Nematodes

This year we continued our tomato trails. After last year's problems with nematodes we decided to test control methods for these pests. We grew and rated heirloom tomatoes in one plot at McClellan Ranch in 1992. In 1993 the City of Cupertino offered us a larger plot so we moved our project and re-grew the most favored varieties. The fruit yields seemed to be lower than in prior years. When we removed the plants at the end of the season we discovered galls on the roots. Dr. Kolodge diagnosed root-knot nematode damage. Based on this information we decided to experiment with different methods of control. Prior to planting tomatoes in 1994 we took samples of the soil and sent them to the nematology department at U.C. Davis for a baseline pest count.

U.C. Publication 4083, "nematode diseases," explains that nematodes are members of the roundworm family. This is a very large family and many species are plant parasites. Although several different species are present in most soils they are plant-specific. The species we are concerned with is classified Meloidogyne, a sedentary endoparisite. The common name is root-knot nematode. Their life-cycle is characterized by burrowing into the roots of host plants for food and to deposit eggs. As the eggs grow, a visible gall is produced on the root, hence the name root-knot. This galling interferes with plant growth. After hatching, the nematodes migrate away from the roots from only a few inches to a foot during a season. They spread to wider areas chiefly by human intervention, through the use of hand tools or farm machinery. It is important to disinfect any tools that are used where nematodes are present to avoid spreading them to unaffected areas

In conjunction with attending the Master Gardener conference at UC Davis, in the summer of 1993, last summer, Ralph Riddle and Roxanne Beverstein toured the nematology laboratory and discussed the project with Dr. Becky Westerdahl and her assistant Cindy Anderson. Dr. Westerdahl told us that nematodes are a serious problem for commercial growers. They affect many vegetable crops such as cucurbits, carrots, lettuce, beans, tomatoes, okra, sweet potato, white potato and turnip. Her lab assists farmers in combating this pest. It was an educational bonus for us. Dr. Westerdahl attended graduate school at U.C. Riverside with Extension Director Dr. Kolodge who is also a nematologist.

Of the nematode control methods practiced, the most widely used by commercial growers employs methyl bromide, a soil fumigant. This product is not available to the public. Currently the only means of control available to home growers is planting species or varieties that have been selected to be resistant. Since methyl bromide is being phased out due to environmental damage, researchers are attempting to find alternative methods for control. One of these products is undergoing field trials to obtain approval for commercial use. It is a chemical that is derived from chitin in the shells of crustaceans. Given its origin it would be considered an organic or "natural" control method. This could potentially be available for use by home growers. Dr. Westerdahl explained that the active ingredient is an enzyme that is supposed to dissolve the shells of nematode eggs preventing them from maturing.

We asked for a sample of "chitinase" from the commercial laboratory involved in the field trials. They sent us a 5 gallon bottle! It contained a very foul-smelling thick, brown, liquid. It was the smallest quantity the lab produced, enough to treat 1/4 acre!

We divided our test area into four plots, each approximately 5ft X 20ft. The chitinase was then diluted to treat one of these 100 sq. ft. plots. We applied the material as directed three times, one month apart, beginning in May. After the first application we planted 8 varieties of tomatoes in the treated plot. Both heirloom and hybridized resistant varieties were included. In the adjacent plot we replicated the tomato planting. Our limited garden space does not permit complete replication as would be done in a classic trial.

For the second control experiment we employed soil solarization. U.C. Leaflet 21346, "Solarization -- A Tool for Verticillium Wilt Control in Orchards," also mentions that this technique reduces nematode populations. Of course this method has the disadvantage that it must be done during the peak of the growing season. You can't use the plot for warm-season plants until the following year. We normally cover our entire experimental garden with mulch, so we cleared the mulch from the third plot and covered it with clear plastic. The edges were then sealed with earth. The soil temperature peaked in the mid 90's in September.

The fourth and last plot we left fallow, covered with mulch. U.C. experiments show that the nematode population can also be reduced by keeping the area free of all plants, so the nematodes have nothing to feed upon. Again, this renders the area unproductive for the season.

The tomatoes grew in both treated and untreated plots with no obvious differences in vegetation. At the end of the season we pulled out the plants and observed galls on the roots of the heirloom varieties from both the treated and untreated plots. The only difference we could discern was that the heirloom plants seemed to produce fewer fruit compared with those grown in other plots in our community garden. We noticed this phenomenon the year before, especially on Brandywine. The soil was re-sampled and tested by the U.C. Davis lab.

Although the nematode count increased in both planted plots, the results show that the chitinase treatment was effective in suppressing the pest. (See Table 1) The results from the solarized and fallow plots are inconclusive since the pre and post season counts were both zero. Nothing had been planted in plot four during the previous year, and plot three had been only lightly planted. It looks like these would be ideal areas in which to try planting tomatoes this year.

Table 1 -- Nematode Counts from U.C. Laboratory

Plot	Pre-Season	Post-Season
1-Treated - planted	100/lit	3,100/lit
2 -Untreated - planted	650/lit	21,150/lit
3 -Solarized	0/lit	0/lit
4 -Fallow	0/lit	0/lit

Conclusions:

Although the chitinase product seemed to work, it would be easier for homeowners to just plant species, or varieties that are resistant to nematodes. Most modern tomato hybrids are resistant. Look for N on the plant label or seed packet. For example VFNT indicates resistance to Verticillium wilt, Fusarium wilt, Nematodes and Tobacco Mosaic virus.

Contributors to this project:

Master Gardeners: Cheryl Berumen, Howard Craven, Ralph Eddy, Elaine Levine, Karla Meadows, Roxanne Beverstein

Plants Donated by: Yamagami's Nursery, Upstarts