Invited Speakers oJfb tsgylau RECENT APPROACHES FOR CONTROL OF PARASITIC WEEDS C.L. Foy and R. Jain Department of Plant Pathology, Physiology and Weed Science. Virginia Polvtechnic Institutc ancl State Universitt,. Blacksburg VA 24061. USA.
Abstract Foy, C.L. and R. Jain. 1986. Recent approaches for control of parasitic rveeds. Arab J. Pl. Prot. 4: 144 - 136
Broomrape ( Orobanche spp.) and witchweed ( Striga have been observed to enhance germination of broomrape spp.) are devastating root parasites of many agricultural seeds in the absence of host plant exudates. Glyphosate her- crops and cause severe yield losses. Conventional weed con- bicide selectivelv controls/ suppresses broomrape in crop trol methods. such as cultivation and the use of selective che- such as broadbean. 14C-glyphosate is translocated from micals, are largely ineffective due to the close ecological and host leaves to broomrape shoots where it accumulates in cou- physiological relationship between host and parasite. A centrations greater than in any part of the host plant, includ- search for genetic resistance to broomrape, O. aegyptiacain ing the apical meristem. An integrated approach using the tomato has riot been very encouraging. Although herbicides seed germination stimulants and translocated herbicides such such as 2,4 - D have been used against witchweed, the only as 2, 4-D in the case of witchweed and glyphosate in the case effective (but expensive) means of broomrape control has of broomrape may prove most useful in controlling these pa- been soil sterilization, by either chemical fumigation or rasitic weeds. Adequate water and nutrient supply to host solarization. Two recent approaches show considerable plants may also reduce parasitization by broomrape and thus promise for controlling these parasitic weeds. Ethylene has prevent yield loss. proved very effective in.stimulating suicidal germination of witchweed seeds but had been far less effective on broom- Additional key words: broomrape, witchweed, methods of rape seeds. Strigol and its synthetic analogues, however, control
Introduction (26). Following the successful establishment of the parasite Broomrapes ( Orobanche spp.) and witchweeds ( Striga seedlings on host roots, the subterranean parts of the para- spp.) are devastating root parasites of many economically site produce a shoot that elongates rapidly and emerges important crop plants. Broomrapes belong to the family above the soil surface. The above-ground portion of the Orobanchaceae which is characterized by plants having no shoot bears the flowers that produce numerous seeds. It has chlorophyll (37). been estimated that about four crenate broomrape ( O. cre- Forsk. H2ORACR) plants parasitizing a.broad bean Witchweeds, on the other hand, belong to the family 4afa ( Vicia fabaL.) plant decrease the seed yield of broad bean Scrophulariaceae which consists of plants having chlor- by one-half (30). ophyll. Both parasitic weed genera have a broad host range. Broomrapes are most damaging to members of the Sola- Several methods of control have been tried against these naceae, Legrlminosae, Asteraceae and Brassicaceae.whereas parasitic weeds. Although herbicides such as 2, 4-D [ (2, 4- witchweeds are most damagingto plantsbelonging primarily dichlorophenoxy) acetic acid l, paraquat (1,1'-dimethyl-4,4' - to the Gramineae family. Both parasitic weed groups are bipyridinium ion) and oxyfluorfen [ 2 - chloro - 1 - (3 - obligate heterotrophs (31), however, and resemble each ethoxy - 4 - nitrophenoxy) - 4 - (trifluoromethyl) ben- other in many aspects of their life cycle. zene ] have been used against witchweed I Striga asiatica (L.) Ktze.# STRLU with some success (15.29,40), Both broomrapes and witchweeds are annuals and repro- I the only truly eflective mc.ans duce by means of seeds which are usually dormant and do of control a-qainsJ the various broornrapes now not germinate readily (31). In order to break the dormancy, seems to be soil sterilization... either by chenrical furnigation. e.g.. with the seeds require a period of preconditioning under suitable methyl bromide (42,43), or by solar heating (20). Soil sterilization moisture and temperature conditions. The preconditioned is very expensive. however. and it is not usually adopted a seeds then germinate most successfully in the presence of on large scale. There is a serious neecl. therefore, root exudates from host (8) and some non-host plants (19). to develop other means of para- sitic weed control that are both eff'ective Seeds stimulated to germinate give out a radicle that pene- and economical. trates the host roots in the vicinity of the parasite seeds Two new approaches are being tested to control broom- (within 2 to 3 mm) and establishes a vascular connection with rapes and witchweeds with the main objectives of (a) reduc- the host to derive water and nutrients essential for its growth ing the seed populations of these parasitic weeds in the soil rxy'l .rtJl qUr il+- - taa and (b) preventing the production of new seeds. These nata) (22). For unknown reasons, nodding broomrape (O. approaches include the stimulation of seed germination of cernua) seeds failed to germinate in these experiments. parasitic means af- the weeds, and their control by chemical Incorporation of GR 7 and GR 24 into the soil under parasites host plants. ter the have established themselves on greenhouse conditions has also proven effective in stimulat- paper with a discussion of progress in the develop- This deals ing hemp broomrape seed germination and thus reducing in- ment of these two approaches as related to broomrape con- fection of tomato ( Lycopersicon esculentum Mill.) plants trol. transplanted in broomrape-infested soil four to six weeks af- Reduction of Broomrape Seed Populations in the ter incorporation of the strigol analogues (3a, 35). The Soil efficacy of these strigol analogues in the field remains to be tested, however, probably because of the continuing limited Broomrapes produce numerous seeds that can remain vi- availability of these specially synthesized chemicals for large- able in the soil for more than 10 years (26,33) . Parasite seeds scale experiments. in the dormant state are quite resistant to most control mea- sures except soil sterilization by fumigation or solar heating. Chemical Control of Broomrape After Attach. Once broomrape seeds have germinated, they require host ment to Host Roots plants to establish themselves and complete their subsequent Chemical control of broomrape after attachment to host development. Thus, it has been recognized that broomrape roots may be aimed at attacking the parasite directly or in- seeds are most vulnerable to destruction immediately after directly before it emerges above the soil surface or by treat- germination but prior to their establishment on host roots. If ing the shoots of the parasite after they have emerged from broomrape seeds are induced to germinate in the absence of the soil. Chemicals used to control broomrape before emerg- host plants, the young seedlinls are unable to support them- ence may either be applied to the soil for a direct or to host selves andsoondie of starvation. The stimulation of parasite plants for an indirect effect on the parasite. In the latter case, seed germination in the absence of host plants has been herbicides capable of translocation within the plant are ap- termed germination> (14) and this concept was first plied "suicidal to the host plant at rates that are relatively safe for the tested on witchweed by usingethylenegas as the seed ger- host plant itself, but are deleterious to the parasite. mination stimulant (13). Table 1. Effect of strigol analogues (growth regulators), GR The observation that broomrape seeds can germinate not 7 and GR 24, on the germination of four species of broom- only in the vicinity of roots of host plants, but also in the rape. vicinity of some non-host plants led to the use of these non- host plants as trap crops to induce suicidal germination of Treatment Conc O. aegyptiaca O. ramosa O. crenata O. cemua seeds of various species of broomrape (6, 7). The results of (ppn) (% germination) field experiments applying this method showed varying however, mainly due to the vast distribu- degrees of success, Conrol 8.0 6.6 0.0 0.0 throughout the tilled layer of soil (3, tion of broomrape seeds GR 7 0.10 17.7 19.8 1.0 0.0 11). GR 7 1.00 2t.3 29.6 4.6 0.0 The failure of trap crops in reducing, consistently, the GR 7 10.00 22.0 2I.9 17.). 0.0 broomrape seed populations in the soil led to the testing of GR 24 0.01 10.8 31.2 0.7 0.0 chemical stimulants for inducing suicicial germination of pa- GR 24 0.10 20.s 32.8 6.0 0.0 rasite seeds. Ethylene gas, which has proven very effective in GR 24 1.00 2t.0 25.3 76.4 0.0 reducing suicidal germination of witchweed seeds, has been SEM 1.5 3.3 1.4 mush less effective in inducing suicidal germination of cren- ate broomrape seeds (12). Strigol, a chemical stimulant iso- Several soil applied herbicides have been tested against lated from the roots of cotton ( Gossypium hirustumL.), the subterranean development of broomrape on host plants however, was found to be very effective in stimulating ger- with variable results (24, 27,28,36). Some interesting results mination of both witchweed and broomrape seeds (10). were obtained, however, with foliarly applied herbicides Several analogues of strigol were subsequently synthesized such as 2, 4-D. When applied to the leaves of broad bean germination (25, and tested for their effect on parasite seed plants parasitized by O. crenata,t4c-2,4-D was observed to 32,34). translocate from the host foliage to the roots and into the Two analogues of strigol, GR 7 and GR 24, have proven attached parasite. The developing tubercle of O. crenata particularly effective in stimulating broomrape seed ger- was found to contain the herbicide in a concentration 14 mination under laboratory conditions (Figure 1). In a pre- times greater than in the host roots five days after treatment liminary experiment, GR 24 was found to be more effective (41). Due io the high susceptibility of most broadleaf crops than GR 7 in stimulating seed germination in three broom- to 2, 4-D, however, the use of this herbicide for broomrape rape species (Table 1). In general, both strigol analogues control is generally not acceptable. The fact that 2,4-D was appeared to be more effective in stimulating seed germina- translocated and accumulated in the parasite in concentra- tion in hemp broomrape ( O. ramosa) than Egyptian tions much higher than in the host, however, suggested that broomrape (O. aegyptiaca) or crenate broomrape (O. ue- if a herbicide could be found that was no more toxic to the
143 -\il .rLJl qUj ii+, 9Hs
H OH
STR IGOL GR 24 fI' MW 346 MW 298 cHs
cHs 3. RAS (HM) 3. RAS (LM) cHs MW 248 MW 248 Figure 1' structures and molecular weights of strigol, GR 24, and high melting point (HM) and low melting point (LM) isomers of a 3- ring analogue of strigol (3-RAS). GR 7 is a mixture of the two 3_RASisomers(5).
host than to the parasite, it could be used selectivelv to con- broomrape in crops by glyphosate, the translocation trol broomrape in crops. and metabolism behavior of glyphosate was evaluated in two Glyphosate I N-(phosphonomethyl) glycine ] is a non_ varieties of tomato plants infected with O. aegyptiaca.The selective, postemergence herbicide that is readily translo_ two varieties of tomato were
and the parasite can be from tomato plants are readily translocated to the attached genetic tolerance to the herbicide laC-glyphosate selectively to control broomrape in O. ramosaplants (37). Our results show that combined, might be used presence tolerance to either glypho- behaves in a manner similar to that of photoassimilates in a crops. The of sufficient varieties is host-broomrape (tomato- O. aegyptiaca) system and that sate or broomrape alone in existing tomato a remote possibility. broomrape acts as a strong
141 - r;ll .rLJl qtii ilx Table 3. Translocation of 1aC-glyphosate in
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laC-glyphosate Figure 3. Translocation of in tomato plants infected with O. aegyptiacathree days after treatment. (A) and (B) are dry mount and autoradiogram of
139 - {rll .rUl ;,Lir ilv 4.5 4 3.5 ro I 5.O x 2.5 E o- E 2.O O t.5 + t.o o.5
200 b t75 t50 x-x BrOOmrope e-o fep6lo shoot E t25 po- roo re Treoted tomolo leoves o 7 I
234 56789tOil1?t3t4ts DISTANCE (cm) rac-glyphosate tac-glyphosate Figure 4. Thin layer chromatographic analysis of (a) standard and (b) in extracts of broomrape, tomato shoots, and treated tomato leaves.
Table potting 4. Effect of media on the growth of tomato Table 5. Effect of porting media on the growth of O. aegyp_ plants. tiacaon tomato plants.
Shoot Root Treatmento Shoot height fresh weight fresh weight Treatmentu Number of Shoot height Fresh weight (cm) (e) (e) infections (cm) (g)
"Rutgers" .Rutgers' Potting medium A 11.98 3.40 1..36 Potting medium A 3.42 10.67 2.57 Potting medium B t2.65 2.65 0.96 Potting medium B 3.00 10.57 2.22 Potting medium C 198.75 2r0.97 9.92 Potting medium C 0.08 0.00 0.32 LSD.05 93.55 48.25 2.17 LSD. 05 0.90 3.00 0.70 "PUZll,, "PUZll,, Potting medium A 18.75 4.33 2.00 Potting medium A 4.2r tt.57 2.83 Potting medium B 12.28 2.4i 1.30 Potting medium B 4.71 10.59 2.55 Potting medium C 220.83 243.78 16.58 Potting medium C 0.21, 0.00 1.05 LSD.05 47.10 55.35 3.59 LSD.05 1.11 2.80 0.69
(a) Potting medium A = clayloam (33.3%),sand (33.3%),Weblite (a) Potting medium A = clay loam(33.3%),sand(33.3%),Weblite (33.3"/"). (33.3%). Potting medium = clay loam (45%), sand (45%), peat moss Potting medium B = clay loam (45%), sand (45%), peat moss (10%). " (10%). Potting medium C = vermiculite (40%), Weblite (40%), peat moss Potting medium C = vermiculite (40%), Weblite (40%), peat moss (20%). (20%). nnll .rL:,tl CUy il.r- - t:S _.4illl r44-136: lAcyJl sLJl 4,-ti,ril+, .;Io}..^Jl str:'Jl a-rK.J c.,-bo.b. 1986 .i,etar,:st.r, .J . ,;*:,,6,9
:lrlU+lf ,r,t-=.>l)lll :t-"" d -,-r* -L+l j iL^i ,S -# :sh LrL iUlu.Jl .;r$lr'-ll ar" ttlrb ilrt6Jl dl laa;;. .rL3a)\.c J.." t oL5 ..,Le"jl ,Ji tJ . J,rUl ;)Le;-dJl jpt r.t1 .;# ;l*; kJ.".--r,F-al:."lt U. c+t .rl",r;r),r.iJl .JJjJl Jr*-- d:Uktt -* 4 ,i i;.L;Jr *t-t-.:t i-Et Ljt J$ :r*'!l i*iisJ e-.- ilqJl gl*-i euir iJt j dl; Jl #Ul ;trri ;, ,Li:t r+Jl -,. J[-alJ * U* d :t".e1t .rt-r.., +*iJl etJi Lt g! ,)i .=/ J,,Ul "? ,5i J" ,*i olf -e. LJ J.iL.Jt o!Jt: .,;Qt j. G.r),*iltr d "ljl.Jt .rL;"it .rtLi JL"-d.-t 4Lt*.Jl ;i;rti)r1t;t ,tl . ltJl d ;_.,:-r+Jl d ilL+ii ;jl;',Jl LrtjJl ;r eJl 2.4D "* J4lliL-.*ll .11;1,'-ll JIJ"J r:Jl -,Ur"rl .:l-i.*: Li 2,4D J& :t*'yl .rl.l*. ,)i ar, d;L .;.-,.-:/ d-95; r; (ilrQl .Jt- d .rLya)ill3 Uyl,;Jl ilt- i a-.jt References ciry'l l. Abu-Irmaileh , B.E. 1979. Effect of various fertiliz- 10. Cook, C.E., L.P. Whichard, B. Turner, M.E. Wall and ers on broomrape (Orobanche ramosa) infestation on G.H. Egley. 1966. Germination of witchweed ( Striga tomatoes. 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