Root Knot Nematode Problems in Flower Crops

In Florida, many of the commercially produced cut flowers and bedding plants are grown directly in the ground. This method exposes plants to soil-borne pests and pathogens. Many of the flowers and bedding plants are susceptible to plant-parasitic nematodes, particularly root-knot nematodes (Meloidogyne spp.). Snapdragon (Antirrhinum majus), lisianthus (Eustoma grandiflorum), sunflower (Helianthus spp.), and gladiolus (Gladiolus spp.) are among those cut flowers particularly susceptible to this soil-borne microscopic worm. Bedding plants most susceptible to root-knot nematodes are coleus (Coleus blumei), impatiens (Impatiens spp.), celosia (Celosia argentea), and snapdragon.

Robert McSorley and K.H. Wang, from the Department of Entomology and Nematology at the University of Florida in Gainesville, are investigating methods for managing nematodes during the flower producers' long growing season. A long growing season may provide excellent opportunity for buildup of nematode populations. Commercial growers may maintain some crops from September to May, with multiple plantings and/or harvests at the same site. In a commercial planting of lisianthus at Sunshine State Carnations farm near Stuart, Florida, fumigated in late July 2001, root-knot nematode populations increased from zero in October, to 18/100 cm³ soil on February 26, 2002, and to 385/100 cm³ soil on April 11, 2002. "Despite the eventual recovery of root-knot nematode populations (at 8.5 months after treatment), fumigation with methyl bromide was effective in maintaining low (18/100 cm³ soil) population levels for 7 months," says McSorley.

In a second study, fumigant treatments were applied on August 21, 2002: methyl bromide (98 percent) + chloropicrin (2 percent) combination mix (450 pounds/acre); metam sodium (75 gallons/acre) + chloropicrin combination mix (150 pounds/acre); and metam sodium (75 pounds/acre). Results were compared to an untreated control. Treatments were replicated four times in a randomized design. Snapdragons were planted on September 30 and October 23. Weed counts, plant heights, and soil samples for nematodes were collected from the experiment at various times.

Weed counts were taken on October 8 and November 14, 2002. There appeared to be a strong effect of fumigants on weeds, but no significant differences among the three fumigant treatments. "The results showed no statistically significant difference between methyl bromide and either metam sodium + chloropicrin or metam sodium alone," says McSorley. Total weed counts on October 8 were 1.25 per 25-foot row for the methyl bromide + chloropicrin treatment, 1.50 for the metam sodium + chloropicrin treatment, and 0.50 for metam sodium. The untreated control had 16.25 weeds per 25-foot row. On November 14, the methyl bromide + chloropicrin treatment had 4.75 weeds per 25-foot row; metam sodium + chloropicrin had 3.75 weeds per row; and the metam sodium treatment had 2.00 weeds per row. The untreated control had 37 weeds per 25-foot row.

Nematode counts were taken on October 9, 2002, November 14, 2002, January 15, 2003, and February 19, 2003. There were some significant effects of fumigants on nematodes, but stubby-root nematodes had recovered in all plots by January. Stubby-root nematodes can be difficult to control by fumigation, but they are not the major nematode pest of flower crops. Control of root-knot nematodes was not significant, though there was a tendency toward higher numbers in untreated control plots. On October 9, there were no root-knot nematodes in the fumigated soil, and 13.2 nematodes/100 cm³ soil in the untreated control. On November 14, none of the fumigation treatments exhibited any root-knot nematodes, while the untreated control had 0.2 nematodes/100 cm³ of soil. On January 15, the methyl bromide-treated soil had 0.2 root-knot nematodes/100 cm3 soil; the metam sodium + chloropicrin-treated soil and the metam sodium-treated soil still had no root-knot nematodes; and the untreated control had 11.9 root-knot nematodes/100 cm3 soil. On February 19, there were, again, no root-knot nematodes in any of the fumigated soils, but there were 9.8 nematodes/100 cm³ soil in the untreated control.

Plant heights of two cultivars, Pot Ivory and Pot Pink, were measured on December 5, 2002, January, 15, 2003, and February 19, 2003. According to McSorley, control plants showed some stunting in comparison with the other treatments, especially early in the season. "The effect on plant growth was probably due to weeds early in the season. The grower removed weeds once data counts were made, so effects on plant performance were probably not as severe as they could have been," says McSorley.

Olie Nissen, of the family-run Sunshine State Carnations, collaborated with McSorley and was enthusiastic about the results. "I was pleased with the results, but the costs are high. I will go with either Basamid or one of the other, less-expensive fumigants and accept the losses," says Nissen.

Last Updated: July 2, 2003