December 14, 2006 | General

Control Of Strawberry Black Root Rot With Compost Socks

BioCycle December 2006, Vol. 47, No. 12, p. 56
For the first time, a raised-bed growing method uses 100 percent mature compost as an alternative to fumigation – significantly reducing root rot symptoms and increasing yields.
Patricia D. Millner

PERENNIAL, matted-row strawberry systems are still used by many small acreage growers and “U-Pick” farms in regions with cold winters and warm summers, where plants produce an abundance of runners (4) that form the matted row. The latter offers small acreage strawberry producers conservative start-up and maintenance costs and fewer management requirements than are required for annual “plasticulture” systems. Soil fumigation with methyl bromide (MeBr) historically has been used to control soilborne plant diseases, pests, and weeds in both systems, but the ban on MeBr has prompted growers and researchers to evaluate various chemical, biological, and cultural alternatives. The plant disease suppressive activity of compost and its value in soil quality improvement contribute to its beneficial use in organic as well as conventional production systems.
Although many diseases are controlled by soil fumigation, black root rot (BRR) has been observed to aggressively reestablish within several months after initial treatment, particularly in older production fields (>10 years) in the mid-Atlantic region, where BRR can be the major factor limiting productivity. The disease involves a complex interaction between abiotic factors (temperature, soil type, and moisture), and one or more of several plant pathogens that survive in soil, old crowns and roots. Plants that become infected with BRR produce few runners, weak branch crowns, and decreased yield. After the first harvest and onset of hot, dry weather, plant vigor declines precipitously and many plants die, even when preplant soil fumigants were used.
Growers with matted-row systems are still interested in using their land in a sustainable, economical, but less input-intensive manner. While hydroponic bag or gutter systems under shelters clearly avoid weeds and soilborne diseases, they rely heavily on nonrenewable, expensive inputs to fill bags, purify recycling solutions, and dispose of spent plastics. In the studies reported here, common and renewable organic materials – compost and vinegar – were evaluated for rapid, low-cost field establishment of raised-row (hereafter referred to as the “GroExx System” compost socks) strawberries in nonfumigated soils with a history of BRR.
Field studies were conducted at three locations in central Maryland: Farm D (Davidsonville) with fine sandy loam soil; Farm G (Germantown) with silt loam soil; and Farm B (Beltsville) with loamy sand soil. All sites had prior histories of red stele, caused by Phytophthora fragariae Hickman, and BRR and had not been fumigated during the seven (Farm B) or two (Farms D and G) years prior to the study.
Preliminary tests (author, unpublished data) with a single drench of 20 percent (v/v) acetic acid (a prospective organic herbicide) showed 100 percent destruction of Rhizoctonia solani and Pythium spp. in pots. Similar control of many annual weeds was achieved in several preliminary pot and field trials with one to several soil drenches and foliar sprays of the same concentration of acetic acid. These results, along with the initial interest of organic growers in use of vinegar for weed control, led to including a soil drench with vinegar in this study. Interest in the use of vinegar drench as a possible alternative to traditional soil fumigants in combination with black plastic mulch (BPM) and soilless propagated transplants led to the inclusion of this combined treatment as a comparison to the matted-row and compost sock systems. The BPM was stretched across the soil surface after drip irrigation tape was placed and operational; film edges were buried in soil. At Farm D, the BPM was laid on a slightly hilled bed, whereas at Farm G, it was applied on a raised bed.
Mature leaf and yard trimmings compost was used to fill 20-cm-diameter compost socks (GroExx/Filtrexx Inc., Grafton, Ohio) at Farms D and G using a pneumatic blower system attached to a flexible hose. Leaf-grass-poultry (layer) manure compost produced at the USDA Composting Facility, Beltsville, Maryland and screened to < 0.32 cm was used to fill compost socks used at Farm B. Cotton-mesh socks were used at Farm D and polyethylene-mesh socks were used at Farms B and G. A drip irrigation system, with emitters spaced 30.5 cm apart and an emitter flow rate of 0.055 liter/min – linear m (4.5 gal/min-1000 linear ft) of row, was placed on bed centers and secured with metal landscape pins.
The data show that, at all three sites, roots were more symptomatic of BRR and plant cover was reduced in soils without compost compared to those with compost sock treatments (Table 1). Overall, compost sock treatments produced significantly (P < 0.05) greater yields, healthier plant roots, and greater plant coverage in the rows than matted row or BPM, regardless of vinegar treatment or location.
Symptomatic plants/roots were hard to find from most of the compost socks. Suppression of BRR in roots was observed visually at harvests and subsequently measured by examining roots and plating root sections after heat stress, 10 and 12 months after plots were established. A vinegar drench, while initially successful in controlling weeds prior to planting and throughout the fall/winter, did not inhibit common annual spring and summer weeds (data not shown) or root disease symptoms the year after planting.
Overall, a vinegar drench in these BRR-infested soils, although an initially promising strategy (preliminary data not shown), did not sustain plant protection against the BRR complex up through the harvest period and subsequent season, and there was no significant subplot or interaction effect. Vinegar treatment contributed to weed control primarily at the time of plot establishment, but this was not associated with significantly improved yields compared to the no-vinegar treatment (Table 1). The only significant difference obtained in yield with vinegar treatment was associated with its use in combination with the BPM at Farm G with “Chandler” strawberry variety (Table 1). However, even BPM with the vinegar treatment did not result in yields as large as those from the compost socks at this location with both “Chandler” and “Allstar.”
These results indicate that compost socks have potential for use in nonchemical protection of perennial row strawberry plants against BRR in Maryland. Unlike plasticulture systems, soilless bag culture, and solarization procedures, weed management must still be incorporated into the crop management program of compost sock and matted-row systems. Differences in compost quality might influence results, but in these tests with two different composts and three locations, healthy roots completely permeated socks filled with either type of compost and did not grow out of the sides or bottom of the socks into the soil. Runners were able to establish on the tops and sides of socks despite the continued presence of the BRR fungal agents in the soil underlying and adjacent to the compost socks. Salts were not observed to be a problem, and irrigation water filtered down through the socks to the soil preventing salt accumulation in the sock.
Recent availability of soilless, container-grown plug plants as an alternative to soil-propagated bare-root transplants offer the prospect of limiting the inadvertent introduction of BRR into new plantings. The plug plants used on Farms D and G, with their smaller size and roots, were easier to transplant into the compost socks than were the traditional bare-root transplants used at Farm B. Pneumatic filling and direct placement of compost socks in the field from a remote truck eliminates additional compaction in the crop area from field equipment. Filling and placement of socks is rapid, e.g., about 300 m/h for 35 percent moisture compost and economical (about $3,000/acre complete for compost, socks, and installation) when compared to fumigation, solarization, or soilless bags.
All of the latter are single-season treatments and do not contribute to soil quality improvement in the long-term as does the addition of stabilized organic matter such as compost. They also require extensive use of nonbiodegradable film that must be removed and disposed of appropriately on an annual basis. Compost socks achieve control over the root zone environment much as other soilless systems do, and they can remain for more than one season. In addition, compost has beneficial effects not only on chemical and physical properties of soil, but also on biological properties that directly or indirectly impact positively on plant growth promotion. By selecting biodegradable material, such as recycled cotton, in manufacturing the mesh sock, disposal costs at the end of the useful life of the beds can be avoided.
In conclusion, this series of field evaluations on unfumigated soils has shown compost socks significantly improved plant and root health, plant coverage of the row, and yields by limiting symptoms of BRR in plants throughout the entire first season unlike the matted-row and the BPM systems treatments. A vinegar drench prior to establishment of planting beds had a positive, short-term effect on weed control, but ultimately did not significantly or consistently reduce BRR symptoms in treatments without compost socks. In general, “Chandler” was more productive on compost socks than was “Allstar,” but further studies are needed to determine the spectrum of cultivar responses to the compost sock system and multi-year effects.
Pat Millner is with the U.S. Department of Agriculture, Agricultural Research Service, based in Beltsville, Maryland. She can be contacted via e-mail at Special thanks to Michael Bzdil, Phil Edmonds, Sally Reynolds, Michele Russell, Jay Radhakrishnan, and David Ingram for technical support and the growers, Tim Hopkins and Susan, Todd, and Wade Butler, and their field staff. Compost socks were donated by Rod Tyler, Filtrexx, Inc. This work was supported in part with funds from a cooperative research project (USDA-CSREES grant) through Michigan State University. Constructive reviews provided by John Teasdale and Dan Roberts are gratefully acknowledged.

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