MSU Extension

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

Potato early-dying disease (PED) is a complex with the lesion nematode, Pratylenchus penetrans and the wilt causing fungus, Verticillium dahliae, usually implicated as the causal agents. Both organisms are pathogens of potato but when present together, they often interact to produce more significant yield losses than they would cause individually. Yield losses of 100 cwt/A are not uncommon in Michigan and because of this yield loss potential, it is highly recommended growers sample fields going into potato production in 2010 for nematodes and vert. This is an important activity in attempts to minimize the risk of potato early-dying disease.

To best assess population densities of lesion nematodes, it is necessary to collect soil and root samples. If a cover crop will be grown, wait to collect root samples until about a month after the crop has been sown. Lesion nematodes feed within root tissue, so if roots are not collected the population density will inevitably be underestimated. If a cover crop will not be sown, still plan to collect roots of the previous crop (e.g., corn stubble) because these nematodes use the roots as overwintering sites. The bottom line is, regardless of the condition of the roots, it is always advisable to include them in a sample. The cost of the analysis (MSU Diagnostic Services assesses a fee of $25 for a standard nematode analysis) is the same for roots and soil as it would be for only a soil sample. For lesion nematodes, always include roots. OK, enough on that emphasis.

When sampling for nematodes and Verticillium dahliae, the more soil probes collected, the better the sample. Both of these organisms are clumped in their distributions, so multiple soil cores are necessary to insure sampling within these “hot spots” where the pathogens are present. Soil should be homogenized as well as possible in a large pail or plastic bag and a pint to a quart ultimately submitted to the lab for analysis. Be sure to place the roots you’ve collected (a handful is a good estimate) in the smaller plastic bag with the soil.

Typically, nematode samples will be completed in seven to 14 days after they arrive in the laboratory. However, a minimum of 21 days are required for vert assays as a small quantity of soil is air-dried for seven days (to eliminate organisms that are susceptible to desiccation) and the fungus is provided time to grow on a selective medium in Petri plates for its subsequent identification. Please keep this time requirement in mind when submitting samples.

The results of the nematode samples and the assays for V. dahliae will be used to assess the risk of loss to next year’s potato crops. If, based on the population densities of lesion nematodes and V. dahliae, the risk of loss to next year’s potatoes is high or severe, another year out of potatoes or soil fumigation will typically be recommended. If most of the risk is solely from high to severe counts of lesion nematodes, re-sampling for nematodes in the spring will be advised to determine if non-fumigant nematicide use is warranted at planting. It is impossible to predict the amount of nematode mortality that will occur over the winter, so collection of another nematode sample is advised to provide additional information.

Fumigation, usually with metam-sodium, is often used to control potato early-dying disease. In Michigan, for optimal results, fumigation should be done in the fall rather than the spring. Therefore, it is important to collect samples early enough in the fall to allow time for completion of the vert assays and still fumigate, if necessary, before the onset of winter. However, keep in mind, that unlike some other soil fumigants, such as 1,3-dichloropropene, metam sodium is very effective at soil temperatures between 40 to 50°F. It is my opinion, soil fumigation is more likely to fail in Michigan when using Vapam, if the product is applied when soil and air temperatures are warm rather than cold.

At the time of this writing, we have already received over 150 samples for nematode and V. dahliae analyses into MSU Diagnostic Services. Approximately, 10,000-20,000 acres of land are sampled annually to avoid potato early-dying disease. For this reason, we are extremely busy in the lab in the fall. You know, most citizens of Michigan would consider the arrival of fall with the changing of color of the leaves of our deciduous trees. Not me. I know fall has arrived when I have 500 or so Petri plates sitting next to my microscope waiting for me to count colonies of V. dahliae.

If you have questions about proper sampling for nematodes and V. dahliae please don’t hesitate to call me at 517-432-1333, or Angela Tenney at 517-353-8563 or Dr. George Bird at 517-353-3890. You can also check our web site, www.pestid.msu.edu. Presently, there is a $25 fee for a nematode analysis and an additional $25 charge for a vert assay if the wet-sieving method is chosen. Routine sampling in the fall for these causal agents of potato early-dying disease is highly recommended.