A Success Story for Biological Control
Bill Burl Brodie
Nematology & Plant Pathology
Ithaca, NY 14853
(prepared from the videotaped presentation*)
For the purpose of this conference and this presentation, I hope that you biological control specialists will allow me to include in your definition of biocontrol the use of host plant resistance. In any case, this strategy is a biologically-based means of control.
We have used host plant resistance to control a very important disease of potato, the golden nematode. This is a quarantined pest that occurs in the U.S. only in the state of New York. So, when we talk about the goal of golden nematode control, it's not about yields, it's not about growing healthy plants, it's about reducing the population of a plant pest to a level where it can't spread.
Dr. Bob Plaisted, a plant breeder at Cornell, Dr. Steve Slack, a plant pathologist at Cornell, Dr. Don Halseth who is a Cornell potato specialist, and I developed and promoted the use of the resistant potato varieties used in this program.
For many years, golden nematode was controlled with the soil fumigants, D-D or Telone, which were applied at 90 gallons/acre. Any chemical applied to the soil at that rate will eventually get into the groundwater, and this occurred in New York.
To prevent this contamination, an effort was made to find a biologically based method to control the golden nematode. To accomplish this type of control, the following aspects of the biology of the potato cyst nematodes (PCN) were considered:
- The PCN is an obligate parasite.
- The PCN 's host range is limited to the Solanaceae.
- PCN can survive 20-30 years in soil without its host.
- PCN hatches primarily in response to potato root exudates.
- PCN requires males for reproduction.
- One generation is produced per year.
A cyst contains 300-400 eggs, each capable of hatching when stimulated to do so by potato root exudates. If no food is present when the nematode hatches, it will die within a couple of weeks. Thus, what we once thought was the nematode's strength can also be its weakness, because the nematode cannot distinguish between resistant and susceptible plants: they hatch equally in the presence of both types of potatoes.
The nematodes hatch and penetrate the roots of the potato plant. If it's a susceptible plant, the nematode will penetrate the root and establish a feeding site (syncitium) involving giant cells and will feed on those giant cells until it produces another generation.
However, with a resistant plant, the nematode hatches and, once inside the plant, a reaction which we haven't yet identified occurs and juveniles are stimulated to exit the roots.
We also found that if nematodes emerge from a susceptible plant they will readily repenetrate either a susceptible or a resistant plant. But if they emerge from a resistant plant, they seem to have lost their taste for potatoes and very few will repenetrate another potato plant, either susceptible or resistant. (Table 1) This is one characteristic of reducing the nematode population.
|Table 1 - Assessment of Repenetration|
Another characteristic of resistant plants is that those nematodes that decide to stay and establish a feeding site find that the cells exhibit a hypersensitive reaction: the cells die that immediately surround the nematode in the plant root and because the nematodes cannot feed on dead cells, they die from lack of food.
Finally, those that do survive, and they are few - not more than 5 per plant - have a diminished capacity for reproduction. On a susceptible plant, there are over 200 eggs produced per cyst, while cysts that are produced on a resistant plant have about 40 (Table 2) . In addition, the juveniles from eggs produced on a resistant plant have a very low infectivity.
|Table 2 - Cyst Size Comparison|
| *Significantly different at P = 0.0001|
**All data values zero; no analysis performed
This scenario for controlling golden nematode with host-resistant plants worked well in the laboratory and in greenhouse pots, but would it work in the fields?
In potato fields, there are hills, where the plants are, and furrows between the plants, where there are no roots. The hills contain about 70% of the soil volume and the furrows contain about 30%. We believed that we could control the nematodes in the hills, but could we control them in the furrows? We need to control all the nematodes because this is a quarantined pest, and in order to effectively manage the population we cannot allow any nematodes to survive.
What we found is this: In the hills where the susceptible variety had grown, there was an increase in the nematode population, and when the resistant variety was grown, there was a population decrease. In the furrows, there was a population decrease when either a susceptible or resistant variety was grown.
We discovered that the hatch-inducing chemical produced by potato roots moves through the soil to the eggs in the furrows, but because the nematodes don't move far enough through the soil, they can't get to the roots and they die. Thus, even with the susceptible plants, there is poor reproduction and the population declines in the furrows. In lab tests, we found that the chemical responsible for hatch stimulation moves 70 cm laterally and 50 cm deep from it point source of production through the soil in sufficient strength to stimulate nematodes to hatch.
From previous research we knew we needed to reduce the number of eggs to less than 0.2 eggs/cc of soil so that the nematodes would be at such a low density they would not spread. We developed a cropping system that alternated yearly plantings of resistant and susceptible potato varieties and nonhost crops that would keep the nematode population low.
Note that the initial density is <1 egg/cc of soil, which is below the detection level of about 1-4 eggs/cc of soil. These results were consistent and repeatable. When we alternately grew resistant and susceptible potatoes at this initial density, the nematode population decreased on the resistant variety but increased to unacceptable levels on the susceptible variety.
Here (above) we planted a resistant potato variety 1 year, a non-host crop the second year, and a susceptible potato variety the third year. With this system, , the population density increased to unacceptable levels when the susceptible variety was grown. This is a density that cannot be detected in the survey that is conducted by the regulatory agency.
When we grew resistant plants for 2 years followed by susceptible plants for 1 year (above), we found that we could keep the population density below 0.2 eggs/cc of soil. However, the small population increase in the year the susceptible variety was grown caused some concern. We didn't know if the population would continue to increase when the susceptible variety was grown in future years.
So we went to a system of 2 years of resistant plants, 1 year of non-host plants, and 1 year of susceptible plants (above). We were able to reach our goal of bringing the population down to <0.2 eggs/cc and maintaining it at that level.
It's possible to continuously plant all resistant plants, and in some cases this is done. We wanted to allow the growers some flexibility, though, and with our system they are only required to plant golden nematode resistant potatoes 2 years out of 4, and they can grow anything they want the other two years.
Next, we tried this cropping system on a more realistic population level instead of at an initial density of <1 egg/cc (above). When a golden nematode infestation is found, the population density is somewhere between 1-4 eggs/cc of soil. This density of 1-4 eggs/cc soil is the initial level we chose to work with. We used the same four systems described above, and we had the same nematode population responses, only on a larger scale because the initial population densities were higher. The only cropping system that reduced the population density below 0.2 eggs/cc and maintained it at that level was the 2 years of resistant, 1 year of non-host, and 1 year of susceptible (below).
This is the program currently used by the regulatory agencies to manage the golden nematode. Only resistant varieties may be grown in newly discovered infested fields. After two years, the field is again surveyed for viable cysts. If cysts with viable nematode eggs are found, resistant potatoes must continue to be grown and the field surveyed annually. When no viable nematodes or cysts are found, the grower may go into the fixed cropping system of 2 years of resistant crops, 1 year of a non-host crop, and 1 year of a susceptible variety.
This system has been in use almost 10 years, and it has worked very effectively. Infestations are practically nonexistent now: they are lower than they have ever been, even when fumigants were used because fumigants did not kill nematodes at lower soil depths.
©All material is protected by Section 107 of the 1976 copyright law.
Copyright is held by Cornell University.
If you intend to use this material, please acknowledge the author and the source of the information.