Allelopathy in Parasite Weed Management

Allelopathy found hopeful utilization in plant protection against parasite weeds. The main world parasitic species are the witchweeds (Striga spp.), broomrapes (Orobanche spp.), and eventually dodder (Cuscuta spp.). Witchweeds and broom-rapes attack many economically important crops especially throughout the semiarid regions. Early detection of parasitic weed infestation and protection is difficult because of the growth habit of a root parasite and huge production of dust-like seeds viable up to 20 years (Kebreab and Murdoch 1999).

The main function of allelopathic substances is in the choice of a host and in the development of coactions. However, the effective compounds and their mechanism were described very rarely. Uncinanone B (4",5"-dihydro-5,2',4'-trihydroxy-5"-isopropenylfurano-(2",3";7,6)-isoflavanone) that induced germination of seeds from Striga hermonthica (Del.) Benth. and uncinanone C (4",5"-dihydro-2'-methoxy-5,4'-dihydroxy-5"-isopropenylfurano-(2",3";7,6)-isoflavanone) that inhibited the radical growth of this parasite are compounds contained in root exu-dates of the legume Desmodium uncinatum (Jacq.) DC and they are one of the first examples of identified allelopathic compounds that prevent against the striga parasitism (Tsanuo et al. 2003). Chang et al (1986) identified a germination stimulant, a p-benzoquinone compound from a natural host (sorghum) of Striga spp.

A chemical stimulant, a root exudate, is required to break seed dormancy of a parasitic weed and initiate seed germination. Upon receiving a signal for germination, a radicle emerges from the seed coat. However, an additional chemical signal is needed for the radicle to penetrate host roots and form a haustorium. Therefore several plants can serve as catch crops or trap crops for the reduction of the parasitic weed seed bank in infested soil.

Catch crops are crops that are susceptible to the parasite and thus become infected. The parasite has the chance to set seed, therefore it is necessary to destroy the catch crop before seed formation of the parasite.

Trap crops or so-called false hosts are crops able to stimulate parasite seed germination but are not themselves parasitized. Each trap crop control at least one species but not all genus species. (Abebe et al. 2005). Dodder (Cuscuta spp.) infestation can be effectively reduced by growing cereals or other grass crops (false hosts) continuously for several years (Dawson 1987). Other examples of suitable trap and catch crops are given in the Table 14.3.

Intercropping with trap or catch crop against parasitic plants is often practiced in resource-poor regions but as parasitic plants it has considerable success. This approach is more effective than use of pesticides, cheap and environmentally

Table 14.3 Examples of effective parasitic weed control

Parasite

Effective crop

Way of use

Reference

Striga hennonthica

Silverleaf (Desmodium intortum (Mill.) Urb.), greenleaf

Trap crops; between rows of

Khan et al. 2002

(Del.) Benth.

(D. Uncinatum (Jacq.) DC.)

maize = "push-pull" strategy

Sudan grass (Sorghum sudanense L.)

Catch crop

Oswald et al. 1999

Cowpea (Vigna unguiculata (L.) Walp.), green gram (Vigna

Intercropping

Khan et al. 2007

radiata (L.) Wilczek). crotalaria (Crotalaria ochroleuca

G. Don), sweet potato (Ipomoea batatas (L.) Lam.)

S. asiatica (L.) Kuntze

Cowpea (Vigna unguiculata (L.) Walp.)

Intercropping within cereal rows

Parker and Riches 1993

Sesbania sesban (L.) Merr., Tephrosia vogelii (Hemsley) A.

Trap crops

ICRAF 1996; Rao

Gary. Crotalaria grahamiana Wight & Arn., Desmodium

and Gacheru 1998

distortion (Aubl.) Macbr., Leucaena leucocephala (Lam.)

de Wit. Senna siamea (Lam.) H. S. Irwin & R. C. Barneby.

Senna didymobotrya (Fresen.),

O. crenata Forsk.

Oat (Arena sativa L.)

Intercropping

Fernandez-Aparicio et al. 2007

Flax (Linum usitatissimum L.), alfalfa (M. L.), cotton

Trap crops

Abebe et al. 2005

(Gossipium spp.), onion (Allium spp.), garlic (Allium

sativum L.), proper (Capsicum annum L.), snap bean

(P. vulgaris L.), maize (Zea mays L.), sesame (Sesamum

indicum L.)

O. ramosa L.,

Maize (Zea mays L.), Snap bean (P. vulgaris L.)

Trap crops

Abebe et al. 2005

O. cemua Loefl.

O. aegyptiaca Pers.

Flax (Linum usitatissimum L.)

Catch crop

Kleifeld et al. 1994

Mung beans (Phaseolus aureus Roxb.)

Catch crop

Kleifeld et al. 1994

O. minor Sm.

Winter wheat (Triticum aestivum), Triticale (Triticum secalotriticum saratoviense Meister)

Trap crops

Lins et al. 2006

friendly (Parrott 2005). The best solution is a combination of several methods - crop rotation and intercropping with trap or catch crops. Both trap crops and catch crops do not bring about immediate complete soil eradication. However, they accelerate the depletion of the seed bank (Mloza-Banda and Kabambe 1997).

Linke et al. (1993) established 30% reduction in Orobanche crenata Forsk seed bank after one catch crop cycle. Maize and snap bean depleted seed bank of O. ramosa L. and O. cernua Loefl. by 72.5% per season (Abebe et al. 2005). Odhiambo and Ransom (1996) found that after 4 years of continuous cropping sudan grass as a catch crop with cowpea or cotton Striga still remain in the soil. Growing trap crops for two consecutive seasons reduced seed bank of Orobanche species by 60%. So, crop rotation with trap crops deplete soil seed bank of parasite weed infestation in 5-10 years (Aalders and Pieters 1987). Therefore using trap and catch crops for the parasite control is limited especially in heavily infested fields (Kleifeld et al. 1994). Large differences in their "trapping" ability exists between varieties, e.g., in lentil L. culinaris Medicus (Fernández-Aparicio et al. 2007).

Allelopathic compounds as a defense could play an important role in resistance of the host plant. According to Jacobs and Rubery (1988), accumulation of phenolic compounds alters the hormonal balance of the parasite, resulting in necrosis. High levels of resistance to O. crenata have been found in the species Lathyrus ochrus DC. and L. clymenum L. (Sillero et al. 2005). Arnaud et al. (1999) described the differences among resistance of sorghum varieties. However, Eizenberg et al. (2003) and Labrousse et al. (2001) described the resistance in several wild sunflower species due to their ability to increase wall deposition, vessel occlusion, or broomrape cellular disorganization. Resistant hairy vetch (Vicia atropurpurea Desf.) blocked O. aegyptiaca Pers. haustorium at the root endodermis layer (Goldwasser et al. 2000). Therefore the mechanism of resistance needs more research.

Against parasitic weeds, plant extracts could be partially effective too. Habib and Abdul-Rahman (1988) found aqueous extracts of Bermuda grass (Cynodon dactylon (L.) Pers.), wall goosefoot (Chenopodium murale L.), Johnson grass (Sorghum halepense (L.) Pers.), and tumble pigweed (Amaranthus albus L.) reduced field dodder (Cuscuta campestris Yunck.) on alfalfa from 83% to 96%, but the same concentration (0.5 g) injured alfalfa up to 43%. Other possibilities of parasitic weed biological control (insects or fungi) were described by Elzein and Kroschel (2003).

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