GREENHOUSE NEMATODE MANAGEMENT

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.