Testing Biofumigation as an Option for Nematode Control

By Jayson K. Harper and Lynn Kime et.al

Some nematodes cause root damage (such as root lesion and root knot nematodes), while others are vectors for viruses (such as the dagger nematode, which can transmit tomato ringspot virus and tobacco ringspot virus, which ultimately causes apple union necrosis and decline and prunus stem pitting). In addition, high ring nematode populations have been associated with increased susceptibility to bacterial canker in stone fruit trees. Newly planted and young fruit trees are particularly susceptible to these types of damage.

Nematodes have been consistently an issue in Pennsylvania orchards, particularly the dagger nematode. In the past, nematodes were often controlled via fumigation with methyl bromide or with chemical nematicides before planting, but these types of treatments are being gradually phased out or have various adverse environmental impacts. With these changes, environmentally friendly biofumigation practices are being encouraged to help manage nematodes. Biofumigation involves the incorporation into the soil of mechanically chopped brassicaceous plants (such as mustard or rapeseed) and graminaceous plants (such as sorghum-sudangrass) that are known to produce cyanide compounds that kill nematodes.

In 2021 the Fruit Research and Extension Center (FREC) obtained funds from the Sunday Endowment administered by Penn State’s College of Agricultural Science to investigate the use of a sequence of sudangrass followed by rapeseed for nematode control. From this experience, we have developed some recommendations for growers who would like to try bioremediation themselves, along with the estimated cost of implementing this set of practices.

Step-by-step Biofumigation Process Tested at FREC

The bioremediation process conducted at FREC began in Fall 2020 and concluded in April 2022. The process was tested at six sites; one large field comprised of a former peach block and organic apple block used for plant pathology research, and five small fields comprised of former apple and pear blocks used for horticulture research.

Fall 2020

1. Removed trees (including roots).
2. Took soil samples for pH, fertility, and nematode analysis. Soil testing for pH and fertility was conducted by the Penn State Analytical Lab ($9/sample for standard testing). Nematode testing was conducted by the North Carolina Dept. of Agriculture ($13/sample for out-of-state).
3. Followed the recommendations from the soil analyses, which indicated the need for phosphorus (100–150 lb/A) and potash (60–80 lb/A)).

Spring–Summer 2021

1. Sudangrass (Pioneer 877F) was planted in mid-June at 35 lb/A with a seed drill.
2. The sudangrass was mowed in mid-July, followed by an application of 75 lb/A of ammonium sulfate.
3. The sudangrass was mowed again in late August. Mowing was followed by disking at the plant pathology site to incorporate the chopped sudangrass into the soil; at the smaller horticulture sites a rotavator was used for incorporation. Each site was cultipacked after incorporation.

Figure 1: (left) Dwarf Essex rapeseed, (right) Sorghum-Sudan. Photos: K. Peter, Penn State

Fall 2021–Spring 2022

1. Rapeseed (Dwarf Essex) was broadcast seeded in mid-September at 8 lb/A. The rapeseed reached the rosette stage before frost and successfully overwintered.
2. In the early spring (mid-April), the rapeseed was chopped and incorporated at each site in the same manner as described for the sudangrass.

Results

Nematodes were sampled three times during the biofumigation process. An initial sample was taken in the Fall of 2020. The second sample was taken in early-mid October 2021, approximately one month after the sudangrass had been incorporated. The final sample was taken in late April 2022, two weeks after incorporating the rapeseed. The number of dagger nematodes present at the different sample timings can be found in Table 1.

Table 1. The number of dagger nematodes initially, after incorporation of sudangrass, and after incorporation of rapeseed (per 100 cc of soil), 7 sites at FREC (Biglerville, PA).

¹Soil sample taken in Fall 2020.

²Soil sample taken in October 2021.

³Soil sample taken in April 2022.

The biofumigation process appears to have been successful at reducing the number of dagger nematodes at most sites. However, the bioremediation process was not able to reduce the number of dagger nematodes below the treatment threshold of 1 dagger nematode per 100 cc of soil for any of the sites. Although reducing dagger nematode levels to below the treatment threshold was not attained, it is encouraging that this level of control is possible, and all fruit growers are encouraged to take steps to decrease nematode populations before replanting orchards.

Some of the lessons learned while conducting this research were:

1. The best time to conduct the initial soil test for nematodes would be mid-September.
2. Availability and access to suitable tillage equipment will determine the effectiveness of biofumigation. The depth of incorporation of plant materials was not as deep or uniform in the large plant pathology site where a disk was used compared to the smaller horticulture sites where a rotovator was used. The weight of the disk harrow available was only sufficient to incorporate plant materials to a depth of 4-5 inches while the rotavator was able to incorporate to the 10 inches needed.
3. The biofumigation process is easier to manage on smaller sites. The plant pathology site (2.7 acres) was too large for the equipment available and should have been divided into smaller sections. The horticulture sites were smaller (0.4-0.5 acre) and the rotovator did a better job with incorporation.
4. Growers should consider using herbicides to control weeds prior to planting the sudangrass and rapeseed. Tillage alone was unable to provide adequate weed control.
5. The biofumigation process does a good job providing additional organic matter to the soil, especially for the gravelly loams common in the fruit-growing areas of Adams County.
6. At the end of the process, growers should test for soil fertility/pH prior to planting fruit trees to determine if lime is needed.

Cost of Bioremediation

The cost of remediation was estimated with enterprise budgeting software based on the list of field operations and 2022 costs for inputs. Details of the cost estimates can be found in Table 2. The rotovator does a better job in incorporating the plant materials, but it also comes at a higher cost due to slower operating speeds and higher horsepower requirements when compared to the disk.

Table 2. Cost of bioremediation per acre, using either a disk or rotavator for incorporation.

Although these costs may seem considerable, the damage caused by nematodes will cause tree loss which can have a major impact on profitability for the entire life of the orchard. In the case of disking, this cost only represents the initial cost of between 46-68 trees (based on a price range of $7.40-$10.90 per tree) or 25 bushels of fruit valued at $20/bushel. In the case of rotovating, this cost only represents the initial cost of between 58-86 trees (based on a price range of $7.40-$10.90 per tree) or 32 bushels of fruit valued at $20/bushel. By comparison, chemical fumigation can cost as much as $2,700-$3,000 per acre (University of California, UC scientists continue the quest for alternatives to chemical fumigation).

Acknowledgment:  The authors wish to thank Tim Baker, Eric Anderson, Carl Bowers, Cody Kime, William Peters, Jared Shelley, Brian Lehman, and Kate Thomas for their assistance throughout the project.