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V. Deleterious Rhizobacteria for Weed Control

V. Deleterious Rhizobacteria for Weed Control

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chamber, three isolates were suitable for field testing. Plots were established at

three sites and after the winter wheat was seeded in the fall the isolates were applied at lo8cfu m-*. At the Washtucna and Eureka sites, the isolates D7 and 2VI 9

significantly reduced the downy brome pouplation. At harvest, downy brome control by isolates D7 and 2V19 significantly increased winter wheat yield at the

Washtucna and Eureka sites (Table IV).Yields were increased 3.5 and 18%, respectively, and the winter wheat population was unaffected (Kennedy ef d.,


Weed density, biomass, and seed production were reduced. Downy brome reduction is shown by the plot treated with D7 (Fig. 9a) and the untreated check (Fig.

9b). There was no benefit from the application of the organisms at the Dayton site.

The wheat was well established at the Dayton site before downy brome growth began so the weed was not as competitive as it was at the other two sites. Also, the

Dayton site was conventionally tilled and seeded, whereas the Washtucna and Eureka sites were no-till seeded. There was no surface crop residue at the Dayton site.

Downy brome control in Kentucky bluegrass by D7 is shown in the foreground

compared with the background (Fig. 10).

Current research efforts on the use of DRB for weed control were summarized

by Kremer and Kennedy (199.5). They stated that weeds cause greater economic

losses on agricultural lands than all other pests combined. They summarized the use

of DRB for weed biocontrol as follows: The DRB strategy is to regulate or suppress

weed growth, the mode of action of DRB is primarily through the production of

phytotoxins, and the DRB should have adequate specificity and efficacy so the weed

is inhibited and the crop is not harmed. DRB have been obtained that are effective

against both narrow- and broad-leaf weed species, and there has been some suc-

Table IV

Winter Wheat Population and Yield from Fields Inoculated withRhizobacteria

and Planted inwinter Wheat at Three Locations in Eastern Washington













4 I OO'*



















Nore. From Kennedy el a / . (1991) with permission.

"NI, noninoculated (Kennedy et al., 1991).

*Statistically different from the noninoculated control from the Same location at the 0.05 level

using Dunnett's LSD.



Figure 9 (a) No-till seeded winter wheat with downy brome in the interrow retarded and inhibited by the application of D7. (b) lnterrow growth of untreated downy brome in no-till seeded winter

wheat (from Kennedy CI a / . , 1991).



Figure 10 Inhibition of the weed downy brome in Kentucky bluegrass seed stand by the application of 1 X lo8 deleterious rhizobacteria organisms per square meter (foreground) compared with the

untreated, check background.

cessful application in the field. Currently, there are several projects under way in

Canada and the United States. They discussed some studies that indicate DRB may

be more effective when combined with low rates of herbicides. The potential use

of DRB with other biocontrol agents was also mentioned. The successful use of

DRB for biological weed control still suffers from unpredictability. However, this

is the case for many biological control agents (Elliott and Lynch, 1995).

The development of successful weed biocontrol approaches using DRB will require additional knowledge in several areas, including (1) establishment of guidelines and procedures for isolating and selecting organisms; (ii) determination of root

colonization mechanisms; (iii) design of effective carriers including the possibility

of using crop residues previous studies have shown strong competitive ability of

winter wheat DRB growing on crop residues-this may be a useful approach for

the use of DRB for weed control; (iv) determination of the mechanism of growth

inhibition (current information strongly implicates phytotoxins but the evidence is

not conclusive); (v) the mechanism regulating specificity must be determined; (vi)

the role of DRB with herbicide use must be explored more thoroughly; and (vii) the

effect of field management practices such as seeding method (till versus no-till),

fertilizer management, etc., on DRB biocontrol strategy requires more investigation. For example, as mentioned previously, preliminary data indicated that NO;



and incubation at 5°C increased the number of TOX+ DRB isolated from the rhizoplane of winter wheat roots. The potential for the use of DRB for weed biocontrol appears good. The approach is environmentally friendly and, if successful,

should be beneficial to the development of sustainable cropping systems.


Crop yields often suffer when conservation tillage systems are implemented

(McCalla and Army, 1961; Papendick and Miller, 1977). These yield reductions

have been attributed to a variety of problems. These include short-chain fatty acids

produced during residue decomposition (Cochran et al., 1977; Lynch et al., 1981),

infection by plant pathogens such as Pythium sp. (Cook et al., 1980); and colonization of roots by deleterious rhizobacteria (Alstrom, 1987; Fredrickson and Elliott, 1985b; Schippers et al., 1987; Suslow and Schroth, 1982). Hairpinning of the

residues around the seed can result in poor seedling growth because of poor seed

zone environmental conditions (Elliott et al., 1984). No-till seeding into heavy crop

residues can cause high crown set when the residues fall back onto the seed row

(Fig. 11). This is very undesirable because the exposed roots are subject to drying,

herbicide effects, and inhibitory substances produced during straw decomposition.

Many of these problems are more severe as residue production becomes heavier.

The solution has been to bum the residues. Burning of residues is causing increased

public concern because of air pollution. Various options such as using the residues

for heat or power production have been explored but are not economically feasible

at this juncture. The management of heavy residues for conservation tillage systems

has been an unyielding problem in many cases. Heavy specialized no-till drills that

can seed through the heavy crop residues and manage them effectively have been

developed. However, the drills are too expensive for many farmers. The development of low-input, on-farm composting of high C/N ratio residues is providing an

innovative residue management option and the potential for assisting the development of sustainable farming systems. Composting of crop residues will overcome

the negative aspects of crop growth using conservation tillage systems where crops

are seeded back into heavy crop residues and will provide added benefits when put

back onto the field. The potential benefits of crop residues to soil structure have

been demonstrated (Elliott and Lynch 1984a).The possible benefits of compost for

alleviating some soil-borne diseases has also been shown (Hoitink et al., 1991).

Compost additions may alleviate problems associated with DRB. These possibilities have not been explored thoroughly. The benefits of compost applications also

include fertilizer content, soil conditioning value, and benefits to soil quality (Bangar er al., 1989; Nelson and Craft, 1992; Thomsen, 1993; Zaccheo e? al., 1993).

Compost applications have shown large benefits for land reclamation (Dick and

McCoy, 1993). Optimum use of crop residues for conservation tillage systems was



Figure 11 High crown set in winter wheat caused by seeding into heavy crop residues and by not

keeping the residues out of the seedling row.

not feasible until the development of low input, on-farm composing of high C/N ratio crop residues. Churchill etal. (1995) have developed the approach for grass seed

straw. Our laboratory studies show the process will work as well with wheat and

rice (Oryzasativa L.) straw. Studies by Honvath and Elliott (1995a,b) and Horwath

et al. (1 995) have explained the mechanisms of the process, which appears to be

rapid delignification. Previously, it was thought that successful composting required a combined substrate C/N ratio of 30/1 or less (Biddlestone et al., 1987). If

the C/N ratio was higher than 30/1, it would have to be cocomposted with an organic material such as manure or sludge to reduce the C/N ratio. Cocomposting

greatly complicates the process for on-farm application.

The low-input, on-farm composting method consists of gathering the straw into

large piles at the side of the field. When rainfall occurs the stacks are turned to allow maximum water intake. The turning with a front-end loader also compacts the

straw, which helps the stack to retain heat. Over winter and early spring the straw

is turned when temperatures cool. The compost is ready for field application after

about three turns in as little as 16-weeks time (Churchill et al., 1995). Even with

two turns with a commercial compost turner, after 16 weeks less than 20% of the

original volume of straw remained (Fig. 12). Studies on the economics of the

process and the value of the compost additions to succeeding crops are incomplete.







YOof original








Figure 12 Percentage of original volume remaining in grass straw wind rows with zero to six turns

(from Churchill e t a / . . 1995, with permission).

In laboratory studies to determine the mechanisms of high C/N ratio substrate

composting, grass straw was incubated under mesophilic and thermophilic conditions. After 45 days of incubation, the loss of lignin C in the Klason lignin fraction

was 25 and 39% under mesophilic and thermophilic conditions, respectively.

Changes in elemental composition indicated that 94% of the lignin fraction had

been altered under both incubation conditions. These data showed that changes in

the lignin fraction were much more extensive than measured by the Klason

method. C mineralization from the straw was 46 and 52% under mesophilic and

thermophilic conditions, respectively. The addition of N decreased the rate of C

mineralization. C mineralization per unit of microbial biomass under thermophilic

conditions was twice that under mesophilic conditions. These data indicate that the

microbial biomass was less efficient under thermophilic conditions, which lead to

greater C mineralization per unit of microbial biomass. These data also established

that the C and N pathways were largely independent. Plate counts of bacteria, fungi, and actinomycetes did not show any definite patterns between the two incubation conditions. These studies indicate that there is the possibility for regulating

the quality of the compost end-product (Horwath and Elliott 1995a,b; Horwath el


1995). Low-input, on-farm composting could be a valuable asset to developing sustainable cropping systems.

Low-input, on-farm composting will allow no-till seeding or shallow conservation tillage on fields that have contained heavy residues. These cropping techniques will be possible without the risk of severe yield reductions of the succeed-



ing crop if the heavy residues can be removed before seeding and composted. It is

also likely that weed control problems will be expedited in the absence of the heavy

residues. Finally, the lack of extensive tillage and return of the residues should enhance soil quality over systems using conventional tillage practices.


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