Grape Replant Disorder—An Integrated Management Approach

S. Schneider, USDA-ARS, Postharvest Quality and Genetics Research Unit, Fresno, CA 93727, H. Ajwa and T. Trout, USDA-ARS, Water Management Research Lab, Fresno, CA 93727, and J. Sims, Dept. of Plant Pathology, University of California, Riverside, CA 92521

Pre-plant soil fumigation with methyl bromide has commonly been used to prevent or lessen “replant disorder” when replanting grapes into vineyards infested with soilborne pests. Methyl bromide is effective against a wide range of soil pests including insects, nematodes, weeds, and pathogens. Accurate diagnosis of the specific problem in a given vineyard has not been necessary, since methyl bromide is effective against such a wide spectrum of pests. Methyl bromide can be used effectively against soil pests over a range of soil types, temperatures, and moistures resulting in greater flexibility of use and less risk of crop loss than is possible with many other soil treatments. Another strength of methyl bromide is its ability to kill old grape roots deep in the soil. If these are not killed, they serve as a reservoir of inoculum for soilborne pests.

Growers who need to replant vineyards that have existing soil pest problems will need alternatives to methyl bromide because of the scheduled ban on import and manufacture in 2005. Unless a “silver bullet” that is effective against a wide range of pests and over a range of soil conditions can be found and made available, the first challenge will be to accurately diagnose the problem(s) in a specific field. Once the problem is identified, a management strategy must be generated that is: 1) effective against the identified pest; 2) effective under the soil conditions found in that vineyard or field; 3) economically feasible; and 4) environmentally acceptable. This strategy might be a single action or, more likely, an integrated approach using multiple biological, chemical, and cultural tools and approaches.

Because of the brief time before the ban on methyl bromide is implemented, adapting the use of existing commercially available compounds as components of integrated systems is being tested as a transitional solution during the more time-consuming development of culturally and biologically based management systems. An additional level of complexity results from the spatial variability of pest populations and soil physical, chemical, and biological factors which will impact the fruit yield and quality. Some management strategies have been hindered by inconsistent performance—sometimes they work, sometimes they don’t. This could be due to the spatial variability of soil factors that affect the efficacy of the management option. Understanding the biological, chemical, and physical interactions will allow growers to select options best suited to their conditions. Management strategies will have to be tailored to the specific pest problems and soil conditions in order to maximize the probability of successful replanting.

Our approach to research on the replant problem in vineyards is to identify and characterize factors contributing to the problem and then mount a two-pronged attack: first, eliminate or minimize factors that negatively impact growth and yield and, second, introduce or enhance factors that positively impact growth and yield. Novel delivery systems such as drip irrigation systems and introduction of beneficial organisms/materials into nursery materials will be evaluated.

In the fall of 1997, a field trial was initiated in a 15-year-old “Thompson Seedless” vineyard at the ARS research location in Parlier, CA. The nine treatments were:

1) a six-month fallow (the untreated control);

2) a combination application of Telone II EC (35 gal/acre) in 60 mm of water through a buried drip tape plus Vapam (26 gal/acre of 42% metam sodium) applied through surface microsprinklers;

3) same as #2 except the Telone was applied in 100 mm of water;

4) a shanked application of methyl iodide (400 lbs/acre), tarped;

5) a shanked application of methyl bromide (400 lbs/acre), tarped (the treated control);

6) 18-month fallow;

7) same as #2 following an 18-month fallow;

8) same as #3 following an 18-month fallow; and

9) 18-month fallow plus a sorghum-sudangrass hybrid cover crop.

Each treatment was replicated 5 times. The telone/vapam treatments were applied in early January, 1998. The methyl bromide and methyl iodide treatments were applied in late April, 1998.

The telone/vapam combinations are novel applications which reduce worker exposure to currently available chemicals. Methyl iodide is not currently registered, but has been shown to be effective in tests on replant disorder in other crops. Fallow treatments remove the existing above ground portions of the vines and the roots that can be easily pulled out, but does not remove all of the roots, especially those deeper in the soil. An 18-month fallow results in a loss of use of the vineyard for an additional year, but also removes the actively growing vine and upper roots as biological factors in the ecosystem for that year.

In July of 1998, each plot was planted with three grape variety/rootstock combinations; own-rooted Thompson Seedless, Merlot on Harmony rootstock, and Merlot on Teleki 5C rootstock. The rootstocks vary in levels of resistance to nematodes, which are thought to play a role in replant disorder. Data on plant growth and nematode populations will be collected for at least five years in order to determine the impact of the treatments not only on vegetative plant growth, but also on fruit yield and quality. Susceptible St. George rootstocks were interplanted between the primary vines to be used as bioassay plants to determine if the nematodes present in the soil are still infective.

Soil samples were collected from each plot at planting to a depth of 5 feet and assayed for plant parasitic nematodes. There was no significant difference between the numbers of rootknot nematode (Meloidogyne spp.) in the untreated control, 18-month fallow (18F), and 18-month fallow plus cover crop (18F+CC). Rootknot numbers were not significantly different between the methyl bromide (MB), methyl iodide (MI), and all four telone/vapam (T/V) combinations, and were significantly less than the untreated control. All nematodes in the MB, MI, and T/V treatments were dead and coiled, whereas the nematodes extracted from the fallow treatments were active. Numbers of dagger nematode (Xiphinema spp.) were slightly higher in the untreated control and 18 month fallow treatments than in the other treatments, but not significantly so. Total dagger nematode populations were relatively low across all treatments. Ring nematode (Criconemella spp.) was significantly higher in the 18F and 18F+CC plots than in all other treatments. Pin nematode (Paratylenchus spp.) numbers were higher in the untreated control, 18F, and 18F+CC treatments than all other treatments.

A qualitative rating of weed abundance was made approximately three weeks after vines were planted (7 months after the T/V treatments and 3.5 months after the MB and MI treatments). The untreated control, 18F, and 18F+CC plots contained a dense cover of weeds. The T/V plots had a few weeds. The MB and MI plots were essentially weed-free.

In February, 1999, the dormant vines were pruned back to 2 nodes above the graft union. Pruning weight per plant for Thompson Seedless vines was highest for MI plots, intermediate for the MB and T/V plots, and lowest in the untreated control, 18F, and 18F+CC plots. There were no differences in Merlot pruning weights for the Harmony and Teleki 5C rootstocks across treatments.

Soil samples were collected to a depth of 24 inches in late May 1999, approximately one year after planting, 18 months after the telone/vapam applications, and 13 months after the methyl bromide and methyl iodide applications. There were no detectable plant parasitic nematodes in any of the plots treated with methyl bromide, methyl iodide, or the telone/vapam combinations. There was no significant difference in citrus nematode populations in the untreated control, 18F, or 18F+CC treatments on each of the rootstocks and all were significantly higher than the chemical treatments for their respective rootstocks.

In the Thompson Seedless plots, rootknot nematode populations were significantly higher in the untreated control than in the 18F and the 18F+CC treatments. There was no significant difference between the ring, dagger, and pin nematode populations on Thompson Seedless in the untreated control, 18F, and 18F+CC plots.

On the Teleki 5C rootstock, rootknot nematode populations were significantly higher in the untreated control plots than in the 18F+CC, and intermediate in the 18F treatments. Ring nematode populations were significantly higher in the 18F+CC plots than in the untreated control, and intermediate in the 18F treatments. Pin nematode populations were significantly higher on Teleki 5C in the 18F+CC treatments than in the untreated control or the 18F. Dagger nematode populations on Teleki 5C were significantly higher in the 18F than in the untreated control or 18F+CC treatments.

On the Harmony rootstock, rootknot and dagger nematode populations in the untreated control and 18F+CC treatments were significantly higher than the 18F treatment. Ring and pin nematode populations on Harmony were significantly higher in the untreated control than in the 18F+CC treatments and were intermediate in the 18F treatments. Soil samples will be collected again in the fall.

Bioassay samples will be collected this summer to determine the populations of rootknot nematode in the roots. For each succeeding year of the test, soil samples will be collected in spring and fall to determine the population levels of the plant parasitic nematodes and plant growth measurements will be made. Evaluation of the soil samples for other biotic and abiotic factors will be conducted as resources allow. When the vines begin bearing fruit, fruit yield and quality will also be evaluated. The entire experiment is being repeated in 1999.

Additional lab and greenhouse tests are being conducted to evaluate various biological, chemical, and cultural management strategies for their negative impacts on pests and positive impacts on plant growth. Agri-50, a colloidal compound, prevented hatch of rootknot nematode eggs at high concentrations and killed infective juveniles at lower concentrations. Greenhouse and field tests are being conducted to evaluate this product in a soil ecosystem. Ceres and Liqui-comp, both microbial products, appear to enhance plant growth in preliminary tests in non-fumigated field replant soils, but did not kill rootknot juveniles or eggs in the lab.

In the short term, novel applications of currently available chemicals appear to be the most likely alternatives to methyl bromide. These will serve as stepping stones during the transition to an integrated systems management approach based on an understanding of the interactions and spatial variability of biological, chemical, and physical factors in the agro-ecosystem. Such a system will include management strategies to reduce or eliminate pests, enhance beneficial organisms, promote good plant growth, kill old roots deep in the soil that serve as pest reservoirs, and protect the environment.

New Editor

Sharon Durham joins the ARS Information Staff as the new editor of the USDA Methyl Bromide Alternatives Newsletter. She can be reached at 5601 Sunnyside Ave., Beltsville, MD 20705–5129; phone (301) 504–1611, fax (301) 504–1641.

Last Updated: October 1, 1999