When I began my foray into the field of biological control of weeds in 1971, the concept of deliberately using pathogens to control weeds was novel and untested and met with skepticism and resistance. Soon, a worldwide network of plant pathologists, weed scientists, microbial technologists, formulation specialists, and regulatory personnel came together to study, develop, and apply pathogens in safe and effective ways of control of a variety of weeds in crops and natural areas. Several new weed–pathogen systems were studied; a few dozen products and pathogens were brought to use, albeit on a very small scale compared to conventional weed-control products; and along the way, some valuable lessons were learned in phytopathology and weed ecology. A seminal body of information was published on the etiology and epidemiology of several diseases of weeds, many new pathogens were discovered and described, and methods were developed for mass production, formulation, and storage of pathogens. Numerous pathogen-produced herbicidal metabolites were discovered and characterized. Protocols were developed, tested, and applied for safe importation and release of exotic pathogens and for registration of microbial herbicides. Spectacular success was achieved with some pathogens used as classical biocontrol agents, and a new class of herbicide, the bioherbicides, came on the scene. Yet some key opportunities were missed. Notably, weed biocontrol research remained largely preoccupied with agent or product development and deployment while great strides were made during this period in phytopathology to understand the genetic–molecular basis of virulence, host range, host specificity, host response to infection, cell death, and pathogen population structure. Nevertheless, the accomplishments in the field of weed biocontrol by pathogens are truly significant. Certainly, we are poised to apply the knowledge gained toward discovery and development of additional weed-control pathogens, but increased effort should be directed also at using pathogen genes, gene products, and genetic mechanisms for weed control. An investment in the latter could help us gain insights into genetically programmed host–pathogen interactions that may be exploited to kill weeds, restrain weed growth, or knock out traits for invasiveness. In our continuing struggle to manage weeds, biocontrol with pathogens should remain a major thrust. Here I present perceptions I have gained from the work that my students, postdoctoral and technical associates, colleagues, and I have done with several weed–pathogen systems.

Jerusalem artichoke has been reported to colonize several ecological niches and agronomic crops in southern Europe. This plant is also of interest because of its high biomass production and its potential to produce ethanol for biofuel. Allelopathy may be an advantageous trait in Jerusalem artichoke under cultivation, as it potentially reduces weed interference with the crop, theoretically allowing a reduction of mechanical or chemical input required for weed management. However, this trait may also be unfavorable if other crops are cultivated in rotation with Jerusalem artichoke or in areas infested by this species. The aim of this study was to investigate the sensitivity of selected diverse crops (wheat, lettuce, corn, tomato, rice, and zucchini) and weeds (barnyardgrass, black nightshade, common lambsquarters, common purslane, large crabgrass, and pigweed) to the presence of Jerusalem artichoke dried leaf tissues in laboratory experiments performed under controlled conditions. The simulated soil incorporation of different Jerusalem artichoke residues (four cultivars and a weedy population) was carried out in a series of laboratory and greenhouse experiments. Jerusalem artichoke reduced the radicle growth of seedling lettuce (60%), tomato (30%), large crabgrass (70%), and barnyardgrass (30%), whereas total germination of these species was less affected. Sensitivity to Jerusalem artichoke residues was species dependent; germination and initial growth of corn were not affected, whereas winter wheat, lettuce, tomato, rice, and zucchini seedlings were more sensitive to residue presence. Our experiments show that both wild and cultivated decomposing Jerusalem artichoke residues, particularly leaves and stems, possess phytotoxic potential. Additional field experimentation remains to be conducted to determine if allelopathy in the field contributes to its invasibility.

Field studies were conducted in Powell, WY, in 2006 and 2007 to determine the influence of season-long interference of various lanceleaf sage densities and durations of interference on sugarbeet. The rectangular hyperbola model with the asymptote (A) constrained to 100% maximum yield loss characterized the relationship between lanceleaf sage density and sugarbeet yield loss. The estimated parameter I (yield loss per unit weed density as density approaches zero) was 3% for both root and sucrose yield loss. Increasing duration of lanceleaf sage interference had a negative effect on sugarbeet root yield. The critical timing of weed removal to avoid 5 and 10% root yield loss was 37 and 52 d after sugarbeet emergence, respectively. Lanceleaf sage interference did not affect percentage of sucrose content. These results indicate that lanceleaf sage is not as competitive as other weeds but that appropriate control measures should be undertaken to minimize sugarbeet yield loss from interference.

Field surveys were conducted on 319 sites of the Western Australian grain belt in 2006 to determine the occurrence and distribution of summer fallow weed species. Sites were located across five growing season regions (north, north central, central, south central, and south) and three annual rainfall zones (high, medium, and low). A total of 51 species (or species groups) from 18 families were identified, with the large majority of species (35%) belonging to the Poaceae family. The most prevalent species found, being present at more than 10% of all sites, were wheat, “melons” (weedy watermelon and paddymelon), rigid ryegrass, capeweed, clover, mintweed, wild radish, fleabane, windmill grass, and rolypoly. Correspondence analysis revealed that the north, central, and southern regions of the grain belt could be predominately segregated according to dominant weed species occurrence; however, no segregation by rainfall zone was apparent. This study has given an overview of summer fallow weed occurrence in the Western Australian grain belt and highlights those weed species that are common and yet lack sufficient research into their ecology and management.

Information linking seed movement, along with changes in seed viability, is critical for understanding weed seed dynamics. Studies were conducted to examine the use of passive integrated transponder (PIT) tags placed in nylon mesh packets in combination with GPS (Global Positioning System) technology to track weed seed movement after tillage. Cylindrical PIT tags 11.5, 12, 20, and 23 mm long by 2 mm wide were evaluated in water and soil. Detection improved as tag size increased because of greater signal strength. Tags with the main axis oriented vertically were recovered at greater depths than when placed horizontally. Average detection depths for 12-mm PIT tags were 29.5 cm in water, 18.2 cm in sand, 24 cm in artificial soil, and 21.2 cm in sandy loam soil. Tests also showed that PIT tags and nylon mesh packets were resilient to intense tillage with a rototiller. No significant differences in displacement because of tillage were observed between free PIT tags and PIT-tagged packets. PIT tag performance was further tested in a 2-yr field experiment conducted between September 2003 and October 2005 at six sites in Nebraska and Wyoming. Tilled and no-till blocks were established at each site. PIT-tagged packets in the tilled block and untagged packets in the no-till block were used. Sample burial depths were 0, 2.5, 7.5, and 15 cm. Sample recovery rate did not differ between tilled and no-till blocks. Time of recovery was the main factor affecting recovery of packets buried at 0 and 2.5 cm in both blocks. Seed predation by small rodents and movement of samples beyond the area of study by tillage implements were the main sources of packet loss. Nevertheless, 2 yr after initiation of the study, more than 85% of the samples were recovered. Future development of PIT tag technology will lead to an enhanced ability to monitor seed movement.

Kochia control in continuous corn became increasingly difficult in experimental plots where isoxaflutole was used PRE for 8 yr. Studies were conducted to determine if poor kochia control resulted from an escape mechanism based on different germination rates or from a difference in sensitivity to isoxaflutole. Germination at constant temperatures showed that the kochia population in the experimental plot had greater seed dormancy compared with populations growing in adjacent fields. Germination at 25 C for seeds collected from the isoxaflutole-treated area was near 20% after 20 d, whereas germination for the other populations was above 80%. The optimal temperatures to release seed dormancy for seeds from the experimental plot were alternating 35/25 C day/night temperatures. The kochia biotype that predominated where isoxaflutole was applied PRE had elevated levels of seed dormancy and required higher alternating temperatures to release dormancy than untreated control kochia. These characteristics were unique and not found in populations never exposed to isoxaflutole. Chlorophyll content was measured to determine if differences in sensitivity to isoxaflutole existed among biotypes. Absorption at 660 nm by photosynthetic pigments was similar among the biotypes at increasing herbicide rates, indicating no differences in sensitivity to isoxaflutole among populations. Reduced kochia control in the experimental plot was due to delayed seed germination, which allowed isoxaflutole to degrade before seeds germinated. The rapid herbicide dissipation from soil can be attributed in part to coarse soils, soil moisture, and the low isoxaflutole rate.

Overreliance on glyphosate can increase the risks of reduced efficacy of the herbicide on weeds and weed resistance, even in regions without glyphosate-resistant crops. That is the reality in Greece, with frequent reports of reduced efficacy of glyphosate against increasingly problematic weeds, such as Conyza spp. The objectives of this study were to determine the occurrence of glyphosate resistance in hairy fleabane populations in central and southern Greece and the effect of weed growth stage on glyphosate efficacy under controlled environmental conditions and to evaluate alternative herbicides in field trials for control of glyphosate-resistant and -susceptible hairy fleabane. Plants from 60 accessions of hairy fleabane, sampled from five prefectures in Greece, were sprayed with glyphosate at 0.36 kg ae ha−1; 15 were classified as potentially resistant. After initial screening, 15 potentially susceptible or resistant accessions were selected and dose–response experiments were conducted. Glyphosate rates required to control some accessions were four to seven times greater than that for control of the reference susceptible accession, AR4. Sensitivity of a resistant hairy fleabane accession to glyphosate was strongly dependent on growth stage, with plants at the seedling stage being most sensitive to the herbicide. A field trial indicated that diquat, glufosinate, or glufosinate + oxyfluorfen controlled glyphosate-resistant or -susceptible hairy fleabane. These herbicides, along with various integrated management strategies, have good potential to manage or slow the spread of glyphosate resistance in this species.

The distribution, growth, and seed germination ability of the exotic lead tree in Penghu Islands, Taiwan, were studied. Survey results have shown that all lead tree plants existing throughout these islands belonged to the Hawaii type. Subsequently a comparison of germination requirements between this and the rare Salvador type, which distributes only in the island of Taiwan, was carried out. The objective of this research was to compare the germination requirements of the Salvador- and Hawaii-type lead tree plants. Lead trees in Penghu Islands have formed a pure forest with a height ranging from 2 to 4 m. All growth stages of the lead tree, i.e., vegetative, flowering, podding, and depodding stages, were found simultaneously in fall, but only vegetative and podding stages were observed in summer. Germination of both the Hawaii- and Salvador-type lead tree was evaluated under different environmental conditions, including temperature, pH, osmotic potential, burial depth, and lighting condition. The Salvador-type exhibited more than 60% seed germination after 14 d at temperatures between 20 and 35 C, whereas the seed germination of the Hawaii type increased from 20 to 60% with temperature increases from 20 to 35 C. The optimal temperature for seed germination of these two types of lead tree was 35 C. Increasing burial depth beneath the soil surface and darkness, as well as an osmotic potential below −0.4 MPa, also reduced seed germination. However, the germination of both types was not affected by pH ranging from 4.0 to 9.0. On the basis of the climatic and soil environment conditions in Penghu Islands, it is reasonable to predict that the summer season, in which lead tree seeds readily germinate on the ground or in shallow soils, is the optimal time for controlling this plant.

Field trials were conducted in the spring of 2007 and 2008 to investigate the critical period of interference between American black nightshade and triploid watermelon. To determine the critical period, the maximum period of competition and minimum weed-free period were examined. American black nightshade (2 plants m−2) was established into watermelon plots at watermelon transplanting and removed at 0, 1, 2, 3, 4, and 5 wk after transplanting to determine the maximum period of competition. American black nightshade (2 plants m−2) was established into watermelon plots at 0, 1, 2, 3, 4, and 5 wk after transplanting and remained until watermelon harvest to determine the minimum weed-free period. To avoid yield loss from exceeding 10% of a crop grown weed-free, the maximum period of competition and minimum weed-free period were found to be 3.9 and 3.6 weeks after transplanting, respectively. Therefore, if American black nightshade is controlled at any time during the critical period of 3.6 to 3.9 wk after transplanting, yield loss should not exceed 10% of a crop grown weed-free.

Presented and illustrated is an easy-to-implement and flexible methodology for the analysis of synergistic and antagonistic effects when the effects are defined as nonlinear functions of means. The methodology augments standard mixed-model analyses with the Delta method for standard errors of nonlinear functions of means. Explained is why standard ANOVA methods that have been adopted in the literature are not recommended. To illustrate the methodology, the joint-action effects of fenoxaprop with companion herbicides in two-component mixtures for weed control in rice were evaluated. The companion herbicides were halosulfuron, bispyribac-sodium, bensulfuron, penoxsulam, carfentrazone, quinclorac, and imazethapyr. Weeds evaluated were barnyardgrass and broadleaf signalgrass. Experiments in Louisiana and Mississippi in 2009 revealed a preponderance of antagonistic effects. The analysis showed that mixtures with bispyribac-sodium, penoxsulam, quinclorac, and imazethapyr were generally the most antagonistic and provided the least control.

If an herbicide application fails to control a targeted weed community sufficiently, farmers may try to eliminate surviving weeds with a follow-up application (hereafter “respray”). Despite the implications of resprays on the spread of herbicide-resistant weeds, respray frequencies and causal factors are poorly understood. A two-part survey of glyphosate-resistant soybean fields and custom application services was conducted in Illinois during 2005 and 2006 to determine the relative frequency of respray requests for postemergence glyphosate, and to identify weed community factors associated with glyphosate respray requests. A meta-analysis was then utilized to project the impacts of weed community factors driving respray requests on crop yield. Glyphosate resprays were requested for 14% of surveyed fields in both 2005 (n = 43) and 2006 (n = 90). In 2005, respray requests were highly associated with both population densities of weed communities visible from roadsides and incidences of skips (i.e., rectangular areas of escaped weeds indicating custom application failure). A skip increased the odds of respray request by more than ninefold, and population densities of weed communities visible from roadsides were, on average, 2.5 times greater in respray-requested fields compared with nonrequested fields. In 2006, respray requests were associated with population densities of weed communities identified by walking through fields. Contrary to 2005, requests in 2006 were concentrated in those fields with low weed population densities. Prior to resprays, weed communities capable of causing substantial soybean yield loss were present in both respray-requested and nonrequested fields in 2005 but in only nonrequested fields in 2006. Although this investigation indicated that custom applicators can take actions to reduce respray requests (i.e., avoiding skips), farmers and custom applicators should be prepared to implement additional weed control after postemergence glyphosate applications because damaging weed communities may remain.

Experiments investigating mortality in the soil seedbank are aided by using only seeds that are initially viable and capable of remaining ungerminated (hereafter “persistent seeds”). However, seed mortality experiments often use heterogeneous populations containing persistent, nonviable, and germinable individuals. In this investigation we developed and compared nondestructive tests for isolating persistent seeds of two weed species characterized by physical seed dormancy (dormancy imposed by a water-impermeable seed coat): velvetleaf and ivyleaf morningglory. Individual seeds were weighed, steeped in water (hereafter “steepate”) for 48 h, and then assayed for imbibition. These seeds were then subjected to persistence assays conducted under controlled conditions (60 d in hydrated soil under 25/15 C day/night temperatures, 14-h photoperiod). Persistent seeds were less likely to imbibe and more likely to produce steepates with low electrical conductivity compared with germinable and nonviable seeds. For velvetleaf, persistent seeds were best segregated by comparing changes in steepate conductivity during 4 to 48 h of soaking, with the corresponding classification and regression tree (CART) model making few false discoveries (false discovery rate for persistence; FDRp = 8.6%, n = 93) and many true positive classifications (true positive rate for persistence; TPRp = 100%, n = 85). For ivyleaf morningglory, both a change in steepate conductivity from 4 to 48 h of soaking and imbibition status after soaking accurately separated persistent seeds (accuracy measures of corresponding CART models: FDRp = 5.6%, n = 150; TPRp = 100%, n = 142). Thus, for species with physical seed dormancy, we recommend use of steepate conductivity and imbibition status after soaking for isolation of persistent seeds. These seeds can then be used to optimize experiments on mortality in the soil seedbank. Nondestructive tests for isolating persistent seeds of species characterized by physiological seed dormancy require further research.

Weeds can act as reservoir hosts of a range of pests and diseases. Information and knowledge on the host status of weeds to common pests and diseases can be used to develop integrated weed and pest management strategies. As part of a survey on the distribution and diversity of root-knot nematodes on crops in Fiji, the root-knot nematode host status of weeds was also studied. Weeds growing in root-knot nematode infested farms (n = 189) and bioassay pot soil samples (n = 277) were identified, and their host status was determined on the basis of a root gall and egg-mass index scale from 0 to 5. A total of 45 weed species were recorded as potential weed hosts of root-knot nematodes with a gall index from 1 to 5. Using the weed and tomato bioassay method, a total of 11 nonhost weed species were recorded with a gall index of 0, relative to infected tomato growing in pot soil samples. Common weeds infected by root-knot nematodes on farms and in bioassay pot soil included slender amaranth, old world diamond-flower, tropic ageratum, sicklepod, mimbra, balsamapple, purple bushbean, little ironweed, ivy gourd, and cutleaf groundcherry. The presence of egg masses on the weed hosts indicated their ability to sustain root-knot nematode populations and, thus, their potential to act as reservoir hosts.