Foliar Nematodes of Strawberry Plants
Note Number: Ag1432
Published: October 2011
Introduction
Plant-parasitic nematodes are microscopic, generally worm-like in appearance with slender, cylindrical, unsegmented bodies, tapering towards the head and the tail. Parasitic nematodes are important in crop production as some of the species damage plants and cause economic yield losses.
The most important plant-parasitic nematode of strawberry is the foliar (leaf) nematode called Aphelenchoides fragariae (Figures 1 and 2).
A. fragariae has a very extensive host range including vegetables, ornamentals (Figures 5 and 6), fruits, broad acre crops, and weeds. A. fragariae is a common and widespread migratory ecto-parasite (outside) and endo-parasite (inside) of leaves, buds and stems.
Other species of nematodes recorded on strawberry plants are root-knot (Meloidogyne spp.), root-lesion (Pratylenchus spp.), stubby-root (Paratrichodorus spp.), spiral (Helicotylenchus spp. and Rotylenchus spp.) stunt (Tylenchorhynchus spp.), dagger (Xiphinema spp.), etc.
This Agricultural Note describes about A. fragariae nematode.
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Figure 1. Aphelenchoides fragariae |
Figure 2. Aphelenchoides fragariae |
Symptoms
Symptoms of plants infested with leaf or foliar nematodes may be confused with diseases (caused by. fungi, bacteria and viruses) or nutritional deficiency. These symptoms may either be widespread, appear in small patches, or in rows within a field. The above-ground symptoms of foliar nematode affected plants include stunted growth, twisting and puckering of leaves, discoloured areas with hard and rough surface, under-sized leaves with crinkled edges, reddening of petioles, short internodes of runners, reduced flowering (trusses with 1 or 2 flowers), and death of the crown bud (Figures 3 and 4). There are no below-ground symptoms (i.e. damaged roots) caused by leaf nematodes.
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Figure 3. Aphelenchoides fragariae infected |
Figure 4. Aphelenchoides fragariae infected |
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Figure 5. Helleborus sp. infected by |
Figure 6. African violet infected by |
Life Cycle and spread
Aphelenchoides species are spread by contact between plants in the presence of water. These nematodes are found to over-winter as juveniles and adults in soil, on dead leaves and dormant buds, but not in roots. They migrate in the film of water on plant leaves. On strawberry the nematodes are ecto-parasitic, living within the folded crown and runner buds. Occasionally nematodes are found within leaf tissue, which they enter either through the stomata pores, or by puncturing the leaf surface with their stylet which is used for piercing and feeding. As nematodes feed on a leaf, they kill cells causing them to darken.
Females lay 20 – 30 eggs during their life span. After hatching, nematodes go through 4 larval stages before becoming adults. The entire life cycle can be completed in 2 – 4 weeks under favourable conditions. They survive in the soil for 3 months, or longer in plant tissue. Generations succeed one another as long as conditions are favourable. An infected leaf may contain multiple generations of nematodes. Foliar nematodes may also over-winter in weeds around strawberry fields. Of all life stages, adults are most infective, and this resistant stage can remain dormant under unfavourable environmental conditions. When moisture becomes available, the nematode rehydrates and moves into a new host to begin feeding. Wet weather during the strawberry growing season is most favourable for spread of foliar nematodes.
Sampling
Pre-planting soil sampling
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Figure 7. ‘Zigzag’ sampling pattern on fallow land |
To make effective nematode management decisions, it is important to determine which nematode species are present. For the purpose of sampling, in fallow fields, or cropped fields, visually divide the field into sampling blocks that represent differences in soil texture or cropping history. Each block should be no more than 2 hectares. Samples should be collected when soil is moist but not too wet or too dry. Collect soil samples at 15 – 20 cm depth. Collect several sub-samples from each block in a ‘zigzag’ pattern (Figure 7) or in a row pattern (Figure 8) so as to cover the entire block. Mix these sub-samples thoroughly and place about 500 g of soil into a plastic bag. Include roots from the crop in the bag with soil. Seal and place label on the outside of the bag.
Sampling existing crop
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Figure 8. Row sampling pattern on fallow land or cropped field |
In an existing infested strawberry crop, examine the field for symptoms described above. Dig up the entire suspected plant, and the surrounding soil, into a plastic bag. For routine testing, sample several leaves, including petioles, buds and young growing points, that are abnormal or showing any symptoms of nematode infestation.
Keep samples in a cool place but do not freeze them. Deliver them as soon as possible to the diagnostic laboratory, for nematode analysis.
Management and Control
The following is a list of activities designed to prevent or contain incidence of foliar nematodes:
- Thoroughly rogue and burn infested plant material
- Avoid contact between plants (in screen houses)
- Avoid the formation of water film on leaf surface, use drip irrigation
- Select planting material from healthy stocks, or use certified planting materials
- Produce runners in sterilized soil, or by tissue culture using nematode free materials
- Disinfect benches and tools before use
- Treat planting material with hot water at 49°C for 20 minutes
- Rotate crop with grain crops such as barley and rye
- Test planting material, and soil of the block to be planted, for nematodes before planting the crop.
References
Maas JL (ed) (1998) Compendium of Strawberry Diseases. Second Edition. St Paul, Minnesota, USA: America Phytopathological Society Press
Nambiar, L and Wainer, J (2011) Collecting Soil and Plant Samples for Nematode Analysis. Agriculture Note AG1444. Department of Primary Industries, Victoria
Acknowledgements
This Agriculture Note was developed by Lila Nambiar and John Wainer, Crop Health Services in October 2011.
