Invited Speakers oJfb tsgylau RECENT APPROACHES FOR CONTROL OF PARASITIC WEEDS C.L. Foy and R. Jain Department of 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 ( 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 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 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 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 which is characterized by plants having no shoot bears the flowers that produce numerous seeds. It has (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 (a Russian varie_ of the plant. "pUZ Due to its nonselective nature, glyphosate is ty reported to be resitant to broomrape) (4). Surprisingly, generally applied in noncropped < areas or prior to planting or tomato plants were as extensively infected with O. after harvesting in croplands for broad spectrum weed con_ aegyptiaca (Figure 2) as ) tomato plants. Auto_ trol. This herbicide has radiographs been found to be very effective of broomrape-infected and against broomrape. Very interestingly, when applied to crop tomato plants showed that the radiolabel from r4C_ plants such as broad bean at very glyphosate low rates (60 to LZO g/ha), applied on tomato leaves was translocated to all glyphosate has been observed to suppress and,/or control the parts of the host plant and particularly to broomrape, where attached parasite without adversely affecting the host plants. it appeared to accumulate in the shoot meristem within three days of treatment (Figure The effectiveness of glyphosates as a foliar application for 3). euantitative estimation of the radiolabel by liquid scintillation O. crenata control in broad bean was first reported by spectrometry in various Kasasian (27). parts of the host plants and broomrape The rate of 200 g,/ha provided complete con_ in shoots (on the basis of total radioactivity trol of the parasite and sufficient safety margin, indicating per gram dry weight of broomrape shoot) showed that less than half the relative tolerance of broad bean to the herbicide. Similar of the radioactivity applied on tomato plants moved out results were obtained with O. aegyptiacacontrol in tobacco of the treated leaves. Radioac_ tivity translocated from the treated ( Nicotiana tabacumL.) (27). Several researchers have since tomato leaves accumu_ confirmed lated in broomrape shoots in much greater that glyphosate could be used selectively to con_ amounts than in the apical meristem of the host. trol broomrape in crops such as broad bean and tobacco (20, Accumulation of radioactiv_ 38, 39). ity in broomrape shoots increased with the time of treatment on the host leaves from three to seven days (Tables 2 and3) In order to understand the nature of selective control of (23). It has already been observed that r4C_photoassimilates 9y'1 .:,tJl \tis ^\--tlz Figure 2. O. aegyptiacaon a tomato plant.

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 and competes directly with thought to be for glyphosate and most the apical meristem of the host Table 2. Translocation of laC-glyphosate in toma- raC-glyphosate did not appear to probably photoassimilates. to plants infected with O. aegyptiaca. be metabolized in either the host or the broomrape tissue (Figure 4) (23). After 3 days After 7 days Although glyphosate appears to accumulate in broomrape Total" 7" oftotal Total" % oftotal in concentrations higher than in the apical meristem of the Plant part DPMxl03 laC applied DPMx103 taC applied host plants, tomato plants treated with the low rates (50 g/ showed injury ha followed by 100 { ha) of. the herbicide Treated leaves 261.0 47 .5 311 .0 56.6 symptoms. Hence, the limiting factor to glyphosate deploy- Apical meristem 3.3 0.6 1.8 0.3 ment as a broomrape herbicide in tomato as well as in some Stem + untreated leavps 6.6 1.2 2.3 0.4 other crops such as pea ( Plsum sativum L.) and carrot Roots 7.6 1..4 8.8 1.6 ( Daucus carotaL.) is the narrow margin of selectivity of Broomrape shoots 13.7 2.5 8.4 1.5 these crops to glyphosate (20). Subsquently, a new approach (96.8)b (101.7)b of controlling broomrape in these crops was suggested by Foy and Jacobsohn (16). According to these researchers, if plants. some tolerance to both glyphosate and broomrape could be (a) Total DPM are means of two (b) in parenthesis are DPM /g broomrape shoot. found in crops such as tomato, an integrated approach where Numbers

141 - r;ll .rLJl qtii ilx Table 3. Translocation of 1aC-glyphosate in , and 11>. Q2.7)b (s1.7)b "PUZ The results of this experiment showed that the growth of both varieties of tomato plants was much more vigorous in (a) Total DPM are means of two plants. potting medium (C) than in potting medium (A) or (B)(Table (b) Numbers in parenthesis areDPM/gbroomrape shoot. 4). Both varieties were equally susceptible to O. aegyp- tiaca. 'fhe number of broomrape infections on tomato plants, however, were negatively correlated with the growth of tomato plants in the three potting media (Table 5). Thus, In order to test this hypothesis, a major screening program the higher nutrient status and water holding capacity of pot- was conducted to determine which (if any) tomato lines had ting medium (C) than those of potting medium (A) or (B) practical levels of tolerance to glyphosate and / ot broomrape may have been responsible for the vigorous growth of toma- (16). Out of the 1522 tomato lines screened in the green- to plants and low infection rate by broomrape (17). Further house to evaluate their tolerance to glyphosat e at 37 .29 /ha experiments, especially under controlled environmental con- applied in a spray volume equivalent to 250 L /ha, about 40 ditions, are required to establish the effect of soil type and tomato lines showed fresh weights of treated plants not signi- nutrient status of the soil. ficantly different from those of untreated plants. All tomato varieties, however, showed injury to varying degrees from Conclusions glyphosate. Repeat screening of selected tomato varieties in Broomrapes and witchweeds cause seveie losses in yield of the greenhouse and field indicated that some tomato lines many economically important crops in many parts of the may have promise for glyphosate tolerance and deserve world. The main objectives in a control program for parasitic attention in future screening programs. weeds include reduction of existing seed populations in the soil and prevention of new seed production by the parasite. Recently, a glyphosate-tolerant gene was discovered in a An eradication program developed for witchweed control in genetically modified strain of a bacterium, Salmonella typhi- the United States is based upon inducing suicidal germina- murium (9).This glyphosate-tolerant gene was introduced tion of witchweed seeds by ethylene and the control of by genetic engineering techniques into tobacco, tomato and emerged parasite plants by certain herbicid6s before the pa- some other crops (18). Such genetically engineered crop rasite is able to produce seeds. Ethylene gas is much less plants are expected to become commercially available in the effective, in inducing suicidal germination of broomrape early.1990's (Sun, 1986). With the availability of glypho- seeds than of witchweed seeds. Certain analogues ofstrigol, satetolerant crops, the use of glyphosate for broomrape con- however, have shown considerable promise for inducing trol in crOps such as tomato and tobacco may become a com- suicidal germination in both broomrape and witchweed mon practice. seeds. Glyhosate has been observed to control broomrape selectively in broad bean in several Reduction in Broomrape Parasitism by Soil Ferti- field trials. The margin of lization selectivity of this herbicide in some other crops such as toma- to, pea and carrot, however, is much lower than in broad The nutrient status of the soil can affect the ability of bean. Screening of numerous tomato lines to low rates of broomrape to parasitize host plants. Farmers in Jordan claim glyphosate has indicated that practical levels of glyphosate that addition of chick manure to the soil reduces broomrape tolerance may exist in some tomato lines. A breeding prog- infestation in their fields (1). High levels of nitrogen have ram incorporating glyphosate tolerance into the commercial been observed to reduce O. ramosainfestations in tobacco varieties or the commercial availability of genetically en- and tomato. High nitrogen levels, however, had some gineered glyphosate-tolerant crops may help to selectively adverse effects on tomato yield (2, 27). Application of potas- control not only broomrape but many other rveeds by sium and phosphorus along with nitrogen overcame the dras- glyphosate in crops such as tomato. Integration of adequate tic effects of nitrogen applications alone on tomato yields soil fertilization may further help to reduce parasitization and drastically reduced broomrape infestations on crop and thus prevent yield loss in crops dtre to this noxious para- plants (1). sitic weed.

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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 tomato plant; (C) and (D) are dry mount and autoradiogram of tomato plant, re,spectively.

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

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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

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