Root-knot Nematode Observed in Some Delta Cotton Fields

Note thin stand in the foreground as opposed to the full stand in the background. Root-knot nematode can produce a thin stand in areas where high populations are present in sandier soils.

Much has been made over the past few years of the loss of Temik for cotton farmers.  Temik offered outstanding nematode management options in addition to some insecticidal properties.  However, with the complete loss of the product from the commercial marketplace some farmers have been forced to rely on seed treatment nematicides as stand-alone products.  However, moving forward, with the EPA’s announcement to allow a generic aldicarb into the marketplace (Meymik 15G produced by AgLogic, LLC) farmers may once again be able to more effectively manage nematodes.  But, much of this will depend on the availability of the product in 2013.  The announcement by the EPA late in 2011 meant that not enough product would be available in the pipeline for farmers to gain access prior to planting the 2012 crop.

In some cases, a thin cotton stand can be the result of a high nematode population.  Yellow foliage, plants having the appearance of nutrient stress, nutritional deficit around the margin of the leaf, and in some rare cases dead plants can all point to a severe nematode problem.   Symptoms in fields with a history of continuous cotton should be investigated.  In addition, in years where conditions of drought prevail for extended periods of time the symptoms associated with nematode injury can be much more pronounced.  However, observation of dead plants may not result in the presence of galls on the roots.  Observing the roots of what appear to be “healthy” plants within the thin field areas sometimes will provide evidence of root galling associated with root-knot nematodes.

Specifically, particular nematodes prefer a specific set of soil characteristics.  RKN prefers soils that predominantly contain sand, while conversely reniform nematodes prefer soils with a greater proportion of silt than sand.  Keep in mind that some fields can contain both soil textural classes and thus have both nematodes present.  But, over a period of time it is more likely that a single nematode will prevail in a given field.

Root-knot nemtaode galls on cotton roots. Note the arrows highlighting the galls on the roots.

Over the past two weeks I’ve observed 2 fields with root-knot nematode infested soils and a field with a reniform nematode issue (to be covered in a separate blog post).  To diagnose the nematode problem, roots were gently removed from the soil profile and observed for the presence of galls.  Pulling plants up without the aid of a shovel will likely leave roots in the ground and make it much more difficult to determine the problem is associated with high nematode populations.  Galls are formed by the female nematode creating a feeding site in the root.  The portion of the nematode that sticks out of the root is the portion of the female that contains eggs that will hatch and produce juvenile nematodes.  On cotton, galls will typically measure less than 1/32nd of an inch and can be observed on lateral roots as well as tap roots depending on the variety planted as well as the overall population present in the soil.  Unlike the production of galls in infested soil planted to soybean, root-knot nematode produces low levels of galling on cotton roots.  In some cases, the galls can be difficult to observe unless the soil is washed off and a microscope is employed to check for the presence of galls.  In the laboratory galls can be crushed to positively confirm the presence of RKN and the eggs.

Economic threshold

The RKN threshold differs between spring and fall soil sampling (post-harvest).  Typically, soil samples collected between December and May are not good indicators of a root-knot nematode population.  However, samples collected in June can be indicative of a nematode problem.  But, spring numbers should theoretically be low.  Two weeks ago a field was observed in Tunica County with thin areas of cotton (see photos).  The difference in the plant stand between what could be considered to be “healthy” plants and infested plants was generally 4-8 inches in height.  In addition, dead plants were present throughout the plant stand that measured 2-3 inches in height.  Dead plants removed from the soil appeared to have succumbed to seedling disease.  However, plants that appeared to be marginally healthy were removed from the soil and a small numbers of galls were observed on the roots using a hand lens.  Plants were returned to the lab and galls were removed to determine if the galls were caused by RKN.  Galls were squashed on a microscope slide and eggs were observed inside the galls.  Soil samples were sent to the nematode diagnostic laboratory and in the worst part of the field, 914 RKN were recovered.  The spring threshold for RKN in MS is 75 RKN/pint of soil; however, the economic threshold listed by state may differ.  For example, Arkansas suggests an economic threshold of 50 RKN/pint of soil while South Carolina breaks the threshold down even further based on soil characteristic.  More importantly, the fall threshold for RKN in MS is 150 RKN/pint of soil.  A quick guide for nematode thresholds in MS is available at:

http://www.entomology.msstate.edu/resources/dept-links/nematology/msunr.html

Economic thresholds were created as a guideline to determine the number of nematodes that produce a loss of yield in a specific crop.  Most of the nematode threshold values are based on observations rather than hard data.  What is even more important regarding the specific thresholds has to do with the particular soil characteristics encountered.  The damage attributed to a specific nematode can differ based on soil characters.

Soil sampling

Root-knot nematode can affect corn, cotton, and soybean.  Our best suggestion is this, if you have an area of a field that has had a reduction in yield over time or plagued by a thin stand this season, collect a soil sample and send it to the Nematode diagnostic laboratory at Mississippi State University.  We realize there are other laboratories that can provide nematode counts; however, sending samples to those labs means that the Extension Service loses valuable data from MS.  By sending samples into the lab we can also help you determine what will be the best future course of action based on the particular nematode present and overall numbers in the soil.  Keep in mind nematode analysis is $11/sample.  While this may be considered an added expense, extracting nematodes from soil is time consuming since the soil samples need to be treated different depending on the particular nematode that may be present and counting the nematodes requires an experienced individual to identify the nematode properly.  Keep soil samples cool and out of the sunlight or heat.  Collecting the samples in a labeled freezer bag and then placing in a cooler or water cooler is a great way to store the samples until delivery.  However, double bagging is best in case the ice melts and the water enters the sample bag.  Wet samples will need to be recollected.

The best method is to collect a composite soil sample, consisting of several soil cores from the affected area.  Walking in a random zig-zag pattern through the area in question, use a soil probe to collect at least 15 cores to a depth of 6-8 inches.  Collect the soil cores and place them in a bucket and hand mix them before submitting the sample is usually best to provide a representative sample of the area.

Management options

At this point in the season the particular management options are increasingly limited.  Aldicarb could no longer be applied in-furrow, fumigants are out of the question, and Vydate application at this stage is too late (typically applied the 1^st through the 7^th leaf stage).  So with that in mind you are essentially limited to planning for the future.  Tolerant varieties are available; however, some of those that are commercially available are marketed as “resistant” and likely only have some level of tolerance to RKN.

Telone applicator

In planning for next season, one alternative is Telone II.  However, site-specific application methods are best employed when using Telone since the product is approximately $16/gallon and the application rate is 3 gallons/Acre.  The MSU Extension Service has conducted site-specific Telone application trials for the past several years.  During 2011, severe RKN infestations near Rosedale, MS were managed with Telone and 200 pounds of seed cotton were the result of the site-specific application compared to untreated strips.  However, several additional methods need to employed to verify that the application is effective and include: a Veris map of the field or yield map to determine the areas of the field where yield loss may be more likely, creation of a zone map to determine the different soil characteristics present within the field, and nematode sampling to determine the specific nematodes and numbers present.  Once those variables have been collected, the application of Telone can be made in the soil zones that may best benefit from the product based on the nematode numbers present and soil electrical conductivity values present.

Seed treatment nematicide products are available for application to cotton seed.  However, the products that are commercially available are best placed in situations where low to moderate numbers of nematodes are present.  Excessive populations, such as the situation mentioned above with more than 900 RKN/pint of soil in the spring appear to not be conducive situations for seed treatment nematicides.  As with any plant disease/nematode situation, the best alternative may be applying several methods to prevent yield loss as a result of the organism.