Abstract
A sand solution technique demonstrated the capacity for a commercial seaweed extract from Durvillaea potatorum and Ascophyllum nodosum (Seasol Commercial®) to significantly suppress infection of broccoli by Plasmodiophora brassicae. In the primary stages of infection, the extract reduced the number of plasmodia formed in the root hairs by 55 %. Later, in the secondary stages of infection, the extract reduced plasmodia in the root cortical cells by up to 84 %. The suppression of infection was found to be independent of the dilution of the extract applied (1:25 and 1:500). The basis for these results is unlikely to be a nutrient or pH effect since the extract had little impact on these parameters, particularly at the lower dilution (1:200). Rather, we hypothesise that the suppression of infection by the seaweed extract was due to its stimulation of resistance mechanisms in the host, which is possibly related to laminarins in the extract and/or the effect of exogenous growth regulators or undiscovered molecules in the extract disrupting the infection process.
Highlights
Clubroot caused by the obligate soil-borne parasite Plasmodiophora brassicae is considered the most important soil-borne disease of brassica crops, such as broccoli, cabbage, canola and others (Dixon 2014; Donald and Porter 2009; Donald and Porter 2014)
The pathology of clubroot infection consists of several phases—primary phases that occur in the root hair and secondary phases that occur in the cortex
In the primary phases of infection, treatment with the seaweed extract reduced the number of plasmodia formed by P. brassicae in broccoli root hairs by up to 55 % compared with the control (Table 2)
Summary
Clubroot caused by the obligate soil-borne parasite Plasmodiophora brassicae is considered the most important soil-borne disease of brassica crops, such as broccoli, cabbage, canola and others (Dixon 2014; Donald and Porter 2009; Donald and Porter 2014). Changes in auxin and cytokinin metabolism are thought to be key mechanisms in the development and formation of clubroot galls (Devos et al 2005; Siemens et al 2006). Concentrations of indole-3-acetic acid (IAA) or conjugated IAA in infected roots alternate from higher to lower levels (Kavanagh and Williams 1981; Ludwig-Müller et al 1993; Devos et al 2005), with the transition possibly correlating with the change from primary to secondary phases of infection (Devos et al 2005). Devos et al (2005) found that infection reduced the total active cytokinin content of root tissue and hypothesised that plasmodia of P. brassicae act as a sink for zeatin in the primary infection phases. Despite the potential role of auxins and cytokinins in the formation of clubroot
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