Chenopodium album is a worldwide-distributed plant species growing in disturbed habitats. It is an abundant and very competitive weed in spring-sown crops, ...
Integrated Pest Management Reviews 2, 71–76 (1997)
Biological control of the annual weed Chenopodium album, with emphasis on the application of Ascochyta caulina as a microbial herbicide P. C . S C H E E P E N S 1 , C . K E M P E NA A R 1 , C . A N D R E A S E N 2, T H . E G G E R S 3 , J. N E T L A N D 4 and M. VURRO 5 2
1 DLO-Institute for Agrobiology and Soil Fertility (AB-DLO), PO Box 14, NL-6700 AA Wageningen, The Netherlands Department of Agricultural Sciences, Royal Veterinary and Agricultural University, Thorvaldsenvej 40, DK-1871 Frederiksberg C, Copenhagen, Denmark 3 Federal Biological Research Centre for Agriculture and Forestry, Messeweg 11=12, D-38104 Braunschweig, Germany 4 Norwegian Crop Research Institute, Fellesbygget, N-1432 A˚ s, Norway 5 Institute of Toxins and Mycotoxins from Plant Parasites, Viale Einaudi 51, I-70125 Bari, Italy
Chenopodium album is a worldwide-distributed plant species growing in disturbed habitats. It is an abundant and very competitive weed in spring-sown crops, particularly if they retain an open structure for a relatively long period. In most crops it is currently controlled by herbicides. In some crops, such as maize, chemical control is difficult, because C. album has become resistant to these herbicides. In other crops, such as sugar beet, the use of herbicides could be reduced considerably if selective control of C. album were possible. Recent experiments using the fungus Ascochyta caulina as a microbial herbicide to control C. album are encouraging (up to 70% control under field conditions). Within the framework of a COST action on ‘Biological Control of Weeds in Europe’, five countries have cooperated in developing a biological control method for C. album. To integrate the biological control of C. album in existing weed management systems is one of the remaining challenges. Keywords: Chenopodium album; Ascochyta caulina; biological control; microbial herbicide. The weed problem caused by Chenopodium album Description of the weed, its distribution and habitats Chenopodium album L. (Chenopodiaceae) is an erect, palegreen annual plant, varying in height from a few centimetres to 2 m and sometimes even up to 3 m. It is strongly taprooted. The stems are smooth, often having ascending, angular or ridged branches. The leaves are simple, alternate, ovate to lanceolate, regularly dented, 1.5– 8 cm in length and 0.5–3 cm broad. The inflorescence is a spiked panicle in the leaf axils or at the termini of stems and branches, with small dense flower clusters crowded on the branches. The seeds are lens-shaped, black or light brown in colour and approximately 2 mm in diameter. The taxonomic features in this polymorphic species could be correlated to its physiological properties such as herbicide resistance (e.g. Arlt and Ju¨ttersonke, 1990). There is no specific system of seed dispersal, so most seeds fall on the soil next to the mother plant. The black seeds but not the brown are dormant. Seeds that mature under long-day conditions are far more dormant than the seeds collected from plants grown under short-day conditions (Gutterman, 1985). Low temperatures or alternating high and low temperatures will break the dormancy. Germination usually occurs in spring. A short photoperiod hastens flowering and maturity. Germination in summer may result in tiny plants that flower and produce a few seeds. 1353–5226
# 1997 Chapman & Hall
Chenopodium album is reported to be one of the 12 most successful colonizing species (Allard, 1965). It is found from 70 8N to more than 50 8S and from sea level up to 3600 m (Holm et al., 1977). It grows in all inhabited areas except in extreme desert climates. It thrives on all soil types and over a wide range of pH values (Andreasen et al., 1991). It occurs in habitats that have been opened up by disturbances. These include crop land, set-aside land and ruderal vegetations. It is rated as one of the most abundant species in a number of spring-sown crops in Europe (Schroeder et al., 1993). Chenopodium album as a weed in crops: an opportunity for biological control Chenopodium album has frequently been reported to be troublesome in sugar beets, potatoes, maize, cereals and vegetables all over the world (Holm et al., 1977). These are sensitive to competition from a few weeks after sowing until closing of the canopy. In most crops it is currently controlled by herbicides. In maize and some vegetable crops C. album is relatively insensitive or has become resistant to the triazine herbicides that are often used in these crops (e.g. Solymosi et al., 1986; Kees and Lutz, 1991; Myers and Harvey, 1993). A selective biological control method comes to view as a possible solution to the problem caused by this weed. In other crops the availability of selective control of C. album would reduce the use of herbicides considerably. An example
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is sugar beet. The current practice is to control the weeds shortly after the emergence of the crop with a mixture of a soil herbicide (metamitron) and a foliar herbicide (phenmedipham). Depending on the weed density, the foliar application is repeated once or twice. These herbicide treatments may be omitted and replaced by one mechanical cleaning. However, C. album is then often the only emerging weed. Thus, a selective control method that reduces or prevents competition of C. album with the crop and that can be implemented in integrated pest management (IPM) is required for adequate weed control. Biological control could provide such a selective control. Because the time that is available to prevent competition is short, inundative use of the biocontrol agent will probably be needed. A plant pathogen used as a microbial herbicide will provide a good opportunity. Alternatively, an isolated phytotoxin from a plant pathogen might be used as a selective herbicide (Strobel et al., 1991). Within the framework of an action of the European Cooperation in the field of Scientific and Technical Research (COST) on ‘Biological Control of Weeds in Europe’ (Anonymous, 1992), five countries cooperated in developing a biological control method for C. album. Weed–Pathogen interactions Focus on Ascochyta caulina During a limited survey in The Netherlands, three pathogens from C. album were identified (Scheepens, 1979): Ascochyta atriplicis (renamed Ascochyta caulina by Van der Aa and Van Kesteren (1979)), Cercospora chenopodii (Ellis, 1976) (synonym of Cercospora dubia, cf. Brandenburger, 1985) and Peronospora farinosa f. sp. chenopodii. The latter was never regarded as a serious candidate for biological control, because downy mildew fungi can thus far only be grown on living host plants or tissue cultures and no methods are available to store the spores for a prolonged time. Cercospora dubia was able to reduce the plant dry weight of C. album by 20% at the most 4 weeks after inoculation (P.C. Scheepens, unpublished data). After inoculation with a mixture of spores and mycelium, the plants had formed new leaves before necrosis developed. In the USA, similar results were obtained with a local strain of this fungus (Winder and Van Dyke, 1985). A larger potential as a biochemical agent was attributed to A. caulina, because it could kill its host within 1 week under appropriate conditions (Scheepens, 1979). Due to the encouraging results, later efforts on the biological control of C. album were focused on this pathogen. Ascochyta caulina is described on Chenopodium and Atriplex species in many countries in Europe and Asia (Van der Aa and Van Kesteren, 1979). In North America, a closely related pathogen, Ascochyta hyalospora, was found (Allan et al., 1986). Knowledge of the life cycle of A. caulina is incomplete. The natural occurrence of necrotic
Scheepens et al.
spots is not frequently observed until late summer. Under moist conditions pycnidia develop on these necrotic spots. If the conditions remain wet, a slimy mass containing twocelled conidia is secreted from the pycnidia. Like all fungi producing pycnidia, the conidia are dispersed by splashing water droplets. The conidia may infect the leaves and stems and give rise to new necrotic lesions. A teleomorph of the fungus has not been found. The fungus may overwinter as pycnidia or mycelia on decaying plant residues (not observed yet) or as mycelia in seeds (Kempenaar et al., 1995). Factors that influence disease development by A. caulina The natural build up of inoculum by A. caulina is too slow to result in substantial growth reduction or even the mortality of C. album as is needed for control of the weed. To estimate the potential of this fungus as a mycoherbicide the factors that may influence infection and disease development have to be elucidated. According to Holcomb (1982), such factors may be related to the pathogen, to the host or the interaction between the host and pathogen. The pathogen-related factors that have been studied for A. caulina are the inoculum production, host range and virulence. The host-related factors are the susceptibility of C. album and other species, the degree of susceptibility of the plant stages and the host genotypes of C. album. The influence of spore density, additives to spore suspension and the environmental factors temperature and wetness duration on the host–pathogen interaction have been investigated (Kempenaar et al., 1996a). The lack of availability of the inoculum at the time that weed control is needed is easily overcome by artificial inoculation. The conidia of A. caulina are readily produced in the pycnidia on artificial substrates (Kempenaar, 1995). By drying cultures on a wheat bran medium and subsequent storage of the dried cultures at 5 8C, spores remained viable for at least 1 year. This method ensured the availability of spores for inoculum experiments throughout the year. Fermentation in a liquid medium, which would be more attractive for production on a much larger scale, has not yet been investigated. The germination of the conidia and infection of C. album leaves and stems take place only if certain moisture requirements are met. At temperatures between 18 and 30 8C during the light period and 12 8C during the dark period, a minimum wetness period of at least 8 h is necessary for the conidia to germinate and to penetrate the host tissue. Under these temperature regimes, a wetness period of more than 24 h was needed to obtain the maximum degree of necrosis (Kempenaar et al., 1996a). At 6 8C during the dark period, the maximum rate of spore germination was not reached even after 32 h. Disintegrating cells in the leaf tissue become visible after 48 h and at 72 h necrosis can be observed macroscopically (Kempenaar et al., 1996a). Eggers and Thun (1988) found
Biological control of C. album
a similar moisture requirement for another isolate of A. caulina and they even doubted whether this requirement would be met under field conditions. At a conidia density of 107 mÿ2 or less, necrotic spots of only a few millimetres in diameter developed. Fungal growth was restricted to the necrotic areas. In contrast, on detached leaves the fungus grows rapidly without obvious limitations. At densities of a magnitude higher, large areas of the leaves and stems become necrotic. As a result of inoculation, the plants either died or resumed to develop healthy new leaves after several days. The degree of necrosis was also increased by the interaction betweeen the conidia density and added surfactant (Sylgard 309) and nutrients (Czapek broth supplemented with yeast extract) to conidia suspensions. The effect on the plant depends on the growth stage, the smallest plants being the most vulnerable (Eggers and Thun, 1988; Kempenaar et al. 1996a). The results of Eggers and Thun (1988) suggest that older leaves become resistant to infection. According to Kempenaar et al. (1995), the fungus can infect the leaves and flowers of C. album during the flowering stage. Infection during early flowering resulted in complete death of the plant. Their observations, together with findings for other Ascochyta species (Maden et al., 1975; Hagedorn, 1984; Gossen and Morrall, 1986), indicate that at this stage the fungus infects the flowers and the scars left by the flower leaves.
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Phytotoxins produced by A. caulina Ascochyta is a well-known toxin-producing genus. Members of this genus produce a range of phytotoxin compounds, such as cytochalasins (Vurro et al., 1992), pinolidoxins (Evidente et al., 1993) and solanopyrones (Alam et al., 1989), that are chemically diverse and possess a broad range of biological activities. Phytotoxins might well be responsible for the necrosis development of C. album plants by A. caulina. Culture filtrates, obtained from growing A. caulina on a semi-synthetic liquid medium, were assayed for their phytotoxic properties using leaf and seedling assays. In the leaf assay, large necrotic spots similar to those caused by the pathogen appeared after 2–3 days. In the seedling assay, necrosis developed after 2 days, mainly on the edges of the leaves. The stems were not affected, indicating translocation and accumulation of the compounds in the leaves. Analysis of partially purified culture filtrate indicates the presence of at least one toxic compound with a hydrophilic nature and a molecular weight less than 3500 kDa. The chemical and biological characterization of the phytotoxin(s) is in progress. Host range of A. caulina The host range of A. caulina appears to be more or less confined to Chenopodium and Atriplex species (Table 1).
Table 1. Development of necrosis on plants of different taxa 1 week after the application of spores of A. caulina (after Kempenaar et al., 1996a) Plant taxon Atriplex patula L. Atriplex prostrata Boucher ex DC. B. vulgaris spp. vulgaris
Brassica oleracea L. sp. capitata C. album Chenopodium ficifolium Sm. Chenopodium glaucum L. Chenopodium polyspermum L. C. quinoa Chenopodium rubrum L. Corispermum marschallii Stev. Pisum sativum L. S. oleracea Triticum aestivum L. Zea mays L. a
Cultivar
Carla Lucy Univers Kyros Egyptische platte ronde
Elsevier Wild type
Eminent Amsterdams reuzenblad Martine Arminda Brazil Mandigo
ÿ, no necrosis; 6, less then 10% of the leaf area; , between 10 and 35%.
Necrosisa
ÿ ÿ ÿ ÿ ÿ ÿ 6 6 ÿ ÿ ÿ ÿ 6 ÿ ÿ ÿ
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Scheepens et al.
Status of biological control of C. album by A. caulina Three strategies to control C. album by A. caulina related to the growth stage of the weed can be envisaged: control of seedlings before emergence, control of juvenile plants and control of flowering plants (Kempenaar, 1995). From a practical point of view, the post-emergence application of the biocontrol agent before the onset of weed competition with the crop is the most attractive strategy. Several field experiments to control C. album in a sugar beet and a maize crop were conducted by Kempenaar et al. (1996b). The application of A. caulina on C. album plants planted in crop rows resulted in the necrosis of C. album, but not of the respective crop. The mean fractions of necrosis 1 week after application varied from 0.35 and 0.75. The mortality after 3 weeks, relative weed dry weight at harvest of the crop and yield reduction of the crop are shown in Table 2. In the weed-free control, the crop yields were 2021 g mÿ2 in 1992 (maize, aboveground biomass) and 1609 g mÿ2 (maize) and 2422 g mÿ2 in 1993 (sugar beet, aboveground plus storage organs). The aboveground biomasses of C. album in the untreated controls in these experiments were 190, 264 and 1072 g mÿ2 , respectively. The mortality of C. album was influenced by the experiment and by extension of the leaf wetness period in one of the three experiments. After 3 weeks, visible symptoms of the disease had disappeared. The weed plants that survived the attack were reduced in biomass. Crop yield reductions occurred in a sugar beet crop, but not in a maize crop. Comparing the growth rates of the weed and crop at different dates between the application of the fungus and the final yield date indicates that the maize crop suppresses the weed to a large extent
from the day on which the fungus was no longer active (Kempenaar et al., 1996b). In a sugar beet crop, some C. album plants became higher than the sugar beet plants, resulting in severe competition for light and a yield reduction of the crop plants. The control of mature plants of C. album by A. caulina was investigated in the same fields as the control of juvenile plants in the previous paragraph. Control of the mature plants may be useful both in preventing interference with harvesting of the crop and seed setting. Alternatively, it may be used in long-term control strategies of C. album, as well as on temporary set-aside land or in field margins. The reduction or prevention of the seed production of C. album by A. caulina in the experiments of Kempenaar et al. (1996b) was influenced by the weather conditions, growth stage of the weed and the presence of a maize crop. The seed production in a maize crop is shown in Fig. 1. Application at the beginning of the flowering period resulted in abortion, whereas application at the end of the flowering period had no effect on the seed production.
12000
107 spores ml21 Untreated
Seeds per 4 plants
Beyond plants of these two genera, a small degree of susceptibility was found on one cultivar of Spinacea oleracea L. Another cultivar of S. oleracea, some other species of Chenopodiaceae including several cultivars of Beta vulgaris L. and species belonging to other families were resistant to the isolate of A. caulina tested by Kempenaar et al. (1996a).
9000
6000
3000
0 A
B
C
Plant stage at inoculation
Fig. 1. Seed production of C. album plants after treatment with A. caulina at (A) the beginning, (B) the mid-flowering or (C) the end of the flowering period (Kempenaar et al., 1995).
Table 2. Results of field experiments to control C. album with A. caulina (after Kempenaar, 1995) Crop and year
Weather shortly after applicationc
Mortality of C. album (growth reduction)a (%)
Yield reduction of crop (potential yield reduction)b (%)
Maize 1992
Cloudy, Cloudy, Cloudy, Cloudy, Cloudy, Cloudy,
30 50 5 30 50 70
0 0 50 30 19 0
showery showery (24 h) Sugar beet 1993 one shower one shower (16 h) one shower (28 h) Maize 1993 showery a Reduction in C. album dry weight mÿ2 , expressed as percentage of untreated b c
(79) (95) (43) (66) (80) (90)
controls. Yield reduction on untreated controls, expressed as a percentage of weed-free controls. Extension of wetness period (number of hours in parentheses).
(19) (19) (73) (73) (73) (20)
Biological control of C. album
The control of C. album by pre-emergence application of A. caulina has the advantage that it is less dependent on the high humidity of the atmosphere. It was investigated in pot experiments in the greenhouse (Kempenaar et al., 1996c). Conidia suspensions were either mixed through the top layer of the soil or sprayed on the soil. Non-sterile soils were used in the experiments. Ascochyta caulina had no effect on the emergence rate of C. album, but it caused disease and mortality of the seedlings shortly after emergence. The disease incidence and severity and mortality were influenced by the conidia density, soil type and soil moisture content. We estimated that 109 –1010 conidia mÿ2 were required for 50% mortality of emerged C. album. Outlook The market for a microbial herbicide to control C. album The biological control of weeds by applying indigenous plant pathogens as microbial herbicides may become an important component of IPM, resulting in less use of chemical herbicides. So far, the commitment of private industry in the development of microbial herbicides has been limited. Potential manufacturers mostly mention commercial factors as limiting, not the lack of efficacy of the control agents (Greaves and McQueen, 1991; Rodgers, 1993). The microbial herbicides available supply only small, regional markets. A microbial herbicide against C. album based on A. caulina would have a much larger market. Chenopodium album is a widely distributed, noxious weed in field crops such as maize, potatoes, sugar beets and vegetables and ornamentals such as tulips and other flower bulbs. It is a noxious weed in these crops because the existing weed control strategies have a weakness against C. album. Regarding the limited host range of A. caulina, it will be safe to use in all crops except S. oleracea and Chenopodium quinoa Willd. Liquid culture filtrates and chromatographic fractions of A. caulina showed toxic effects similar to the symptoms caused by the pathogen itself, suggesting the involvement of toxins in disease development. The fast appearance of large necrotic spots on the host leaves justifies further investigations on the possible practical use of these metabolites alone or as synergists together with the pathogen against C. album. Critical issues for the further development of A. caulina as a microbial herbicide Under field conditions, a mortality rate of 70% of C. album plants and a substantial growth reduction of the surviving plants have been achieved by one application of A. caulina (Kempenaar et al., 1996b). This level of control appears to be sufficient in competitive crops such as maize, but a higher efficacy will be necessary in less competitive crops. The results in the field were achieved with only one isolate
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of A. caulina. A small test in the laboratory with two additional isolates suggests the existence of considerable variation in virulence between the isolates. The first option to increase the efficacy will be a survey for more virulent isolates in the whole distribution range of the fungus. An alternative approach would be the addition of synergists that decrease resistance in the host plant (Sharon et al., 1992). The variability in the environmental conditions is too great to ensure enough efficacy at any time. In particular, the need of A. caulina and most other fungi for a prolonged period of high humidity shortly after application is very critical. The selection of fungal strains that are adapted to adverse weather conditions might be an option to overcome the environmental constraints. The correct timing of application is primarily determined by the need to reduce the competitiveness of the weed and it might be difficult to obtain the right weather for application in that period. As irrigation of the crop is not possible or not realistic in many cases, formulation will be the option to create the right environment in which spores of the agent organism can germinate and infect the host plant (e.g. Womack et al., 1996). Biological control of C. album and IPM In most situations, C. album is not the only non-crop species present. If C. album is controlled selectively, plants of other species may replace it. Although they will in general not be as competitive as C. album, in large numbers they can be damaging to the crop. Biological control as a single measure is not meaningful to farmers; instead an integrated approach to control all the weeds in a given crop should be used. Such an approach may include mechanical control and chemical control of other species. In both sugar beet and maize, mechanical weed control (harrowing) up to the two-leaf stage of the crop could replace the application of a soil herbicide. Massive emergence of C. album is expected after that time. A microbial herbicide based on A. caulina could be used to control this weed. If seeds of other weeds germinate as well, the fungus may be tank mixed with half the regular dose of a sulphonyl ureum herbicide. It should be ensured that the biological control is compatible with other measures in a cropping system. For instance, in a potato crop the application of fungicides could interfere with the control of C. album by A. caulina. In that case, the timing of the fungicide application or the use of selective fungicides that do not inhibit A. caulina could solve the incompatibility problem. In maize and sugar beet crops, no fungicides are usually applied at all, so no compatibility problems with mycoherbicides will arise. References Alam, S.S., Bilton, J.N., Slawin, A.M.Z., Williams, D.J., Sheppard, R.N. and Strange, R.N. (1989) Chickpea blight: production of
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the phytotoxins solanopyrones A and C by Ascochyta rabiei. Phytochemistry 28 (10), 2627–30. Allan, W.G., Watson, A.K. and Wymore, L.A. (1986) Evaluation of a Chenopodium album pathogen for potential use as a mycoherbicide. Canadian Journal of Plant Pathology 9, 272. Allard, R. (1965) Genetic systems associated with colonizing ability in predominantly self-pollinated species. In H. Baker and G. Stebbins (eds) The genetics of colonizing species, p. 49. New York: Academic Press. Andreasen, C., Streibig, J.C. and Haas, H. (1991) Soil properties affecting the distribution of 37 weed species in Danish fields. Weed Research 31, 181–7. Anonymous (1992) COST Cooperation. Objectives, Structures, Operations. Luxembourg: Commission of the European Communities, Office for Official Publications of the European Communities. Arlt, K. and Ju¨ttersonke (1990) Die infraspezifische Struktur von Chenopodium album L. in Beziehung zur herbizidresistenz. Weed Research 30, 189–99. Brandenburger, W. (1985) Parasitische Pilze an Gefa¨sspflanzen in Europa. Stuttgart: Gustav Fischer. Eggers, Th. and Thun, K. (1988) Biologische Beka¨mpfung von Chenopodium album mit Ascochyta caulina? Zeitschrift fu¨r Pflanzenkrankheiten und Pflanzenschutz 11, 225–37 (in German). Ellis, M.B. (1976) More Dematiaceous Hymphomycetes. Slough, UK: Commonwealth Agricultural Bureaux. Evidente, A., Lanzetta, R., Capasso, R., Vurro, M. and Bottalico, A. (1993) Pinolidoxin, a phytotoxic nonenolide from Ascochyta pinodes. Phytochemistry 34, 999–1003. Gossen, B.D. and Morrall, R.A.A. (1986) Transmission of Ascochyta lentis from infected lentil seed and plant residue. Canadian Journal of Plant Pathology 8, 28–32. Greaves, M.P. and McQueen, M.D. (1991) Bioherbicides: their role in tomorrow’s agriculture. In I. Denholm, A.L. Devonshire and D.W. Hollomon (eds) Proceedings of the SCI Symposium ‘Resistance ’91: Achievements and Developments in Combating Pesticide Resistance’, pp. 295–306. London and New York: Elsevier Applied Science. Gutterman, Y. (1985) Flowering, seed development, and the influences during seed maturation on seed germination of annual weeds. In S.O. Duke (ed.) Weed physiology, Vol. 1, pp. 1–23. Boca Raton, FL: CRC Press. Hagedorn, D.J. (1984) Compendium of Pea Diseases. St Paul, MN: American Phytopathological Society. Holcomb, G.E. (1982) Constraints on disease development. In R. Charudattan and H.L. Walker (eds) Biological control of weeds with plant pathogens, pp. 61–72. New York: John Wiley & Sons. Holm, L.G., Pluckett, D.L., Pancho, J.V. and Herberger, J.P. (1977) The World’s Worst Weeds (Distribution and Biology), pp. 84–91. Honolulu: University Press of Hawaii. Kees, H. and Lutz, A. (1991) The problem of triazine resistance in annual weeds in maize and horticultural crops. Gesunde Pflanzen 43, 216–20 (in German). Kempenaar, C. (1995) Studies on biological control of Chenopodium album by Ascochyta caulina. PhD thesis, Wageningen Agricultural University, Budapest.
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Kempenaar, C., Horsten, P.J.F.M. and Scheepens, P.C. (1995) Effect of application of Ascochyta caulina to flowering Chenopodium album plants on propagation of the weed. In Proceedings of the Ninth Symposium of the European Weed Research Society ‘Challenges of Weed Science in a Changing Europe’, pp. 535– 42. Kempenaar, C., Horsten, P.J.F.M. and Scheepens, P.C. (1996a) Spore germination and disease development of Chenopodium album after application of Ascochyta caulina as a mycoherbicide. European Journal of Plant Pathology 102, 143–53. Kempenaar, C., Horsten, P.J.F.M. and Scheepens, P.C. (1996b) Growth and competitiveness of common lambsquarters (Chenopodium album) after foliar application of Ascochyta caulina as a mycoherbicide. Weed Science 44, 609–14. Kempenaar, C., Wanningen, R. and Scheepens, P.C. (1996c) Control of Chenopodium album by soil application of Ascochyta caulina under greenhouse conditions. Annals of Applied Biology 129, in press. Maden, S., Singh, D., Mathur, S.B. and Neergaard, P. (1975) Detection and location of seed-borne inoculum of Ascochyta rabiei and its transmission in chickpea. Seed Science and Technology 3, 667–81. Myers, M.G. and Harvey, R.G. (1993) Triazine resistant common lambsquarters (Chenopodium album L.) control in field corn ( Zea mays). Weed Technology 7, 884–9. Rodgers, P.B. (1993) Potential of biopesticides in agriculture. Pesticide Science 39, 117–29. Scheepens, P.C. (1979) Bestrijding van onkruiden met microorganismen. Gewasbescherming 10, 113–17. Schroeder, D., Mu¨ller-Cha¨rer, H. and Stinson, C.A.S. (1993) A European survey in 10 major crop systems to identify targets for biological control. Weed Research 33, 449–58. Sharon, A., Amsellem, Z. and Gressel, J. (1992) Glyphosate suppression of an elicited defense response. Increased susceptibility of Cassia obtusifolia to a mycoherbicide. Plant Physiology 98, 654–9. Solymosi, P., Lehoczki, E. and Laskay, G. (1986) Differences in herbicide resistance to various taxonomic populations of common lambsquarters (Chenopodium album) and late flowering goosefoot (Chenopodium strictum) in Hungary. Weed Science 34, 175–80. Strobel, G.A., Kenfield, D., Bunkers, G., Sugawara, F. and Clardy, J. (1991) Phytotoxins as potential herbicides. Experientia 47, 819–26. Van der Aa, H.A. and Van Kesteren, H.A. (1979) Some pycnidial fungi occurring on Atriplex and Chenopodium. Persoonia 10, 267–76. Vurro, M., Zonno, M.C., Evidente, A., Capasso, R. and Bottalico, A. (1992) Isolation of cytochalasin A and B from Ascochyta lathyri. Mycotoxin Research 8, 17–21. Winder, R.S. and Van Dyke, C.G. (1985) Cercospora dubia (Reiss) Wint. on Chenopodium album L.: greenhouse efficacy and biocontrol potential. Proceedings of the Annual Meeting of the Southern Weed Science Society 39, 387. Womack, J.G., Eccleston, G.M. and Burge, M.N. (1996) A vegetable oil-based invert emulsion for mycoherbicide delivery. Biological Control 6, 23–8.
