Progress in cultural control: indigenous practices and possible improvement
Several techniques have been developed to encapsulate fungal propagules in a solid matrix which can also help to buffer the organism from environmental constraints like rapid desiccation or microbial competition (Walker and Connick, 1983; Connick et al. 1991). Some efforts and progress were made to apply simple encapsulation methods suitable for pre-planting soil applications to the biocontrol agents pathogenic to parasitic weeds. Amsellem et al. (1999) successfully used the ‘Stabileze’ formulation method of Quimby et al. (1999), a formulation of absorbent starch, corn oil, sugar and silica, to formulate conidia and mycelia of FOXY and FARTH to control O. aegyptiaca grown in polyethylene bags. Another successful example of granular formulation called ‘Pesta’, containing propagules of Foxy 2 and FOO was prepared, and showed high efficacy in controlling S. hermonthica and O. cumana in the greenhouse (Kroschel et al. 2000, Müller-Stöver, 2001; Elzein, 2003) (Fig. 1). The method consists of mixing fermented fungal biomass (wet or dry) in a matrix composed of agricultural commodities (semolina, kaolin, and sucrose) using an extrusion process (for more details see Müller-Stöver 2001; Elzein 2003). With an application of 0.5 g of the formulated ‘Pesta’ granules per kg of soil, the authors achieved the same efficacy in controlling Striga and Orobanche as with the use of 10 g of inoculum propagated on wheat grains. (Table 2a, 2b, Fig. 2). Such a reduction (95 percent) in the amount of fungal inoculum as a result of adopting formulation technology could be economically very significant when large-scale application is considered. Using 0.5 g granules per kg of soil, approximately 300 kg of ‘Pesta’ granules, at a cost of approximately US$86 (ingredients only), is required for the control of 1 ha infested with O. cumana or S. hermonthica . Nevertheless, and from a practical perspective, one possibility to reduce the dose may be the proper placement of the formulated product onto the target weed, e.g. in the planting hole, in-furrow application, or coating the sorghum seeds with the antagonist before delivery. For example, if 0.5-1g ‘Pesta’ granules is applied per seed pocket, only 14-28 kg (US$4-8) granules will be required for the control of 1 ha infested with Orobanche or Striga .
In maize, considerable progress has been achieved at the International Institute of Tropical Agriculture (IITA) and by the International Maize and Wheat Improvement Center (CIMMYT) in developing open-pollinated varieties, inbred lines and hybrids that have both reduced host plant damage symptoms (tolerance) and reduced parasite emergence under artificial infestation with S. hermonthica (Kim, 1994; Berner et al. 1995; Lane et al. 1995). Some of these varieties have also shown moderate levels of resistance to S. aspera (Willd.) Benth. and S. asiatica . Resistant varieties have been developed with adaptation to the lowland and mid-altitude ecologies, and with a range of maturity, grain colour and grain texture characteristics (for more details see Kling et al. 2000). In order to ensure that the developed varieties are adapted to farmer circumstances and satisfy end-user preferences, the best varieties are being actively extended to farmers through the efforts of the regional maize and Striga networks and several collaborative projects, including the African maize stress project, and these are now being evaluated.
In rice, Oryza glaberrima lines ‘ACC102196’, ‘Makassa’, and ‘IG 10’, as well as O. sativa lines ‘IR49255-BB-5-2’ and IR47255-BB-5-4’ showed partial resistance to S. aspera and S. hermonthica under field conditions in Cote d’Ivoire (Riches et al. 1996; Johnson et al, 2000).
improved soil fertility enabling maize plant to better compete with Striga and reduce the damaging potential of Striga . Thus, an appropriate agroforestry system can provide a significant supplementary strategy to deplete the Striga seed bank through suicidal germination, even during the fallow phase.
Received: 30 November 2017; Accepted: 09 February 2018;
Published: 07 March 2018.
Alternative control systems were developed in tomato by Eizenberg et al. which showed that the addition of biochar to the pot soil resulted in inhibition of P. aegyptiaca infection. Biochar did not affect the tomato exudation of stimulants of parasitic germination, nor changed the penetration ability of the parasite. The major cause for the decrease in germination percentage was physical adsorption of the stimulant molecule by the biochar. In addition, Venezian et al. studied the influence of plant growth regulators on P. aegyptiaca development and its control efficacy showing that maleic hydrazide is a potent inhibitor of the early stages of parasitism, namely attachment and the tubercle stage.
Citation: Rubiales D, Fernández-Aparicio M, Vurro M and Eizenberg H (2018) Editorial: Advances in Parasitic Weed Research. Front. Plant Sci. 9:236. doi: 10.3389/fpls.2018.00236
Most of the parasitic weed damage in the crop occurs during early stages of their underground infection, and by the time the parasite is visible emerged from the soil surface, any control method is useless as the damage to the crop has already been done. Fernández-Aparicio et al. reviewed the underground biology of broomrape weeds and the control strategies to inhibit their parasitism. The vulnerability of some underground events key for the intractable parasitism, such as crop-induced seed germination or haustorial development were reviewed as inhibition targets for plant protection programs. Root parasitic seed germination is triggered by host-derived signals upon which it invades the host root, and strigolactones are the main germination stimulants. Cheng et al. studied the role of endogenous strigolactones and their interaction with ABA during the infection of tomato by P. ramosa. These findings may have implications for future control strategies, including improvement of parasitic plant resistance.
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
The reviewer RA declared a past co-authorship with one of the authors HE to the handling Editor.
Only moderately broomrape resistant faba bean cultivars are available to farmers. Rubiales et al. dissected resistance components in faba bean accessions against a number of infective and non-infective broomrape species. The intermediate levels of resistance of cv. Baraca acted against all broomrape populations and species studied, confirming previous reports on the stability of its resistance, whose identified mechanisms were due to (1) low induction of seed germination; (2) negative tropism of germinated seeds with radicles growing away from faba bean roots; (3) necrosis of radicles that had successfully contacted faba bean roots; (4) necrosis of formed broomrape tubercles.