Until recently, phytopathogenic bacteria have not been considered potential biological weed control candidates because they lack the ability to penetrate intact plants. This deficiency can be overcome by providing entry wounds or using surfactants. Spray application of Pseudomonas syringae pv. tagetis (5 × 108cells/ml) in aqueous buffer with a surfactant produced severe disease in Canada thistle, common ragweed, Jerusalem artichoke, sunflower, and certain other members of the Compositae under field conditions. Spray application of the bacterium without surfactant was ineffective on all reported hosts. Xanthomonas campestris pv. poannua controlled annual bluegrass in bermudagrass golf greens when applied by spray during mowing. The bacterium entered through mowing injuries, causing lethal, systemic wilt. Application of the bacterium to annual bluegrass in the absence of fresh mowing injuries failed to produce symptoms. Under field conditions, this previously unknown pathovar's host range was limited to a single subspecies of annual bluegrass, but inundative application to freshly mowed turf resulted in infection of diverse annual bluegrass biotypes. In field trials, six monthly applications resulted in greater than 70% control. The preceding examples are among the first attempts to use foliar phytopathogenic bacteria for biological weed control. Efficacy of these bacterial bioherbicides and of future biocontrol strategies employing bacteria is dependent on facilitated host penetration.

Several sequences of POST herbicides applied each year in fall alone, in spring alone, or both in fall and again in spring controlled Canada thistle stands in spring wheat by severely decreasing root biomass and the numbers of adventitious root buds to a depth of 50 cm over four years. These treatments included dicamba applied at 1.7 or 2.2 kg ae ha–1 for the first two successive falls followed in wheat by either chlorsulfuron at 30 g ai ha–1 plus nonionic surfactant, MCPA plus bromoxynil at 280 plus 280 g ha–1, or 2,4-D amine at 560 g ha–1 applied annually for each of four consecutive years from the start. Chlorsulfuron at 30 g ha–1 applied alone in spring for each of four years also reduced and prevented Canada thistle root growth as effectively as a sequence of fall-applied dicamba followed by spring-applied chlorsulfuron in spring wheat.

Sugarbeet tolerance to and Canada thistle control from tank mixtures of clopyralid plus other postemergence herbicides and/or petroleum oil adjuvant applied at various times were determined. Herbicides were applied to 5 to 20 or 20 to 70-cm tall Canada thistle. Clopyralid at 0.14 and 0.21 kg ai ha-1 alone or with petroleum oil adjuvant did not injure sugarbeet. Tank mixing clopyralid with desmedipham/phenmedipham plus endothall injured sugarbeet 7 d after application in 2 of 3 yr but not when observed 14 d after application. Herbicides applied early postemergence in 1987 and late postemergence in 1988 controlled Canada thistle poorly due to rainfall 1 h after application and drought conditions, respectively. The addition of petroleum oil adjuvant enhanced control of Canada thistle by clopyralid at 0.14 kg ha-1, and control equaled that of clopyralid at 0.21 kg ha-1 alone. Addition of desmedipham/phenmedipham plus endothall to clopyralid did not reduce Canada thistle control. Petroleum oil adjuvant added to clopyralid at 0.14 kg ha-1 increased Canada thistle control to that achieved by clopyralid at 0.21 kg ha-1 plus desmedipham/phenmedipham plus endothall.

Alfalfa seed yield and Canada thistle densities were measured in 10 fields in southern Alberta to assess the effect of thistles on seed yield. Thistle density and seed yield from each field were fitted to the Weibull, hyperbolic, and linear equations and a random coefficient and linear equation were used to obtain an “average yield loss curve”. Estimated average losses in alfalfa seed yield were >34% and >48% at thistle densities of 10 and 20 shoots m–2, respectively.

Ceutorhynchus litura dispersed 9 km in 15 yr from its point of release in 1972 with incident of infestation of Canada thistle plants at one study site reaching 92% in 1987. Plants generally survived larval feeding in the spring, but holes created by departing larvae provided entrances for arthropods, nematodes, and diseases. Subsequent mortality rates were high for belowground Canada thistle shoots. However, the number of new shoots from underground roots the following spring usually offset the mortality that had been indirectly caused by C. litura.

A random mail survey of Ohio growers was conducted to determine the geographic distribution of weeds across Ohio and the influence of production practices on the severity of agronomic weed problems. Statewide, the ten most severe weed problems, in descending order, were Canada thistle > foxtail spp. > velvetleaf = common ragweed > common cocklebur > giant ragweed = annual morningglory spp. > quackgrass = common lambsquarters = Pennsylvania smartweed. Distribution of most weeds varied geographically. Infestations of common and honeyvine milkweeds, hemp dogbane, and tree seedlings increased as tillage was reduced. Infestations of common cocklebur and common ragweed decreased as tillage was reduced. As crop rotation became more diverse, infestations of velvetleaf, common cocklebur, giant ragweed, Pennsylvania smartweed, and jimsonweed decreased. Where rotations included forage or hay crops, infestations of large crabgrass, common chickweed, wild carrot, and common dandelion increased.

The interaction of cultivation and photoperiod on the initiation of Canada thistle rosettes, and the effect of growth stage on control of Canada thistle with glyphosate were determined in a series of field experiments. Under the natural photoperiod occurring in the southern Canadian Prairies, rosettes of Canada thistle were initiated by cultivation to remove the shoot growth during the last week of July. Regrowth remained as rosettes without any stem elongation and formed dense clusters of leaves. Application at the “August rosette stage” improved the effectiveness of the glyphosate treatment and resulted in consistent control of Canada thistle with less than half as much herbicide as recommended for control when applied at the bud-stage. The number of shoots of Canada thistle on plots treated with glyphosate was less than on the intensively summer-fallowed check plot. Check plots received 5 cultivations during the summer-fallow season. One year after application of glyphosate at the rosette stage, the reduction in shoot density was 99% compared to the summer-fallowed check. By year three, without glyphosate applications in years two and three, the benefits of applying glyphosate at the rosette stage rather than the bud-stage were very evident. When applied at the bud-stage the shoot density on plots treated with glyphosate at 2.25 kg ha−1 was 24 m−2 compared to only 10 shoots m−2 when applied at the rosette stage, (35% vs 72% control). Glyphosate at 0.9 kg ha−1 or less than half of the rate recommended for application at the bud-stage, applied to Canada thistle in the rosette stage, resulted in consistently fewer shoots 2 and 3 yr after treatment. Physical removal of shoot top growth, simulating an in-crop herbicide treatment, improved the consistency of control. Two years after application of 0.9 kg ha−1 of glyphosate at the rosette stage, the control of Canada thistle was 98% compared to the summer-fallowed checks.

Continuous corn was grown for 5 consecutive yr using conservation tillage methods in the Maryland Piedmont. Herbicide treatments were applied annually at common usage rates and were compared for their effects on weed cover, species stability, and grain production. Weed flora dominance shifted from giant foxtail the first year to smooth pigweed the following year. Smooth pigweed then dominated the flora for the duration of the study. Horsenettle was ubiquitous in all treatments but never became competitive. Canada thistle occurred in measurable densities in all plots that had not received atrazine applied preemergence. Grain yields were correlated negatively with densities of smooth pigweed in 1983 (r = −0.58, P = 0.01), and Canada thistle in 1983 and 1984 (r = −0.63 and −0.62, respectively; P = 0.05). Grain yields were correlated positively with midseason precipitation (r = 0.85, P = 0.01).

In a three county area, the density of musk thistle was higher than plumeless thistle in pastures in the easternmost county, equivalent in the central county, and lower in the westernmost county. Bull thistle was detected frequently, but density was always low. Detection of Canada thistle decreased from east to west.