In 1998, a three-year study was started in a southern Georgia nursery to evaluate treatments to replace methyl bromide (MC33), an ozone-depleting soil fumigant scheduled for phase-out by 2005. During the third year of the study, patches of stunted seedlings began to appear in different sections of the nursery fields. The needles of the stunted seedlings were yellowed, and the root systems were much smaller than normal, with few lateral or fine roots.
Soil samples from the affected areas were sent to a nematode laboratory: soil and roots were also examined for fungus pathogens, but the cause of the stunting could not be determined. Examining unwashed pine seedling roots under a dissecting microscope, Fraedrich and Cram found large needle nematodes of the Longidorus genus that had escaped the notice of the nematode laboratory. Growth chamber experiments on container seedlings inoculated with Longidorus resulted in root damage similar to that in the stunted field seedlings.
Finding Longidorus was a surprise: the nematodes have been identified in soil from areas where southern pines are grown, but Fraedrich and Cram could not find any published reports of needle nematodes damaging the roots of loblolly or other southern pine species. The USDA-ARS Nematology Laboratory in Maryland was unable to identify the species of Longidorus found in Georgia: the nematode is presently listed as "undescribed" and believed to be a new species.
The new Longidorus is seven to eight millimeters long -- quite large for a plant-parasitic nematode -- and occurs in relatively small numbers around the roots of stunted pine seedlings. These factors help explain why the first nematode laboratory did not find Longidorus in the soil samples sent from the affected areas. Extracting large nematodes from soil samples requires specific techniques not routinely used by nematode laboratories.
"Damage to pine seedlings by needle nematodes is a previously unknown problem in southern nurseries," said Fraedrich. "Nursery managers and pest management specialists who suspect nematode damage should alert testing laboratories to examine soil samples specifically for Longidorus."
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