Hot Water Treatment Enhances the Bioherbicidal Efficacy of a Fungus

Heat is a valuable entity that has served a variety of needs throughout human history, but only relatively recently has heat been used for weed control, where it may serve as an alternative to chemical herbicides. Much of the research involved with heat to control weeds has been via four general methods: controlled burning, flaming of weeds, hot water treatment and steam application.

Synthetic herbicides have been a tremendous asset to weed control in agriculture; however, weed control problems still exist, and many weeds have developed resistance to herbicides. An alternative to synthetic herbicides for weed control is the concept of bioherbicides. Bioherbicides are microbes and/or microbial phytotoxins used to control weeds. 

In this paper, the authors hypothesized that hot water treatment, followed by a bioherbicide application, might promote the efficacy a given pathogen for control of certain weeds. And the authors chose to examine the effects of hot water applications and CT (applied or formulated with corn oil emulsions and Silwet L-77) under greenhouse and field conditions for the control of hemp sesbania, an important weed in row crops in the southern U.S. In all greenhouse experiments, treatments were arranged in a randomized block design with four replicates (48 plants per replicate) and the experiments were repeated over time. In the field experiments, all treatments were replicated 4 times and the experiment was repeated in successive years.

The results indicated that hemp sesbania (Sesbania exaltata) plants (>30 cm tall) sprayed with hot water (45°C – 95°C), followed by spray applications of fungal spores of Colletotrichum truncatum (CT) at 1.0 × 10⁷ spores/ml^-1 and 22°C – 25°C, suspended either in: 0.2% Silwet L-77 surfactant (SW); unrefined corn oil (CO)/distilled water (1:1, v:v); or 0.2% SW in CO, were controlled by 80% - 95%, 12 days after treatment (DAT) under greenhouse conditions. These treatments also reduced dry weight accumulation of this weed. Plants treated with hot water without CT were also injured at temperatures ≥35°C (5% mortality), and 60% mortality at 95°C. Artificial dew treatments (25°C, 12 h), imposed on plants after the treatment protocols above, had little or no effect on weed mortality or dry weight reduction compared to treated plants without dew. Under field conditions, 85% control of hemp sesbania was achieved 12 - 15 DAT when a pre-treatment with hot water (65°C) was followed immediately with a CT application at the spore concentration as described above. Plants in field tests treated with CT without a hot water treatment were visually unaffected, with no mortality or plant biomass reductions recorded 15 DAT.

In conclusion, these results suggest that use of hot water may be an important tool for improving the infectivity and bioherbicidal potential of some plant pathogens. Future research will be needed to transform this concept into an economical and efficient technology for weed control.

Article by C. Douglas Boyette, et al, from USA.

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