Nematode problem is common in greenhouse production. Many
farmers still are not able to identify nematode problems in their crops with
certainty. The most common species of nematodes identified in greenhouse
production are root knot (Meloidogyne), lesion (Pratylenchus), Burrowing (Radapholus),
and leaf stem, or foliar nematodes (Aphelenchoides or Ditylenchus).Nematode
problems affecting greenhouse crops and commodities can be avoided if
appropriate pest management practices are considered.
Nematodes, a group of specialized worms, live almost
everywhere. Some are PLANT PARASITES. Most greenhouse farmers do not know about
plant-parasitic nematodes as they are very small (less than 1 mm long) and can
be seen only through a microscope. Nematodes infest roots, rhizomes, bulbs,
stems and also live in soil. Unlike other pathogenic organisms, it is very
difficult to identify plants affected by nematodes. These plants generally look
sick and symptoms may be confused with other root diseases (e.g. yellowing
caused by Fusarium wilt) or nutrient problems.
Occasionally infested plants are stunted with
burning/drying of tip leaves. Planting pieces infested with nematode looks
shriveled, with sunken or swollen patches on the skin surface. Infested plants
have swollen, distorted roots. Infested rhizomes have warts on the surface.
Heavy nematode infection can greatly reduce yields. Select planting material as
free from nematodes as possible.
Causes of nematodes build-up in soils
In most cases, problems with these nematodes arise from
planting of infected seed or planting-stocks, in systems utilizing bare ground
or raised ground beds.
Secondly, contaminated soil is used as a component of the
growing medium. Subsequent spread of nematodes within the greenhouse usually
occurs via movement of infested soil or plant material by workers, water, or
greenhouse equipment. In some greenhouse systems, nematodes can be introduced
into and circulated through the drainage system and irrigation water to other
healthy plants within the greenhouse. This explains how once introduced,
nematodes can quickly spread and become headache in greenhouse production.
Nematode Management Practices and Considerations
For greenhouse production, nematodes must be managed
through exclusion and avoidance (prophylactic strategy). Once nematodes are
introduced and damage becomes visible, it is currently not possible to
chemically or non-chemically resolve the problem to avoid potentially
significant yields losses thereafter. Some of the most important sources of
contamination and hence major factors to consider include: 1) seed and planting
stocks, 2) irrigation water, 3) soil and potting media, and 4) general
cleanliness.
Site Selection
Among the common considerations for greenhouse location
like: proximity to water source, security from thieves and destruction,
windbreaks etc, there are also important pest management considerations.
Greenhouses location should be shielded from pest
migration paths especially from neighbouring farms. This will be achieved by
guarding the greenhouse with nets and avoiding very open fields.
Ensure the soils to be used are free from nematodes of
economic importance because nematodes are very hard to eradicate once
introduced ion the greenhouse. In addition, greenhouse structures should be
stored such that they will not be exposed to running water downstream that might
be transporting nematodes or other soil borne diseases with them.
Irrigation
Water from shallow wells (hand-dug water holes) has been
noted to bear high quantities of nematodes. Similarly, water from ponds and
other stagnant sources that rely on surface runoff have known to harbor
nematodes. In the event that these waters are used, there should be proper
treatment prior to use for greenhouse irrigation and/or for preparation of
spray mixtures. This treatment should be extended to recycled water from the
greenhouse. The simplest method of water treatment for use in the greenhouse is
heating to temperatures beyond 90°C.
Nematode-Free Planting Materials
In most cases, greenhouse problems with both soil borne
and foliar nematodes arise from planting of infected seed or planting stocks.
As a result, nematode-free transplants or plug plants that rely upon soilless
substrates from production are increasingly used to exclude foliar and soil
borne species of nematodes, but also to expedite plant establishment and crop
production.
In addition, hot
water dips have been developed as a control strategy to eliminate
plant-parasitic nematodes form both roots and foliar tissues of infested plant
materials to be used as transplants (e.g., strawberry, ginger etc).
To destroy nematodes in plant material, dip seed pieces
in hot water (50°C) for 10 minutes, then remove, and cool. Fungicide dip
treatments can be done following the hot water treatment. Plant the treated
seeds within 2-3 weeks of treatment.
Sanitation
Other cultural measures that reduce nematode problems in
the greenhouses include rapid destruction of infested crop root systems
following harvest. Discarding infected potted plants will help prevent spread
of nematode. Entry points for greenhouse structures should also contain
sanitizing stations for hands, shoes, boots, tools, and other equipment.
Soil pasteurization (Soil heating)
On a relative temperature scale, nematodes and water
molds would be considered relatively intolerant of high temperature, being
effectively killed by 30-minute exposure at temperatures as low as 50°C. Within
the range of 50-60°C, most plant pathogenic fungi, and bacteria are killed.
Certain weeds and viral pathogens compose the most tolerant group, requiring
30-minute exposures at temperatures of 60°C or greater for pest elimination. A
30-minute exposure of 90°C or greater is generally recognized as the
recommended soil pasteurization temperature and the period of exposure to
provide a broad spectrum measure of soil borne pest and disease control. The
following section defines specific pest control systems that rely on heat to
control nematodes in greenhouse production.
Soil Solarization (Heating with the Sun)
Soil solarization involves application of a transparent
polyethylene plastic mulch over moist soil for a 6 to 12 week period to heat
bare (but ploughed), greenhouse soils to temperatures lethal to nematodes and
other soil borne pathogens prior to planting. Soil temperature rises because of
the trapping of Sun’s radiation under the clear polyethylene plastic. To be
effective, soils must be made thoroughly wet to increase susceptibility of soil
borne pests and heat reception and retention of soil.
Use of Resistant Host Plant
Most crops have
resistant varieties to nematodes. Use of nematode-resistant crop varieties is
often viewed as the foundation of a successful integrated nematode management
program.
The surest way of managing nematodes is through prevention
of entry into your greenhouse soils.
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