Cucurbit Nematode Control – How To Manage Nematodes In Cucurbit Plants

By: Mary Ellen Ellis

Melons, squash, cucumbers, and other members of the cucurbit family are susceptible to infestation with nematodes. Cucurbits with nematodes may suffer varying degrees of crop loss, depending on the severity of the infection. Management strategies are important in keeping this pest under control.

Signs of Nematodes in Cucurbit Plants

Unfortunately, most cucurbits are highly susceptible to infestation by nematodes, microscopic worms that live in the soil. Of particular concern are root knot nematodes and sting nematodes. Among the cucurbits, watermelons are the least susceptible to damage from the root nematodes, but any plants in this family may be attacked and damaged.

When nematodes of cucurbit crops attack the roots, the symptoms of infestation include stunted growth, leaf yellowing, and premature wilting. When the plant is given water, it takes longer than usual to recover. The roots develop galls, or knots, when infested with root knot nematodes. Sting nematodes cause the roots to be stunted and grow as dense mats.

When symptoms begin to appear depends on environmental conditions and the degree of infestation. If plants have good conditions, you may not see signs of nematodes until later in the season, but if conditions are not great and the nematodes are plentiful, plants may start to decline very early, even as seedlings.

Cucurbit Nematode Control

It can be difficult to completely eliminate nematodes, but good management practices can help you keep infestations manageable and recover good crop yields. Crop rotation is one of the most important practices in cucurbit nematodes control. If you see an infestation this year, plant something that is not susceptible in that area next year.

Another thing you can do before planting next year is to prepare the soil using an appropriate pesticide. It also helps to plow the soil deeply, which reduces nematode populations. You can try to heat the soil using clear plastic on a sunny day, but this is less effective at killing nematodes than using pesticides. Control weeds that play host to these nematodes to further manage infestations such as pigweed, nightshade, and nutsedge.

To try to save your crop this year when your cucurbits are showing signs of infestation, provide plants with the most optimal conditions. Extra water and fertilizer can help the plants continue to produce a decent yield. The nematodes prevent the plants from absorbing as much water and nutrients as they normally would, so providing more can help them grow and produce more.

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What are nematodes?

Nematodes are unsegmented roundworms, different from earthworms and other familiar worms that are segmented (annelids) or in some cases flattened and slimy (flatworms). Many kinds of nematodes are found in Florida soil. Most nematodes are beneficial, feeding on bacteria, fungi, or other microscopic organisms, and some may be used as biological control organisms to help manage important insect pests. However, plant-parasitic nematodes (Figure 1) feed on live plants and are detrimental to the garden.

Plant-parasitic nematodes are very small, and most can only be seen using a microscope (Figure 2). All plant-parasitic nematodes have a stylet or mouth-spear that is similar in structure and function to a hypodermic needle (Figure 3). The stylet is used to puncture plant cells and then inject digestive juices and ingest plant fluids. Most of the plant-parasitic nematodes that are important in vegetable gardens feed on plant roots. Some plant-parasitic nematodes, called ectoparasites, remain in the soil during their entire lifecycle and feed by inserting only their stylet into the root (Figure 4). Other nematodes enter the plant with part or all of their body and are called endoparasites. Some endoparasites, called migratory endoparasites, burrow around inside the root (Figure 5). Other endoparasites, called sedentary endoparasites, establish permanent feeding sites inside the root and remain in one place. As it matures, a sedentary endoparasite's body changes shape and adult females are usually swollen (Figure 6).

Size of a lance nematode (one of the larger plant-parasitic nematodes) compared to a human hair.

W. T. Crow, UF/IFAS Entomology and Nematology Department

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Diagram of a plant-parasitic nematode stylet.

R. P. Esser, Florida Department of Agriculture and Consumer Services, Division of Plant Industry used with permission.

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Ectoparasitic nematodes from a soil sample.

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Migratory endoparasitic nematodes (stained red) tunneling within a root.

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Development of sedentary endoparasitic root-knot nematodes within roots. A. Second-stage juveniles enter root, cause a feeding site, and then no longer move. B. Juveniles swell and molt several times. C. Adult female nematode is swollen and starting to lay eggs. D. Root tissue pulled back to show adult female nematode.

Panels A, B, and C, N. A. Sikora UF/IFAS Panel D, Theresa Friday UF/IFAS Extension

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Managing Nematodes Of Cucurbit Crops: How To Treat Cucurbits With Nematodes - garden

Plant-parasitic nematodes are at their most vulnerable during their active phase in soil when searching for the roots of host plants. Once endoparasitic species have penetrated a root, control with chemicals is more difficult as nematicidal compounds have to be non-phytotoxic and preferably systemic.

A nematicide that can be safely applied to growing plants and is translocated to the roots in sufficiently large amounts to kill endoparasitic or ectoparasitic nematodes has not been discovered. Oxamyl, a systemic compound that is translocated basipetally, is the only commercial product that is used as a foliar treatment, but its use as a liquid formulation is restricted in many countries for toxicological reasons.

There are several nematicides that can be used effectively for nematode pests of annual crops (van Berkum and Hoestra,1979), but there appears to be little prospect for management of nematodes in many susceptible perennial crops without repeated application of nematicides (Tables 1 and 2). Only in certain cases will such treatments be justified economically. Since the discovery and wide-scale use of fumigant nematicides 50 to 60 years ago, a number of products and formulations (Table 1) have been developed for use against several nematode pests, and these are available in most regions of the world (Hague and Gowen, 1987). Only in comparatively recent times have the dangers associated with the manufacture and use of these products become apparent. This has resulted in restrictions on use and sometimes withdrawal from the market. It seems that the age of the traditional fumigants and nematicides has passed, and the opportunity for managing nematodes with synthetic chemicals with broad biocidal activity is declining.

The development of new classes of nematicides with novel activity, that are effective when used in soil or applied directly to crops and are environmentally benign and specific to target pests, is perhaps an idealistic hope. Such compounds will, by definition, require considerable research effort, and if they are specific only to certain nematodes are unlikely to be considered an economic proposition for the traditional agrochemical company.

The elimination of nematodes from some crops is essential for certain export requirements, particularly of high-value horticultural products. Chemical treatment with fumigants or nematicides may be the only technique available, and from the plant quarantine standpoint it is important that their use is retained.

The use of chemicals in protected cropping may still be preferable to other techniques such as steam treatment for economic and practical reasons. The use of soil-less growing media in some north European countries has resulted in a decreased demand for chemical treatments. In southern Europe, the Mediterranean region and North Africa, many horticultural and salad crops are grown in soil under polythene and soil treatment with methyl bromide, dazomet or non-fumigant nematicides is widely practised. The cost of such treatments may be as much as US$500 per hectare per year and can only be justified if the crops are of high market value.

Fumigants. Several general purpose fumigants give excellent control of nematodes in soil. The efficacy is related to their high volatility at ambient temperatures. All fumigants have low molecular weights and occur as gases or liquids. As they volatilize, the gas diffuses through the spaces between soil particles nematodes living in these spaces are killed.

Nematicides currently available on world markets

Granular or emulsifiable liquid

Granular or emulsifiable liquid

Granular or emulsifiable liquid

Granular or emulsifiable liquid

Granular or emulsifiable liquid

Granular or emulsifiable liquid

Fumigants perform best in soils that do not have high levels of organic matter (which deactivates the toxicant) and that are free-draining but have adequate moisture. In general, fumigants are most effective in warm soils (12° to 15°C) as dispersion is temperature related.

Methyl bromide, the most dangerous of the fumigants still in common use, has to be applied beneath a polyethylene sheet. In some countries this is done with specialized machinery that treats and covers the soil in one operation. The cover is removed some days later and the crop is sown or planted when all traces of the fumigant have dispersed.

Examples of recommended nematicidal dosages and treatments for some important crops 1

Incorporated in 30-cm bands

Incorporated in bands and irrigated Time interval before planting

Tylenchulus semipenetrans

Annual treatment applied along drip-line

Annual treatment applied along drip-line

(e.c. formulation)

Applied around plant 2-3 times per year

Helicotylenchus multicinctus and/or

Applied around plant 2-3 times per year

Applied around plant 2-3 times per year

Applied around plant 2-3 times per year

Applied around plant 2-3 times per year

1 Information taken from literature. Products may be unavailable for use in some countries. Economic and environmental justification should be evaluated before use. The omission of compounds does not imply that they are not equally suitable for nematode control.

2 kg a.i./ha unless otherwise stated.

Liquid fumigants EDB, metam-sodium and 1,3-D are applied to soil that has been prepared for planting. The soil surface is compacted with a roller after treatment which helps to seal the fumigant in the soil. Compounds releasing methyl isothiocyanate (dazomet, metam-sodium) work best in soils at >15°C. In cooler soils, the period between treatment and planting may have to be extended to allow sufficient time for the product to disperse.

The liquid fumigant DBCP is the only volatile compound that can be applied to growing plants without causing phytotoxicity. However, its manufacture has now ceased for toxicological reasons and its use is banned in many countries.

Non-volatile nematicides. A number of organophosphate and oximecarbamate nematicides were developed in the 1960s, which had the advantage that application was relatively simple (Wright, 1981). As a consequence nematicide use became more widely practised. These compounds (Table 1) are active at dosages of 2 to 10 kg a.i./ha which are smaller than the 200 to 300 litres/ha required for treatment with liquid fumigants. Most of the early formulations of these products were as granules that, when applied to the soil surface (or preferably incorporated in the top 10 cm of soil), release the active ingredient, which is spread through the soil by rainfall or irrigation. The efficacy of soil penetration depends on the amount of moisture, organic matter and soil structure. Heavy soils with relatively small pore spaces are more difficult to treat than sandy soils which have larger pore sizes. Some chemicals, particularly the organophosphates, are absorbed in organic matter, in which case efficacy may be impaired (Bromilow, 1980).

In general, distribution of the active ingredient or its toxic degradation products is less efficient than that of fumigants and results with granular nematicides have sometimes been inconsistent. To be effective, nematicides have to persist long enough for nematodes to be exposed to lethal concentrations, which may be as low as 1 to 2 m g/litre. Extended persistence is, however, not desirable if there is a risk of residues in the crop or the active compounds contaminating groundwater.

Persistence of soil-applied nematicides depends on the soil characteristics. In warm countries, relatively high soil temperatures may accelerate the natural degradation of nematicides, and in protected crops where even higher soil temperatures than out-of-doors may occur, the effective life of a nematicide might be as short as one to three weeks (Bromilow, 1980). The repeated use of products of similar structure can lead to the selection of a soil microflora that metabolizes these compounds and decreases their persistence.

All nematicides are eventually degraded if they remain in the topsoil where there is greatest microbial activity. Once nematicides or their degradation products are flushed through the upper soil layers their persistence may be extended. It is the problem of toxic products in groundwater that has led to the prohibition of fumigant and non-fumigant nematicides in some countries. The permitted level of pesticide residues in drinking-water in the European Union is 0.1 m g/litre. In regions of intensive agricultural production these tolerance levels may be exceeded at certain times of the year.

Nematicides are highly toxic compounds that have very low LD50 values. This is particularly important for operators of application machinery and people at risk from exposure to the chemicals during their application. The liquid formulations of some of the non-fumigant nematicides are emulsifiable concentrates. Their use should therefore be restricted to skilled operators who take adequate safety precautions. This may not always be the case where basic levels of education are poor or where operators cannot read the instructions on the labels of the products. The application of nematicides to crops too near to harvest is another risk which pesticide residue monitoring may not be sufficiently well coordinated to prevent.

The incidence of pesticide poisoning and mortality in some countries (Kottegoda, 1985) serves as a grim warning of the risks that arise when pesticides are widely used under poor management.

The development cost of new products is more than US$20 million and the costs for registering these products are increasing as the criteria for their use are tightened. Conventional compounds (organophosphate or oximecarbamates) are unlikely to be developed if their toxicities are high.

New classes of nematicidal compounds are constantly being sought but there are currently no promising materials close to commercial development. Avermectins, which are of microbial origin, have been developed for veterinary use and are powerful anthelmintics. Their efficacy against plant-parasitic nematodes is well established, however, because the compounds are complex they cannot be used successfully as soil treatments.

Watch the video: How to Control Root Knot Nematode

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