Nematodes

Plant parasitic nematodes are microscopic, unsegmented roundworms. The two species most commonly associated with damage in California strawberries are the foliar nematode, Aphelenchoides fragariae, and the northern root-knot nematode, Meloidogyne hapla. The northern root-knot nematode is found in the soil or as a sedentary endoparasite (an immobile life stage inside the plant tissue) in roots. The foliar nematode is a parasite of aboveground plant parts (stems and leaves) and may be endo- or ectoparasitic. Symptoms caused by A. fragariae are sometimes called spring crimp, spring dwarf, or strawberry crimp. However, these names are misleading and should not be used in California because nematodes and associated symptoms may be present during seasons other than spring and other symptoms may be more important than crimp in recognizing the presence of the nematode.

Although A. fragariae and M. hapla have been most frequently associated with damage in California, strawberries are also hosts for the following nematodes:

• root lesion (Pratylenchus penetrans),
• stem (Ditylenchus dipsaci),
• dagger (Xiphinema americanum),
• needle (Longidorus elongatus),
• foliar (Aphelenchoides ritzemabosi, A. besseyi), and
• root knot (Meloidogyne incognita and M. javanica).

All of these nematodes are potential pathogens to strawberries in California and their identification in strawberry plantings or in land to be planted to strawberries should be cause for concern.

The presence of either foliar or northern root-knot nematodes may cause plant stress and reduce yields. Past (and current) fumigation practices have resulted in these nematodes not being widely present in strawberry production fields. However, with the increasing use of organic methods, which do not include the use of fumigants, infestations and damage may become more common. Control of these two pests by nursery stock producers is critical because it greatly reduces the risk of spreading these nematode pests throughout California. Additionally, an infestation will prevent nursery producers from receiving government certification, thereby greatly reducing the value of the planting stock.

Plant symptoms can be indicative of a nematode problem but are not diagnostic because similar symptoms could result from other problems as well, such as nutrient deficiency, soil texture, or soil moisture. The symptoms may either be widespread or may appear in small patches within a field.

Foliar Nematode

Aboveground symptoms include stunted growth, reddened leaves, small curled or crinkled leaves (crimp), deformed buds and flowers, and a reduction in flowering and fruiting. Research conducted in California on the Chandler cultivar, as well as older cultivars such as Douglas, Fern, Pajaro, and Selva showed that a reduction in flowering and fruiting may more reliably distinguish a foliar nematode infestation from insect infestations, which also produce leaf symptoms similar to those described above. Similar data on more current cultivars are not available, but the same may be true. There are no reported belowground symptoms with this species.

Root-knot Nematodes

Aboveground symptoms include wilting during hot days, stunting, chlorosis, and reduction of yields. Root galls formed near the root tips and prolific root branching at and above these galls are the primary belowground symptoms of this pest.

To make proper management decisions, it is important to determine which nematode species are present. Take plant or soil samples (or both) and send them to a diagnostic laboratory for identification.

In an existing strawberry crop:

1. Examine the field for symptoms described above.
2. Dig up the entire suspected plant, place the plant and surrounding soil into a plastic bag.
3. Take a separate sample from an area without symptoms for comparison.

In fallow fields or fields in other crops:

1. Visually divide the field into sampling blocks that represent differences in soil texture, drainage patterns, or cropping history, but are not larger than 5 acres in size.
2. Take a separate sample from each block so that each can be managed individually.
3. Sample when the soil is moist and sample at the rooting depth of the current or previous crop.
4. Collect at least 10 subsamples from different parts of the field.
5. Mix these subsamples well and place about 1 quart of soil per sample into a plastic bag. If possible, place roots from the crop in the bag with the soil.

Seal bags, place a label on outside of the bag, keep the sample cool (do not freeze), and transport as soon as possible to a diagnostic laboratory. Inform the laboratory that you want to know if the nematodes listed as pests above are present so they can use appropriate extraction techniques. Request a diagnosis to species. Keep in mind that nematode sampling and extraction techniques are typically 30 to 50% effective in finding species that might be present. Therefore, a negative finding does not rule out the possible presence of plant parasitic nematodes.

The use of certified planting stock (produced in fumigated fields) has been critical in avoiding the introduction and spread of parasitic nematodes into strawberry production fields. Combined with fumigation of production fields, this has resulted in nematodes generally not causing important problems in strawberry. For growers not satisfied with the degree of nematode control that has been achieved with the standard techniques, use of additional techniques (such as hot water treatment of infested planting stock) could increase the level of control obtained.

For growers wishing to minimize the use of chemical nematicides, using a combination of other techniques should provide some degree of nematode control. However, the same dollar input will probably not equal the control or yields achieved with fumigation, although the costs could perhaps be offset by higher returns for organically produced strawberries. Alternative techniques often require more advanced planning than the use of traditional chemical management techniques, may not be applicable to all growing situations, and should first be attempted on a small scale.

Nonchemical alternatives to nematicides include:

• select a planting site free of plant-parasitic nematodes
• clean equipment to minimize nematode transfer
• avoid nematode-infested irrigation water
• use hot water treatments of planting stock
• plant when environmental conditions are unfavorable for nematodes
• remove plants with symptoms indicating foliar nematode infestation
• rotate with broccoli and incorporate the crop residue into the soil
• fallow the field
• biological control

It is not possible to provide guidelines applicable statewide for using combinations of these techniques that might provide adequate nematode control. Growers wanting to utilize nonchemical management techniques should discuss possible solutions with their local farm advisors.

With the phase-out of methyl bromide, the most effective soil fumigation is a sequential application of chloropicrin or 1,3-dichloropropene/chloropicrin followed 5 to 7 days later by metam sodium or metam potassium. This combination of pesticides can provide effective control of weeds as well as soilborne pathogens, soil insects and nematodes. 1,3-dichloropropene (Telone) is a true nematicide and, as such, is more effective against nematodes than chloropicrin. For more information on fumigation, see the DRIP FUMIGATION section.