Application timing of herbicides, glyphosate and atrazine, sway respective epidemics of foliar pathogens in herbicide‐tolerant rapeseed

Abstract

Rapeseed (Brassica napus) production in Australia relies heavily on triazine‐or glyphosate‐tolerant cultivars. For 14 triazine‐tolerant cultivars, disease development of Neopseudocercosporella capsellae (white leaf spot), Alternaria brassicae and A. japonica (Alternaria leaf spot), and Hyaloperonospora brassicae (downy mildew) were all dependent upon herbicide application timing (p < 0.001), with significant differences between cultivars (p < 0.001) and a significant interaction (p < 0.001) between herbicide application timing and cultivars. Atrazine applied preinfection by N. capsellae, A. brassicae, or A. japonica enhanced disease incidence, severity, and leaf collapse, while atrazine application postinfection for these same pathogens reduced all three disease parameters. However, for H. brassicae, application of atrazine after, and especially prior to, infection resulted in lower disease incidence, severity, and leaf collapse. Application of glyphosate on five glyphosate‐tolerant cultivars for N. capsellae resulted in significant differences (p < 0.05) between glyphosate application treatments, and between host cultivars in terms of incidence and consequent leaf collapse. For A. brassicae, A. japonica, and H. brassicae, glyphosate resulted in significant differences (p < 0.001) across application timings between cultivars, and a significant interaction (p < 0.001) between herbicide application timings and cultivars. Glyphosate applied on glyphosate‐resistant rapeseed after, and especially prior to, attack by H. brassicae, reduced downy mildew. These are the first studies to highlight how the timing of application of triazine or glyphosate in relation to pathogen infection is critical to the susceptibility of rapeseed to white leaf spot, Alternaria leaf spot, and downy mildew. This new understanding offers fresh possibilities for improved management of these diseases in herbicide‐tolerant rapeseed crops.

The ilv2 gene, encoding acetolactate synthase for branched chain amino acid biosynthesis, is required for plant pathogenicity by Leptosphaeria maculans

“…The timing of herbicide application may factor into susceptibility to disease. Murtza et al [ 24 ] tested the impacts of the herbicides glyphosate and atrazine on canola susceptibility to the foliar fungal diseases Neopseudocercosporella capsellae , Alternaria brassicae, A. japonica , and Hyaloperonospora brassicae. They found that the timing of the herbicide application impacted the fungal pathogen disease severity and incidence.…”

Molecular Targets of Herbicides and Fungicides─Are There Useful Overlaps for Fungicide Discovery?

“…Likewise, glyphosate reduced infection of GR soybean with Asian soybean rust ( Phakopspora pachyrhizi ), ,, GR alfalfa with alfalfa rust ( Uromyces striatus ), and GR cotton with Rhizoctonia solani . Results were more variable with GR rapeseed and three diseases, two fungal and one oomycete: white leaf spot ( Neopseudocercosporella capsellae ), Alternaria leaf spot ( Alternaria brassicae and Alternaria japonica ), and downy mildew ( Hyaloperonospora brassicae ) . Einhardt et al insinuated that nickel ion and glyphosate act synergistically in reducing Asian soybean rust in GR soybean, but a more than additive effect was not proven .…”

“…69 Results were more variable with GR rapeseed and three diseases, two fungal and one oomycete: white leaf spot (Neopseudocercosporella capsellae), Alternaria leaf spot (Alternaria brassicae and Alternaria japonica), and downy mildew (Hyaloperonospora brassicae). 70 Einhardt et al insinuated that nickel ion and glyphosate act synergistically in reducing Asian soybean rust in GR soybean, but a more than additive effect was not proven. 71 Glyphosate applied at the V3 or V6 growth stages of GR soybean suppressed charcoal rot (Macrophomina phaseolina) in a tilled site and had no effect in a no-till site.…”