Vermont Vegetable and Berry News - April 1, 2005
Compiled by Vern Grubinger
University of Vermont Extension
(802) 257-7967 ext.13
ROW COVERS FOR EARLY STRAWBERRY PRODUCTION
(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):
MONITORING IS KEY TO GREENHOUSE PEST CONTROL
(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
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.
AVOID GREENHOUSE AIR POLLUTION
(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
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
COMING SOON: “Reports from the Field”
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