.
Indicator plants are a good way to monitor for the presence of sulfur dioxide and ethylene in a greenhouse. Tomato seedlings are often used because they germinate quickly and can be grown on a year-round basis. They respond very quickly, in as little as three hours.
Following are areas of the heating system that frequently cause pollution problems:
Firebox Leaks. Continual expansion and contraction of the metal in the heat exchanger of a furnace can stress the welds, resulting in cracks that are a prime source of pollution, especially in older units. Placing a furnace candle or smoke bomb inside the firebox and observing any escaping smoke can be an effective way to check a furnace. An alternative is to insert a trouble light into the firebox at night and look for light rays in the heat exchanger area. Cutting into the outside of the metal furnace enclosure and welding the split seam can repair some units. In other cases the whole firebox must be replaced.
Chimney connector. The stovepipe or connector pipe should be severely fastened to prevent leaks. On stovepipe, sheet metal screws can be used to fasten the joints. If the sections do not fit tightly, fill the cracks with pipe cement.
Chimney height. To get adequate draft for combustion and to reduce the potential for backdrafts, the top of the chimney must extend above the peak of the greenhouse and any nearby obstructions. Heating codes recommend a height of three feet above the ridge of the greenhouse or two feet above a 10-foot horizontal line to any part of the structure. A cap on the chimney can help to reduce down drafts, a common cause of fumes inside the greenhouse.
Makeup air. Today's tight greenhouses require an outside source of makeup air to feed the combustion process. On a cold night with the heating system operating almost continuously, the oxygen can be depleted in two or three hours in a tight house if no makeup air is provided. Some of the newer furnaces and boilers have a built-in port to connect a pipe to an outside air source. For heating units without this port, outside air can be brought into the area of the burner using PVC drainpipe or galvanized stovepipe. Pipe size should be at least as large as the vent on the furnace. The pipe should extend from the furnace through the side or endwall and up above the expected snow line. Attach the pipe to the greenhouse for support. A cap and screen should be placed on the exterior end to shed water and keep out animals. Take the time to make an inspection. It can improve the efficiency of the heating system and reduce the potential for pollution problems.
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