Kate Everts, Vegetable Pathologist, University of Maryland; firstname.lastname@example.org
All cucurbits, including watermelon and cantaloupe, are hosts of root knot nematodes (RKN). RKN are plant pathogenic roundworms, which live in the soil. Although many other species of nematodes are nonpathogenic to plants or even beneficial, RKN can invade host roots and result in yield losses to cucurbits. Symptoms of RKN are often overlooked because the stunting, reduced vigor and wilting of the host plant can be caused by many other biotic or abiotic causes. To determine if plants are infected with RKN, observe plant roots for large galls, knots, or swellings. Also, look for damage that occurs in patches in the field. Plants are most susceptible to damage from RKN at the seedling stage.
Preventative management of plant-parasitic nematodes, using rotation, cover crops, transplants that are free of nematodes, nematicides and soil fumigants, is an important measure and more effective than trying to manage an outbreak. Use of green manure and soil amendments is also beneficial. We have found the soil incorporation of large amounts of organic matter, such as sorghum-sudangrass green manure in combination with poultry compost, reduces populations of root knot nematodes. Some rapeseed cultivars, such as ‘Dwarf Essex’ and ‘Humus’ also are suppressive to nematode populations.
As noted above, management of RKN is best done prior to planting. Where damage has been observed in the past, several soil samples should be taken from soil within the root zone, mixed together, and sent to a diagnostic lab for identification. If RKN is present in damaging levels, be sure that appropriate cultural practices are in place. If chemical management is necessary, it is best conducted preplant with fumigants such as Telone or Vapam.
Once the crop has been established the available options are less effective. However, the following can be used, Vydate L can be applied at 0.5 to 1.0 gal/A and incorporated into top 2-4 inches of soil, or at 2.0 to 4.0 pt/A apply 2 weeks after planting and repeat 2-3 weeks later. Velum Prime, which is in a different chemical class, can be applied at 6.5 to 6.84 fl oz/A through drip irrigation at 5-day intervals (see label for details).
I discussed this problem with Dr. David Langston at Virginia Tech, who has worked extensively on nematodes. His opinion is that although both Vydate and Vellum Prime may have some effect on nematodes when applied after transplant, the effect will be modest. If a grower is committed to a rescue treatment, keep in mind that watermelons are relatively tolerant to RKN and either forego treatment, or apply the least expensive option.
No matter what the treatment decision, remember that the damage from RKN can be mitigated, to some extent, by providing plants with adequate nutrition, moisture, and protection from stress.