The determination of host range is an important component in developing a plant pathogen for use as a bioherbicide. The safety of non-target economic and wild plants must be assured before experimental release and commercial use. In contrast to other methods of weed control, the genetic variability and genetic stability of both the weed and the biological control agent must be considered. Schemes to determine host range generally assume a close phylogenetic relationship between the weed host and its co-evolved pathogens. Therefore, testing generally is based on inoculation of genetically related plant species and progresses to more distantly related species until the host range is circumscribed. Several potential weaknesses in these schemes will be illustrated with examples using specific biological control agents. Future tests must place greater emphasis on testing taxa representing the full range of genetic diversity within the biogeographic area of intended use.

Few plant pathogens are both lethal and specific enough to be effective weed control agents. In short, highly specific organisms seldom kill. Two genetic approaches to overcome this problem are to delimit the host range of lethal pathogens or to enhance the efficacy of host-specific ones. Narrowing the virulence or survival of a deadly pathogen seems more plausible than imparting new characters to a nonlethal organism. Our approach has been to genetically restrict the host range or to decrease the survival and/or spread of Sclerotinia sclerotiorum (Lib.) de Bary, a highly virulent and aggressive pathogen of several weeds. Working with this fungus, three classes of induced mutants which meet criteria for delimitation were obtained: auxotrophic mutants that only attack plants when applied concomitantly with an exogenous source of the required nutrient; mutants unable to form sclerotia, structures required for long-term survival and precursors to fruiting bodies; and mutants with reduced virulence and/or host ranges. These studies demonstrate the validity of genetically improving bioherbicides and greatly expanding the number of fungi that may be useful as bioherbicides.

Field studies were conducted to determine the effect of several herbicides on soybean thrips numbers in soybean. Adult soybean thrips numbers in the preplant-incorporated and preemergent herbicide treatments were observed to be either greater than or equal to, but never less than, numbers in the control plots. Larval soybean thrips numbers were not observed to be affected by the preplant-incorporated or preemergent treatments. The postemergence herbicide treatment of naptalam plus dinoseb consistently reduced both adult and larval soybean thrips numbers. The other postemergent treatments did not affect numbers consistently.

Experiments were conducted in each of two seasons to determine possible effects of diclofop, difenzoquat, dinoseb, and mecoprop on the incidence of take-all disease and grain yield of winter wheat. All of the herbicides, especially mecoprop, reduced incidence of take-all. Treatments increased grain yields the first year but not the second, compared to the inoculated weed-free control. None of the herbicides tested increased incidence or severity of take-all disease in either of the two seasons. The technique of inoculating disease-free soil was successful in obtaining uniform and reproducible incidence of disease.

Chlorsulfuron, metsulfuron, clopyralid, 2,4-D, picloram, and dicamba plus 2,4-D were spring-applied to musk thistle in rosette, bolting, bud, and early flower growth stages. All treatments reduced viable achene production at all musk thistle growth stages at Mcgrew, NE, and Longmont, CO. However, differences occurred at Evergreen, CO, among treatments and growth stages. No achenes developed at Evergreen when herbicides were applied during the rosette stage with dicamba plus 2,4-D, picloram, or metsulfuron at 21 g ai/ha or during bolting with chlorsulfuron or metsulfuron at 21 g/ha. Chlorsulfuron applied during early bloom reduced viable achene production over 99% compared to untreated plants and the picloram treatment.

Field experiments were conducted in Oklahoma to determine the effects of row spacing, cultivar, seeding rate, and water or ammonium polyphosphate fertilizer injection in the row at seeding, on the competitiveness of hard red winter wheat with cheat. Decreasing row spacing from 23 to 8 cm increased yield of weed-free wheat at two of three locations and cheat-infested wheat in six of ten experiments. Increasing seeding rate from 265 to 530 seeds m-2 increased wheat yield. Injecting water at 20 ml m-1 of row at seeding did not increase wheat emergence or yield. Cheat seed production was not consistently suppressed by any one cultivar. Juvenile growth habit was unrelated to wheat competitiveness.

Pyridate was evaluated alone and as a component in herbicide systems for weed control and peanut tolerance. Pyridate at 1.0 and 1.5 kg ai ha-1 applied postemergence controlled 79 and 96% of Florida beggarweed, respectively. Pyridate was less effective on sicklepod than 2,4-DB. Peanuts exhibited no visible foliar injury or yield reduction with pyridate at 1.5 kg ha-1, the highest rate evaluated. Comprehensive weed control and maximum yield required that pyridate be tank mixed with 2,4-DB at 0.22 kg ae ha-1.

Field experiments were conducted over a 3-yr period to evaluate the efficacy and crop yield response of selected herbicides to control loose silky bentgrass in fall rye. Loose silky bentgrass was controlled 18 to 100% with trifluralin preemergence incorporated at 0.4 to 0.6 kg ai ha-1, chlorsulfuron preemergence at 0.024 kg ha-1 and metsulfuron early postemergence at 0.006 to 0.024 kg ha-1. Rye yield was unaffected by these herbicides 2 of 3 yr. In a farm demonstration trial trifluralin preemergence incorporated at 0.4 kg/ha controlled loose silky bentgrass.

Postemergence herbicides alone and in combination were evaluated for Venice mallow control in corn and onion in Colorado during 1986 and 1987. Season-long control of 88 to 98% in corn was obtained with bromoxynil plus cyanazine or atrazine; 2,4-D amine plus dicamba or bromoxynil; or linuron, bromoxynil, or dicamba applied alone. Oxyfluorfen plus bromoxynil in a split application controlled Venice mallow 80 to 99% all season in onions. Uncontrolled Venice mallow caused higher yield reductions in onions than in corn.

In a petri dish assay, radicle growth of trifluralin-resistant (R) green foxtail exposed to trifluralin concentrations of up to 0.4 ppm (w/v) was not inhibited. Radicle growth of trifluralinsusceptible (S) biotypes was completely inhibited. Shoots were more sensitive to trifluralin than roots, with shoot growth being inhibited for both R and S biotypes at trifluralin concentrations of 0.2 ppm or more. Best discrimination between R and S green foxtail biotypes was achieved by measuring radicle length after incubation of pregerminated caryopses at 0.3 ppm trifluralin in the dark for 5 d at 22 C.

Linuron, metribuzin, oryzalin, and metolachlor were applied at recommended (1X) and two-thirds (0.67X) rates to evaluate control of redroot pigweed and common lambsquarters in conventional and rye-stubble reduced-tillage potato production systems. Regardless of tillage, common lambsquarters control was satisfactory during both seasons at both rates of linuron, metribuzin, and oryzalin. Redroot pigweed control by these three herbicides, although excellent in 1988, was poor in RT plots during 1987. Yields did not differ between tillage systems. Reduced weed control with metolachlor during both seasons, and possible crop injury with linuron in 1987 resulted in significant yield reductions.

Field experiments were initiated in 1973 at Fargo and Williston, ND, to evaluate wild oats seed longevity in soil at six burial depths. The number of viable seed decreased rapidly during the first 7 mo from 99% to 21 and 15% at Fargo and Williston, respectively, when averaged over depths. The number of viable seed decreased gradually over the 7- to 168-mo period at Fargo; however, at Williston, viability remained relatively constant for 84 mo and decreased rapidly after 108 mo. At both locations, several wild oats seed were viable 168 mo after burial. More viable wild oats seed were recovered from deep than shallow depths regardless of sampling date. Seed viability at the 6- to 10-cm burial depth was lost more rapidly with nitrogen fertilizer than without nitrogen fertilizer. The percentage of dormant viable seed remained constant for 33 mo at Fargo and 60 mo at Williston regardless of burial depth, but decreased gradually thereafter. Dormant seed percentages at the various depths did not relate to differences in seed viability at these depths.

Alachlor, chloramben, cinmethylin, diphenamid, fluazifop, fluazifop-P, metolachlor, metribuzin, napropamide, oryzalin, and sethoxydim were applied on transplanted sweet potato at Tifton, GA, during 1982 to 1985. The weeds most prevalent were large crabgrass, Florida pusley, smallflower morningglory, and yellow nutsedge. No significant phytotoxicity was observed from any herbicide treatment over the 4 yr. Cultivation at 4 wk is advantageous to eliminate early flushes of weeds which escape control and reform the beds. Alachlor or metolachlor at 4.5 and 3.4 kg ha-1, respectively, and metribuzin controlled weeds best. Although no serious phytotoxicity was evident, these treatments tended to cause lowered yields of marketable roots while marginally affecting total yields in most cases. Metribuzin was the only compound which controlled late-season broadleaf weeds such as smallflower morningglory effectively.

Combinations of thifensulfuron with carbaryl, chlorpyrifos, malathion, and methomyl caused soybean injury that exceeded the expected injury based on effects of herbicide and synergists applied alone. Thifensulfuron-induced injury and growth reductions generally increased as insecticide rates increased from 140 to 560 g ai ha-1. TBPT and fenarimol also synergistically enhanced thifensulfuron injury in soybeans. Yellow foxtail control by thifensulfuron was enhanced slightly by all insecticides except carbaryl. Insecticides did not enhance kochia control by thifensulfuron.

The effect of the isopropylamine salt of glyphosate and the ethyl ester of quizalofop on the rhizome bud viability of five quackgrass biotypes was tested. Two bioassays used were: direct measure of the growth of the bud on an agar medium and determination of the respiratory activity with tetrazolium chloride. The response to herbicide treatments differed among biotypes. The growth bioassay showed lower viability than the tetrazolium bioassay. Quizalofop reduced viability more than glyphosate. Viability of untreated buds increased with increasing distance from the base of the rhizome. The viability of herbicide-treated buds was more or less equal regardless of the position. It appeared that both herbicides were translocated to all the rhizome buds.

Subterranean clover as a living mulch was evaluated for weed control and its effect on field corn silage and grain yield in 1986 to 1988. Treatments included combinations of subterranean clover living mulch, rye dead mulch, and no mulch with three superimposed tillage practices (conventional, minimum, and no-tillage). Results indicated that subterranean clover living mulch effectively controlled ivyleaf morningglory. Little control of fall panicum was obtained in 1986; however, living mulch combinations effectively controlled fall panicum in 1987 and 1988. Weed biomass was reduced significantly by all living mulch combinations, but all other combinations resulted in higher weed biomass than living mulch. Corn silage and grain yields from the no-tillage plus living mulch treatment were comparable to or higher than those obtained with the conventional tillage plus no mulch treatment.

The effects of different periods of green foxtail competition on the components of potato yield were investigated. Delaying the removal of green foxtail for 2 wk following crop emergence reduced total and marketable tuber yield by 19 and 29%, respectively, when averaged over 2 yr. Similarly, the number and average weight of marketable tubers were reduced by 24 and 6%, respectively. The proportion of unmarketable tubers comprising the total yield increased as the duration of weed competition was increased. Successively longer periods of green foxtail competition further decreased the measured parameters.

Chlorsulfuron was applied to wheat at rates of 0, 10, 20, and 40 g ai ha-1 in three locations at Lethbridge and Brooks between 1980 and 1984. The soil pH and organic matter contents at Lethbridge and Brooks were 8.0 and 2.1% and 7.4 and 3.0%, respectively. Rotational crops were seeded for up to 7 yr after chlorsulfuron application. The criteria used to indicate when susceptible crops will grow safely were no significant yield loss at the 40 g ha-1 rate and the magnitude of the upper confidence limit for this difference. The following times after application before crop yields were not affected were required at Lethbridge: barley, 2 yr; canola, 3 yr; peas, beans, 4 yr; flax and potatoes, 5 yr; alfalfa and sugarbeets, 6 yr; and lentils at least 7 yr. The effect of chlorsulfuron residues in the soil on rotational crops appeared to be similar at Brooks and at Lethbridge. Estimated chlorsulfuron concentrations were related to the tolerance of rotational crops.