Prior to planting vegetable crops or planting of perennial crops, it is advisable to test the soil for plant parasitic nematodes. Soil testing can reveal the identity and population pressure of several potentially harmful nematodes. Ideally, soil samples should be taken at harvest or postharvest in the root zone of the previous crop.

This information is invaluable in preparing for fall planting, including choosing appropriate soil fumigation or treatment methods. In the Imperial Valley, plant parasitic nematodes of concern include the needle nematode (Longidorus africanus), root knot nematodes (Meloidogyne incognita, M. javanica, and M. arenaria), lesion nematodes (Pratylenchus spp.), stubby root nematodes (Paratrichodorus and Trichodorus spp.), and Sugarbeet cyst nematodes (Heterodera schachtii).

There is a wide crop host range for many of these nematodes. The common name of the Sugarbeet Cyst nematode should not be mistaken for the host range. The Sugarbeet Cyst nematode is able to infect most cole crops in addition to sugarbeet, causing stunted growth and mid­day wilting. The damage caused by the needle nematode may be very familiar to carrot growers ­causing a pronounced forking of the carrot taproot. Lettuce growers will recognize needle nematode damage as galling on lettuce roots, and stunted seedling growth. Root knot nematodes cause galling symptoms on the fibrous or tuberous roots of susceptible plants, reducing yield and marketability of some crops. Symptoms of plant parasitic nematodes vary according to the nematode and host. However, more common symptoms include yellowing, mid­day wilting, stunting, and circular patches of poor growth.

• Sampling methods

The decision to fumigate should be based upon the identity and population number obtained from nematode sampling of the soil from the previous crop, and a history of nematode infestations in the field.

Each crop has an economic threshold guideline that should be used to determine if treatment is necessary for each type of plant parasitic nematode. Samples should be taken at harvest of spring crops prior to subsequent fall crops. Nematode analysis labs are able to quantify and identify multiple types of nematodes from one sample. The sampling technique is very important for revealing the nematode populations inhabiting the soil. Nematodes like to migrate into ‘hot spots’ in the field, and usually at or below the root zone. Therefore, it is important to take soil from 12 inches to 24 inches depth, representative of the rooting depth of the current or previous crop.

Most importantly, randomly sample soil from throughout the space of the field. When sampling, discard surface soil if it is too dry, and include the moist soil underneath (UC­IPM). Sampling using a shovel should suffice in clayish soil; sandier soils may rely upon a soil core probe if available. A reliable method of ensuring that you have taken a representative sample is to divide the fields into blocks (not more than five acre areas) and randomly sample and combine the soil from a block into a plastic bag. Each of the soil samples from each block should be refrigerated (not frozen) prior to shipping, and it may be helpful to include an icepack during shipping to keep the soil temperature cool. There are nematode analysis labs that are able to correctly and quickly help with nematode analysis from the soil.

• Management

Fumigant treatments that are available for use in California include Metam Sodium (Vapam, Sectagon 42), 1,3­Dicholorpropene (Telone II, Telone­EC), and a 1,3­Dicholorpropene/Choloropicrin mix (InLine), Oxamyl (Vydate) and Myrothecium Verrucaria (Ditera). Please check the label for crop use restrictions and recommendations.

Other more environmentally friendly management techniques include green manure crops, and soil solarization or the combination of the two techniques. Green manure is a crop (usually mustard) that is grown in the soil for approximately two months, and then prior to flowering incorporated into the soil to release isothiocyanates that have fumigant activities. However, care should be taken prior to planting green manure, as many of these brassicaceous plants are good hosts for some root knot species and may increase levels of root knot to a level that may not be reduced sufficiently following green manure incorporation. It should also be noted that soil solarization may only heat up the top 12 inches of the soil, and any nematode populations inhabiting lower depths will later migrate to the root zone.

An alternative biofumigation technique that doesn’t require growing a crop prior to planting is the use of seed meal or mustard meal soil amendments. Mustard seed meal is a commercial waste byproduct from defatting mustard oil from the mustard seed. The leftover seed tissue has high levels of glucosinolate chemicals that break down into isothiocyanate products with fumigant activities. However, care should also be taken in selecting the mustard seed meal, as there are different levels and types of glucosinolates that occur in various varieties of mustard plants that may or may not have a nematicidal effect on the target plant parasitic nematode.

Crop rotation may be of help for some crops infected by Sugarbeet cyst nematode. Crucifers, beets, spinach and some related weeds are good hosts for the Sugarbeet cyst nematode (UC­IPM). The length of rotation away from these crops is dependent on the size of the nematode population sampled after harvest (UC­IPM). Conversely, root knot nematodes or needle nematodes have a very wide host range, including most of the crops grown in the Imperial Valley and rotation is not a viable option. Knowledge of previous plant parasitic nematode problems, and sanitation of equipment and tools is also very important for nematode management.

As the new plant pathology advisor in Imperial County, I would like to invite growers and PCA’s to advise me on nematology issues and ideas for research that need to be addressed in the Imperial Valley. In addition to my other plant pathology projects, I am looking forward to helping solve the prominent nematology issues that are concerning the growers and PCA’s.

Please feel free to contact the UCANR Cooperative Extension office with your concerns and ideas.

• Telephone: 760­352­9474

• E-mail: [email protected].