Canola Growth Stage at Time of Infection Determines Magnitude of White Leaf Spot (Neopseudocercosporella capsellae) Impact

Abstract

White leaf spot (Neopseudocercosporella capsellae) is a persistent and increasingly important foliar disease for canola (Brassica napus) across southern Australia. To define the role of plant growth stage on development of the disease epidemic, we first investigated the response of different canola cultivars (Scoop and Charlton) at five Sylvester-Bradley growth stages against N. capsellae. White leaf spot disease incidence and severity was dependent upon plant growth stage and cultivar (both P < 0.001), with plants being most susceptible at plant growth stage 1,00 (cotyledon stage) followed by plant growth stage 1,04 (4th leaf stage). Then, second, to quantify the impact of this disease on canola yield, we investigated the in-field relationship of white leaf spot disease incidence and severity with seed yield loss following artificial inoculation commencing at growth stage 1.04 (4th leaf stage). White leaf spot significantly (P < 0.001) reduced seed yield by 24% in N. capsellae inoculated field plots compared with non-inoculated field plots. We believe that this is the first time that serious seed yield losses from this disease have been quantified in-field. The current study demonstrates that N. capsellae disease incidence and severity on canola is determined by host growth stage at which pathogen infestation occurs. Emerging seedling cotyledons were highly susceptible, followed by less susceptibility in first true leaves to emerge but then increasing susceptibility as plants subsequently age towards the 4th leaf stage. This explains field observances where white leaf spot readily establishes on emerging seedlings and subsequently becomes more prevalent and severe as plants age.

Temperature and relative humidity shape white leaf spot (Neopseudocercosporella capsellae) epidemic development in rapeseed (Brassica napus)

“…White leaf spot, caused by Neopseudocercosporella capsellae (teleomorph Mycosphaerella capsellae ), has been documented on a wide range of cultivated and wild crucifers (Boerema & Verhoeven, 1980; Gudelj et al, 2004), including oilseed, vegetable, and forage brassicas (Amelung & Daebeler, 1988; Crossan, 1954; Deighton, 1973; Gunasinghe et al, 2020; Koike et al, 2007; Petrie & Vanterpool, 1978; Sumner et al, 1978). Where severe, rapeseed ( Brassica napus ) yield losses can range from 15% to 24% (Barbetti & Khangura, 2000a; Khangura et al, 2014; Murtza et al, 2021). Similarly, in France in the mid‐1980s, white leaf spot was widespread on foliage and decreased yields of rapeseed by up to 15% when subsequent pod infection occurred (Penaud, 1987).…”

“…In a previous study, N. capsellae isolates collected in 2016 from Victoria, Australia, caused more severe white leaf spot symptoms on all cultivars tested than isolates collected from Western Australia and South Australia (Murtza et al, 2019); this again demonstrated the association of both geographic location and environmental conditions with differences in pathogen virulence. Recently, Murtza et al (2021) highlighted how white leaf spot disease incidence and severity, at constant 15 ℃, was affected by plant growth stage and rapeseed cultivar, with plants being most susceptible at the cotyledon stage, followed by the fourth true leaf stage.…”

“…Recently, Murtza et al (2019) demonstrated that disease severity caused by different isolates of N. capsellae inoculated onto three different B. napus cultivars varied with the geographic location (with different climatic conditions) from which the isolates were obtained. In addition, Murtza et al (2021) demonstrated the influence of both plant growth stage and cultivar on development of white leaf spot at a constant 15 ℃, with plants most susceptible at the cotyledon stage, followed by the fourth true leaf stage of growth. Following from the above findings, we undertook studies to determine the effects of the two temperatures (17/14 ℃ and 14/11 ℃ day/night) and different durations of high humidity postinoculation (0, 48, 96, and 144 hr) on disease progression of white leaf spot caused by N. capsellae on B. napus (cultivars Charlton and Scoop) at two different plant growth stages (Sylvester‐Bradley growth stage 1,00, the cotyledon stage, and 1,04, the fourth true leaf stage; Sylvester‐Bradley & Makepeace, 1984).…”

“…In a previous study of N . capsellae , Murtza et al (2021) used a high humidity duration of 72 hr. Therefore, for our study, we selected a range around this duration to observe the effect of moisture from no humidity (0 hr) to the highest duration of humidity (144 hr).…”

“…capsellae isolates collected in 2005 in both field and controlled environment conditions (Gunasinghe et al, 2014), but has shown differences in the expression of resistance in different trials against N . capsellae (e.g., Gunasinghe et al, 2016c; Murtza et al, 2021). The Sylvester‐Bradley growth stages 1,00 and 1,04 were chosen as they had earlier been found to be susceptible growth stages against a mixture of N .…”