Chemical management of Orobanche in Toria (Brassica campestris L. var. Toria)

Introduction

Orobanche is a serious weed affecting various crops across the country, particularly rapeseed, mustard, tomato, brinjal, and other solanaceous crops. Its impact on yield can be devastating, often leading to near-complete crop failure in some areas. This depends on factors such as infestation severity, soil fertility, moisture availability, environmental conditions, and the competitiveness of the crop. Toria (a type of mustard) is an important cash crop in Assam due to its adaptability to the local soils, compatibility with rice-based cropping systems, and minimal moisture and nutrient requirements. It is also relatively low-maintenance in terms of pest and disease management. In Nagaon district, Toria performs exceptionally well during the winter season, making it a preferred crop for farmers. However, Orobanche infestations have been increasing over the years, significantly affecting crop yields as reported by many farmers. Orobanche is an annual, highly branched, achlorophyllous, and harmful root parasite that relies entirely on host plants for survival, reproducing solely through seeds (Punia et al., 2012) [2] . Upon germination, its seed-dependent on host plant signals-develops a germ tube or radicle, which elongates chemotropically to form a haustorium. This haustorium attaches firmly to the host plant's vascular system, turning the parasite into a powerful metabolic sink, often referred to as a "super-sink." This enables it to effectively compete with the host plant for water and essential nutrients, leading to moisture and nutrient starvation, plant stress, stunted growth, and a notable reduction in crop yield and quality in infested fields (Punia et al., 2012) [2] . The level of damage from Orobanche varies based on infestation intensity, environmental factors, soil fertility, and the competitiveness of the host crop. In severe cases, it can lead to complete crop failure, forcing some farmers to abandon mustard cultivation entirely due to this parasitic weed. The study was conducted over three consecutive winter seasons (2020, 2021, and 2022) at various farmer-managed field locations in the Nagaon district of Assam. The primary objective was to evaluate the tolerance of mustard plants to Orobanche infestations and to assess the efficacy of glyphosate applications in reducing Orobanche shoots. This research was motivated by previous positive findings indicating that glyphosate could reduce Orobanche infestations and improve mustard crop resilience. Based on those findings, a glyphosate dose for Toria (a type of mustard) was formulated from prior experiments on mustard crops.

Experimental Design and Treatments

The experiment was designed as a field trial with glyphosate applications planned at specific stages of crop growth to maximize efficacy and minimize crop stress. The application of glyphosate was divided into two stages to ensure optimal control of Orobanche without harming the Toria plants.

First Application of Glyphosate

 Timing: The first application of glyphosate was administered 30 days after sowing (DAS) of the Toria crop.  Dosage: Glyphosate was applied at a rate of 25 g/ha.  Objective: This initial application aimed to target early Orobanche growth, preventing it from establishing a significant presence in the crop. At 30 DAS, Toria plants are in the early vegetative stage, which is critical for preventing Orobanche attachment and minimizing parasitic competition for resources.

Second Application of Glyphosate

 Timing: The second application was scheduled between 50 and 55 DAS, during the active growth stage of the Toria crop.  Dosage: Glyphosate was applied at an increased rate of 50 g/ha.  Objective: The purpose of the second application was to suppress any Orobanche shoots that may have survived or emerged after the first application. At this stage, Toria plants are actively growing and resilient, with sufficient soil moisture, which minimizes stress on the plants and supports herbicide efficacy.

Results and Discussion

Based on the data presented in Table 1, glyphosate application was shown to reduce Orobanche infestation by up to 90%, while increasing Toria crop yield by 50-60%. These results align with findings by Punia et al. (2018) [1] , who reported that sequential, low-dose glyphosate applications at specific intervals provided effective Orobanche control and significantly enhanced mustard seed production. Punia and colleagues attributed this tolerance in mustard plants to glyphosate's degradation into non-toxic metabolites. Glyphosate, when absorbed by mustard foliage, is translocated to the roots, where it inhibits EPSP synthase synthesis. This inhibition disrupts the production of aromatic amino acids, effectively preventing Orobanche parasitization of host plants. This mechanism allows glyphosate to target Orobanche without harming the mustard crop, demonstrating its potential as an effective treatment strategy.

Conclusion

Based on the above trails conducted in farmer's field, it can be concluded that foliar application of glyphosate at 25 g/ha at 30 DAS followed by 50 g/ha at 55 DAS would be very helpful in reducing the Orobanche infestation by reducing the further increase in weed seed bank without any crop suppression and also residual effect on any succeeding crop. The results of our present study very convincingly show that glyphosate, if used as recommended, may prove to be very effective in significantly reducing parasitic weed Orobanche infestation, and increasing yield of toria grown in weed-infested fields. However, the dosage of glyphosate should be kept in concern as it is required in trace amounts.

References

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