In strawberry and raspberry production plant growth and development are affected by a number of nematode species. Nematodes are microscopic, unsegmented round-worms. They are found world-wide in water, soil, decaying organic matter, plants and animals. Plant parasitic nematodes are closely associated with host plants for most of their lives and they tend to favour sandy or sandyloam soils.
Life cycles among nematodes are similar. All types develop from eggs and undergo a series of immature stages and molts before they become adults, when they average 0.4 mm to 0.8 mm in length. Some species of nematodes spend their lives within the plant, while other species scavenge their food through root surfaces. Nematodes feed by use of a stylet with which they puncture cells and suck out the contents. They are so small that even when present in large numbers they cannot be seen without the aid of laboratory instruments. With severe infestations they can number in the thousands per gram of dry root.
NEMATODES AFFECTING STRAWBERRIES AND RASPBERRIES
The species of nematodes which have been found associated with strawberry and raspberry production in New Brunswick are: lesion, root knot, dagger, needle, pin, stunt, ring and spiral. The number of nematodes identified in a soil sample determines if treatment is required prior to planting. These threshold numbers vary with the type of nematode found; for lesion' nematodes, 500 per kilogram of dry soil is considered a level where preplant treatment would be beneficial. The treatment levels for pin, root knot, and needle nematodes per kilogram of dry soil are 5000, 500, and 200 respectively, although treatment for needle nematodes is usually only considered when they are found in combination with dagger nematodes in the same field. Spiral, stunt and ring nematodes are frequently found in samples from strawberry and raspberry plantations; however their impact on yield, if any, is unknown.
Nematode-feeding on plant rootlets (Figure 1) causes a general decline in root growth and overall plant vigour. The wounds created by the feeding activity can also serve as an entry point for root decay fungi. High numbers of nematodes such as the root lesion are sometimes found associated with fields having root rot disease. Some nematodes in the dagger and needle group are known to transmit viral diseases; however these diseases are not a problem in the province.
FIG.1: Root Lesion Nematode (Pratylenchus sp.) in a Section of Strawberry Root (magnified approx. 10OX)
- note dead plant Tissue on right
Symptoms of nematode activity are similar in many crops. Root function is impaired, resulting in poor growth (Figure 2), low yield, deficiency symptoms and a poor root system often with decay. Root knot nematodes cause roots to have very small swellings, about 2 mm in diameter, which can be seen with the unaided eye. Lesion nematodes make fine reddish-brown "scratches" 1.5 - 3.0 mm in length, on the main roots of young plants or on new roots from the crowns of older plants. A laboratory analysis is needed to confirm the presence of nematodes and assess if they are present at levels requiring treatment.
FIG.2: Growth of Lesion-Infested Strawberry Plant (on left) vs. Healthy Plant (on right)
SOIL SAMPLING FOR NEMATODES
Sample during the month of August with a narrow spade or a soil sampling tube and include some feeder roots if possible. Sample randomly at a depth of 5 - 30 cm preferably in the root zone of the existing crop. Collect 50 samples per hectare and mix thoroughly in a clean container. Seal about one litre of this mixed soil in a plastic bag to keep the sample from drying out and store at a temperature of 10 to 150C. As soon as possible submit the sample for analysis with a tag identifying sampling date, area sampled, present crop and crop to be grown. Currently we are utilizing nematode laboratories outside the province for analysis, however regional offices can assist you in sending your sample.
Nematodes usually have a wide range of hosts and are seldom confined to a single plant species. Some of the nematodes infesting strawberry and raspberry plantations such as the root lesion nematode can also feed and proliferate on weeds, cereals, trees, and vegetable crops. For this reason rotation does not give an adequate reduction in the nematode population. Rotation with cereals can be successful in reducing root knot nematode as they do not thrive on these crops. Good weed control will eliminate those weed species which support nematode populations.
Treatment during cropping is not considered practical and therefore your nematode assessment and treatment, if required, must be carried out prior to planting. Plant parasitic nematodes can be introduced into a field with nursery plants, another important reason for purchasing certified stock. Certified stock plant production in the Maritimes requires that the nurseries fumigate for nematode control.
Fumigation prior to planting is an effective way to control nematodes. However for both environmental and economic reasons it should only be carried out where soil sampling has been done and nematode numbers are above economic thresholds. Fumigation is preferably done in the fall of the year prior to planting. The soil should be worked several times to a depth of 25 cm in the weeks prior to fumigating to insure that all plant material is decomposed. For a field which was previously in strawberry or raspberry production the only way to be sure that undecomposed plant material is not left in the soil at the time of fumigation is to summer fallow the field and harrow or disc several times during the fallow season. A harrow may be used initially to rake old crowns, canes, or roots to the edge of the field.
Fumigation should be delayed in the fall
until late September or early October. It can be carried out earlier
but more attention must be paid to obtaining a good seal. The soil
must be moist throughout the fumigation depth for at least one week
prior to fumigating. The soil should be at 50 to 80% of its
water-holding capacity. (A handful of soil in this condition will
form a ball when squeezed that will break apart when touched. If it
will not form a ball it is too dry and if it will not break apart it
is too wet.) Do not fumigate dry soil . The soil should be cultivated so that it is level and with no clods or clumps. The gas that is formed when the fumigant is injected will not penetrate clumps or undecomposed plant material.
Make sure the fumigator is calibrated and that all shanks are delivering the fumigant. The shank depth should be set so that the fumigant treats all the soil in the plough layer. This prevents discing or harrowing from pulling unfumigated soil up into the treated area. For most operations the shanks should be set 20 cm apart and the fumigant injected by setting the shanks 15 to 20 cm deep in the soil. Following the application drag the soil to fill the shank cuts, as fumigant will escape through these slits. Roll the soil to compact and seal and, if necessary, apply water to seal the soil surface and prevent loss of the fumigant to the air. Apply only enough water to wet the soil to a depth of 1 to 1.5 cm. Leave the soil undisturbed until the following spring.
If possible return the empty barrel to the fumigant supplier; otherwise puncture the empty fumigant barrel to prevent it from being re-used and to allow any residual fumigant to evaporate. A barrel treated this way can be deposited at a sanitary landfill after one week.
Cultivate the soil several times at oneweek intervals prior to planting to allow complete aeration of the soil. Do not cultivate deeper than was fumigated. Do not plant if you can still smell the fumigant. If only part of a field is fumigated, cultivate so that the fumigated soil does not become contaminated with unfumigated soil.
Fumigants currently sold in this area contain dichloropropene and related hydrocarbons. In addition, some formulations may contain chloropicrin and methyl isothiocyanate. All products will give an excellent kill of nematodes even when used at the recommended low application rate. Higher fumigation rates are used for fumigating soil for disease control or weed suppression. Fumigation rates therefore fall within a range depending on your reason for fumigating and your soil type (heavier soils require a higher fumigant rate). Obtain the recommended rate from the fumigant supplier for your particular application. Some suppliers will assist you in obtaining the fumigator if you make arrangements adequately in advance. FUMIGANTS ARE HAZARDOUS PESTICIDES - READ AND FOLLOW ALL SAFETY PRECAUTIONS.
The authors would like to extend thanks to Dr. John Potter, Nernatologist of the Agriculture Canada Research Station in Vineland, Ontario for his contributions of photos, technical and revision work.
For more information on nematodes, see:
1. Tremblay, R. and J. Baker. "Nematodes in New Brunswick Strawberry Fields", Adaptive Research Reports. NBDA. 1990. Vol 12 32.
2. Tremblay, R. and C.Hollis. "Nematodes in Now Brunswick Strawberry Fields", Adaptive Research Reports. NBDA. 1991. Vol 13.
1 Lesion (Pratylenchus spp.); root knot (Meloidogyne hapla); dagger (Xiphinema spp.); needle (Longidorus spp.); pin (Paratylenchus projectus); stunt (Tylenchorynchus spp.); ring (Criconemoides spp.); spiral (Helicotylenchus spp.)