The germination of caryopses on water or GA3 solutions was used to characterize the degree of primary dormancy present in the seed progeny of 10 genetically pure lines of wild oats (Avena fatua L. # AVEFA). These 10 lines represented a range of types from lines exhibiting no dormancy to those with a high degree of dormancy. Repeated propagation of these pure lines under constant environmental conditions identified several genetically inherited characters that were associated with the inherited differences in degree of primary dormancy. Correlation of the seed development period, number of primary caryopses produced, and caryopsis moisture content to the degree of primary dormancy shows: a) Lines characterized by low degrees of primary dormancy were, in general, those that had a short seed development time and produced few heavy seed low in percent water; and b) lines characterized by high degrees of primary dormancy were, in general, those that had a long seed development time and produced many light seed high in percent water. The adaptive advantage of the coupling of physiological and morphological characters with degree of dormancy is discussed and the possibility that some characters are determined by others is indicated.

A comparative study of the growth and development of several populations of dinitroaniline-susceptible (DS) and -resistant (DR) goosegrass [Eleusine indica (L.) Gaertn. # ELEIN] biotypes was conducted under noncompetitive field conditions. Plant height and width, number of tillers, aboveground dry weight, relative growth rate (RGR), number of inflorescences/plant, total inflorescence dry weight, individual inflorescence dry weight, number of spicate branches/inflorescence, and flag leaf length and width were recorded periodically during the growing season. There were no significant differences between biotypes in most characteristics with the exception of total inflorescence dry weight. The DS biotype produced more total inflorescence dry weight than the DR biotype at 8 and 13 weeks after transplanting. Significant variation among DS and DR populations occurred in 37 and 33 of 56 evaluations, respectively. When variation existed within a biotype, populations exhibited broad ranges of response for the various growth parameters. With the exception of total inflorescence dry weight, DS and DR biotypes generally exhibited similar ranges of variability.

Stranglervine (Morrenia odorata Lindl. # MONOD) was identified in 1957 at a commerical citrus grove in Florida. It is believed that this plant pest was introduced into Florida as an ornamental plant from South America. In the early 1960's, it became recognized as a serious weed pest that competed with the citrus trees for sunlight, water, and nutrients; girdled tree limbs; and interfered with spraying, harvesting, and irrigation practices. By the 1970's the vine was distributed throughout most of the citrus-growing areas in Florida. Control measures were limited to herbicides and mechanical cultivation. It was estimated that the cost of controlling this pest was $124/ha/yr. In many instances, control measures were inadequate to keep the vines from becoming established in the trees (11, 12, 13). Studies reported by El-Gholl (4) further demonstrated the inherent ability of this weed to persist.

Germination of dormant wild oat (Avena fatua L. # AVEFA) seed was examined in response to various concentrations of ethephon (2-chloroethylphosphonic acid), KNO3, or both together in the dark at temperatures ranging from 7 to 32 C. Both chemicals significantly (P = 0.05) promoted germination and their effects were additive. Maximum stimulation of germination in response to all treatments occurred at 7 C, with a gradual decrease in reponse as temperature or duration of after-ripening, the seed that retained dormancy during storage remained responsive to ethephon also occurred during storage of mature seed at 3 to 32 C. The rate and extent of this dormancy loss increased with an increase in storage temperature. Regardless of the temperature or duration of after-ripening, the seed that retained dormancy during storage remained responsive to ethephon and KNO3 treatments. Suitable combinations of after-ripening, ethephon, and KNO3 induced over 90% germination of seed that was dormant at maturity. Experimental evidence has been obtained that suggests that this percentage may represent the total number of viable seed in the population. While ethephon promoted seed germination, it severely stunted root and shoot growth of the resulting seedlings. It appears that strategies could be developed to combine the use of ethephon and nitrate with the natural process of after-ripening to enhance and synchronize wild oat germination.

Percent germination and germination rates of seeds of atrazine (6-chloro-N-ethyl-N′-(1-methylethyl)-1,3,5-triazine-2,4-diamine)-resistant and -susceptible biotypes of redroot pigweed (Amaranthus retroflexus L. # AMARE) and Powell amaranth (A. powellii S. Wats. # AMAPO) from southern Ontario were compared in petri dishes using a 100-cell germination plate. The plate provided 100 separately controlled, alternating temperature combinations ranging from 0 to 45 C in increments of 5 C. The experiment was conducted under alternating light and dark conditions for 21 days and under constant dark conditions for 7 days. Initial seed viabilities exceeded 90% for all four biotypes. Under alternating light and darkness, the resistant biotype of Powell amaranth had significantly lower total germination and a slower rate of germination than the susceptible biotype, whereas the biotypes of redroot pigweed did not differ in germination response. Germination rate and total germination were positively correlated for all four biotypes. In constant darkness, the atrazineresistant biotypes of both species had significantly lower germination percentages compared to the susceptible biotypes. In general, percent germination of Powell amaranth was greater than that of redroot pigweed for day and night temperatures <25 C, whereas redroot pigweed had greater germination at higher temperatures. Greater seed dormancy of the resistant biotypes under some light and temperature combinations may have implications for the persistence of resistant populations in the field.

The use of nematodes as biological control agents has been met with skepticism, partly due to the newness of the approach and also to the potential difficulties of using a parasitic worm as a control organism. Most of the attention directed towards nematodes as biological control agents has been focused on several species that act as insect parasites. Considerable headway has been achieved with several of these parasites, especially with those parasitic on wood-boring insect larvae. The insect gallery of wood-boring larvae provides an optimum microclimate for the nematode to survive and seek out its larval insect host. A system where this strategy has proved successful involves the use of the insect parasitic nematode Neoaplectana carpocapsae Weiser as a biological control agent for carpenterworms (Prionoxystus robinae Peck) in fig (Ficus caria L.) orchards in California (6). Similar systems are being developed both here and abroad with the same nematode or a closely related genus or species. Many of these systems show promise (5).

Under simulated no-till conditions in the greenhouse, rye (Secale cereale L. ‘Wheeler’) residues reduced emergence of lettuce (Lactuca sativa L. ‘Ithaca’) and proso millet (Panicum miliaceum L. # PANMI) by 58 and 35%, respectively, over that obtained with a wood shaving control mulch. Rye shoot tissue inhibited lettuce germination 52% more than root tissue. Petri dish bioassays of residue in soil confirmed phytotoxicity on cress (Lepidium sativum L. ‘Curly’), lettuce, barnyardgrass (Echinochloa crus-galli L. Beauv. # ECHCG), and proso millet. Radicle elongation was a more senstive measure of phytotoxicity than germination per se. Phytotoxicity increased as the distance from seeds to rye residues decreased. While rye shoot residues caused 40% greater inhibition of cress radicle elongation in sterile soil than nonsterile soil, barnyardgrass was equally inhibited in both soil regimes. Phytotoxicity decreased in nonsterile soil, indicating that the compounds were degraded by microorganisms. Phytotoxic compounds in shoots were water extractable. Cress and barnyardgrass responded similarly to both aqueous rye extracts and to residues added to soil.

The relationships between microorganisms and velvetleaf (Abutilon theophrasti Medic. # ABUTH) seeds in contact with soil were studied to determine microbial effects on velvetleaf seed viability. The characteristic seed microbial association persisted on velvetleaf seeds placed on the soil surface during 32 days of incubation. The fungal association comprised of Alternaria alternata (Fr.) Keissl., Cladosporium cladosporioides (Fres.) de Vries, Epicoccum purpurascens Ehrenb. ex Schlecht, and Fusarium spp. was detected on over 50% of the seeds during incubation. Establishment of soil microorganisms on 50% of seeds occurred at only one sampling date. Such establishment was largely unsuccessful due to the effective barrier presented by seedborne microorganisms. Seedborne and soilborne microorganisms were unable to reduce viability of velvetleaf seeds in contact with soil as the total viability of all seeds tested exceeded 90% during incubation. Velvetleaf seedborne microorganisms may augment seed longevity on and in soils by acting as a barrier to potential seed decomposers originating from soil.

Seedlings from golden delicious apples (Malus domestica Borkh. ‘Smoothee’) were grown in the greenhouse with orchardgrass (Dactylis glomerata L. # DACGL), Kentucky bluegrass (Poa pratensis L. # POAPR), red sorrel (Rumex acetosella L. # RUMAA), and Korean lespedeza (Lespedeza stipulacea L. # LESSL) at four densities and four fertility levels. Orchardgrass and red sorrel inhibited tree growth more than Kentucky bluegrass on a per plant basis. Korean lespedeza inhibition of apple growth was associated with the amount of root nodulation on the Korean lespedeza. Apple shoot dry weight over all fertility levels, ground cover species, and densities was predicted by a regression model based on increase with fertility and decrease with ground cover dry weight. These results indicate that the competitiveness between equal number of ground cover species is due to differences in biomass production. Addition of nitrogen to the system increased nitrogen levels in apples but did not completely overcome apple growth inhibition.

‘Ransom’, ‘Govan’, and ‘Bragg’ soybeans [Glycine max (L.) Merr.] were seeded in 30-, 61-, and 91-cm row spacings to achieve a uniform population of 323 000 plants/ha. In 1979 and 1980, shade development within the row was similar for all row spacings, but 15 cm from the row the inflection point occurred earlier when soybeans were seeded at the 30-cm row spacing. In 1979, shading 30 cm from the row was similar with the 61- and 91-cm row spacings, but in 1980 the 61-cm row spacing provided earlier shading. Shading within the row and 15 and 30 cm from the row was similar for all cultivars in 1979, but Govan and Bragg shaded row middles earlier than Ransom at the 91-cm row spacing. In 1980, shade development in the row was similar for all cultivars, but delayed shading was observed between the rows with Ransom. In 1979, maximum soybean seed yields were produced with 2, 2, and 0 weed-free weeks at the 30-, 61-, and 91-cm row spacings, respectively. In 1980, 2 weed-free weeks prevented soybean seed yield reductions at all row spacings. In 1979, Ransom, Bragg, and Govan required 4, 2, and 0 weed-free weeks, respectively, for maximum seed yields. In 1980, all cultivars achieved maximum seed yields with 2 weed-free weeks.

Giant foxtail (Setaria faberi Herrm. # SETFA) seed germination was about 55% in water but increased to >90% after pretreatment in polyethyleneglycol 8000 (PEG) at water potentials of −0.3 MPa or less. Pretreatment at 20 C required about 24 h exposure to PEG for maximum effect, but at 35 C shorter times (4 to 8 h) gave a partial effect. Treatment with PEG can begin at the time of seed wetting or after the seeds have imbibed water for 24 h. Continuous exposure to a water potential of −0.5 MPa PEG (or less) imhibited germination. Other methods of applying temporary water stress to the seeds, such as slow hydration in a humid atmosphere, or drying after partial hydration, increased germination in a manner similar to PEG treatment. The relatively slow response kinetics with PEG suggest that increased germination was caused by physiological adaptation to water stress, possibly by observed adjustments in seed water potential. It is suggested the water stress mechanism could operate under field conditions where wetting-drying cycles are common.

Biological weed control research at Arkansas has been with indigenous fungal pathogens applied as inundative inoculum to weed populations (9, 19, 22, 32, 34, 39). We term this concept the mycoherbicide approach to biological weed control (35). Our experience with this concept began in 1969 with the discovery by R. J. Smith, Jr., of a previously undescribed fungal pathogen on northern jointvetch, [Aeschynomene virginica (L.) B.S.P. # AESVI] (20, 22, 29). It was ultimately commercialized as COLLEGO™ in 1982 by the Upjohn Co. for use by Arkansas rice (Oryza sativa L.) and soybean [Glycine max (L.) Merr.] farmers. Nothing has been found nor can be foreseen from the experience during discovery, development, and deployment of COLLEGO™ that would preclude extension of the mycoherbicide concept to controlling other weeds in annual crops or noncrop areas (29). Nevertheless, the concept does have limitations. An overview of the COLLEGO™ effort plus a statement of some research interests engendered by it will be useful for assessment of the practical potential of this new approach to biological weed control.

The importance of seeds in the reproduction and maintenance of yellow nutsedge (Cyperus esculentus L. # CYPES) populations was evaluated. Isozyme analysis using starch gel electrophoresis was performed on 20 individuals of each of 10 widely separate populations in California. Genetic variation among individuals served as an indicator of the relative importance of asexual and sexual reproduction in each population. Eight enzyme systems were assayed from which 12 loci were resolvable, with four of those loci exhibiting variability. A maximum of four isozyme genotypes appeared in any population; only nine of 81 total potential genotypes were identified. Five populations were isozymically uniform, apparently composed of a single genotype. The remaining five populations were genotypically variable; these frequently deviated strongly from the genotype frequencies expected of a sexually reproducing population. These results indicate that sexually produced seeds are unimportant in the maintenance of yellow nutsedge populations in agricultural environments. Although viable seed may be produced, tubers appear to be the primary mode of reproduction in this species.

Research was conducted to determine percentage excretion and germination or viability of common crupina (Crupina vulgaris Cass. # CJNVU) achenes fed to sheep (Ovis aries), cattle (Bos taurus), deer (Odocoileus hemionus), horses (Equus cabullus), and Chinese pheasants (Phasianus colchicus). No achenes were found in the feces of sheep. Horses (nonruminant), deer, and cows (both ruminant animals) excreted 5, 3, and 25%, respectively, of the achenes consumed. The mean percentage achene excretion for pheasants was 23%; however, excretion was highly variable among individuals. All achenes excreted by any test species were detected within 5 days after consumption. The percentage germination or viability of excreted achenes ranged from 29% (cows) to 81% (deer). The percentage of consumed achenes that were excreted and viable ranged from 2 to 7% for large animals, whereas it was 17% for pheasants.

Two distinctive leaf forms have been described in the morningglory species Ipomoea hederacea (L.) Jacq. These are identified as ivyleaf morningglory [I. hederacea (L.) Jacq. # IPOHE] and entireleaf morningglory [I. hederacea (L.) Jacq. var. integriuscula Gray # IPOHG]. Controlled matings were made between pure breeding forms of plants with these two leaf types. The first generation progeny (F1) were all ivyleaf and the second generation (F2) was segregated in a 3:1 ratio (ivyleaf to entireleaf). The data fit a single gene model for determination of leaf shape, with ivyleaf being the dominant allele. Despite the dominance of the ivyleaf trait, the abundance of the entireleaf phenotype in the Mississippi Delta was higher than that of ivyleaf. Ipomoea hederacea is a facultative self-pollinator and other annual morningglory species found in the same area are also self-pollinated.

Red clover (Trifolium pratense L.) was direct seeded into a grass sod after spraying with dalapon (2,2-dichloropropanoic acid), glyphosate [N-(phosphonomethyl) glycine], or paraquat (1,1′-dimethyl-4,4′-bipyridinium ion) at rates of 5.6, 2.2, and 0.6 kg ai/ha respectively. Dalapon applied a few days prior to planting did not improve red clover stands in comparison to seeding in unsprayed sod. Applying glyphosate a few days prior to seeding in spring and summer invariably increased red clover stands, but paraquat was effective in only one of four spring or summer applications. Glyphosate applied the fall preceding a spring or summer seeding produced red clover stands superior to those after all spring treatments. Where glyphosate or paraquat was applied, water content of the upper 20 cm of the soil profile was greater during the first 10 days after treatment, relative to unsprayed plots or those treated with dalapon. By 30 days after application, the glyphosate-treated sods used less soil water than those treated with paraquat. Applications of glyphosate and paraquat greatly suppressed water utilization by oats (Avena sativa L.) in the greenhouse, based on water potential of the potting soil. Dalapon had little effect on water use by oat seedlings in the greenhouse.

The response of wild garlic (Allium vineale L. # ALLVI) to fall and spring applications of chlorsulfuron {2-chloro-N-[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl) amino] carbonyl] benzenesulfonamide} was evaluated in the field in 1982 and 1983. In natural stands, application of 20 g ai/ha in the spring reduced aerial bulblet production by 99%. Applications in the fall were much less effective (20 g/ha reduced aerial bulblet production by 59%), but were still better than the standard treatment of 2,4-D applied in the spring (840 g ae/ha reduced aerial bulblet production by only 45%). In planted stands, chlorsulfuron was applied preemergence at two locations at 10 and 20 g/ha in the fall, and postemergence in early fall, late fall, early spring, and late spring. All treatments reduced aerial and underground bulb production, but spring applications were much more effective than any of the fall treatments. Application of 20 g/ha of chlorsulfuron in late spring (mid-to-late April) reduced aerial and underground bulb production by greater than 95%. At this time the old bulb reserves were exhausted, and the scape and new bulbs were at an early and rapid stage of development.

The impact of chemical or biological herbicides in weed control may or may not be absolute. The traditional goal in chemical herbicide evaluation has been the identification of materials and application rates resulting in absolute (100%) kill of susceptible weeds. However, many insects or plant pathogens affecting weeds do not kill their weed hosts but decrease their growth and or reproduction in the field. Obligate plant pathogens dependent upon their weed hosts for their own survival derive no evolutionary advantage from killing their weed hosts and seldom do so under natural conditions. Under field conditions where weeds are subjected to stresses imposed by competition with crop and/or other wild plants, these biological agents can decrease the impact of a weed on crop or wildland productivity to tolerable economic levels. The recognition that the impact of chemical and/or biological herbicides on weeds need not be absolute to be effective has been accompanied by a search for an accurate, efficient, and economical method to measure quantitative changes in weed growth or health. While much of this report will concentrate on measuring impact of plant pathogens on weeds or other plants, all comments and techniques will apply also to other stresses such as herbicide injury.

Field experiments were conducted to study the phenology and growth of broadbean (Vicia faba L. ‘Alameda’) and crenate broomrape (Orobanche crenata Forsk. # ORACR) in relation to planting date. The establishment of crenate broomrape on broadbean planted between mid-December and mid-January coincided with the flower initiation on the crop. Establishment occurred during the vegetative phase, several weeks before flowering from a mid-November and mid-October planting. In general, no consistent relationships were observed between growth of the host and attachment or subsequent growth stage of the parasite. The maximum number of crenate broomrape plants successfully attached to the host as well as the duration of the underground stage of crenate broomrape decreased as broadbean planting was delayed. Broadbean dry weight decreased as infection rate increased. This effect was more intense in early than late plantings.

Field experiments designed to determine the efficacy and interaction of hexazinone [(3-cyclohexyl-6-(dimethylamino)-1-methyl-1,3,5-triazine-2,4(1H,3H)-dione] and nitrogen on weeds and lowbush blueberry [Vaccinium angustifolium (Ait.)] growth and yield were established at five locations in 1980 and 13 locations in 1981. Hexazinone was applied after pruning, preemergence with nitrogen, using a split-block design, with locations serving as replications. Grasses, goldenrod (Solidago spp.), and meadowsweet [Spiraea latifolia (Ait.) Borkh. # SPVLF] were reduced significantly as the rate of hexazinone was increased. Blueberry injury, plant stand, number of flower buds, and yield all increased with increases in hexazinone rate. Variations in blueberry yields occurred among locations due to differences in plant stand and productivity, as well as climatic and edaphic factors. Within each nitrogen rate, increases in the rate of hexazinone resulted in a quadratic increase in blueberry yield. Within the hexazinone treatments, increasing nitrogen had no effect on yield. Blueberry yield reached a maximum of 3926 kg/ha at a hexazinone rate of 1.83 kg/ha. Partial budgeting was used to evaluate the effect of combinations of hexazinone and nitrogen on net income. Response function analysis determined yield and profit-maximizing combinations of hexazinone and nitrogen. Partial budgeting indicated that hexazinone alone or combined with nitrogen increased net income, but nitrogen without hexazinone decreased net income. Response function analysis indicated that hexazinone at 1.79 kg/ha without nitrogen maximized profits.