Biofumigation for control of Globodera pallida and Rhizoctania solani

Potato cyst nematodes, Globodera pallida and G. rostochiensis are a major problem for British potato growers, They are currently estimated to cost over £35 million y-l in lost yield and nematicides. Due to environmental concerns, some of the most widely used nematicides in the 1990s have now been withdrawn from use within the European Union and more environmentally benign control measures are sought. The soil-borne fungal pathogen Rhizoctonia solani is another constraint on potato production for which current control measures are not entirely satisfactory. Biofumigation, the incorporation of brassica green manures into soil to control soil-borne pests and pathogens, has the potential to control various organisms in a number of cropping systems. The mechanism most commonly implicated in pest or pathogen suppression by brassica green mulches is the production of volatile secondary metabolites called isothiocyanates, which are produced upon enzymatic hydrolysis of parent glucosinolates. In some cases, suppression through biological mechanisms or by volatile toxins not derived from glucosinolates has also been observed. The principal aims of this project were to evaluate the potential of biofumigation to control G. pallida and R. solani and to determine the mechanisms involved in any suppression. An in vitro toxicity assay was developed and used to screen five isothiocyanates and aqueous leaf extracts of 22 brassica lines for toxicity to G. pallida second- stage juveniles (J2). G. pallida J2 were highly serisitive to isothiocyanates and to extracts of several of the brassica lines tested. The toxicity of benzyl, 2-phenylethyl, 2-propenyl and 3-(methylthio)propyl isothiocyanate was similar, with ED80 values ranging from 11 to 18 μM, while 2-methylbutyl isothiocyanate was less toxic. The plants causing greatest suppression in vitro were Raphanus sativus, Nasturtium officinale and Brassica juncea. The fungistatic activity of the isothiocyanates toward R. so/ani was determined in vitro. The rank order of toxicity of the isothiocyanates to the two organisms was similar but the fungus was more tolerant than the nematode. A method was developed for quantifying viable potato cyst nematode eggs by measuring the abundance of actin mRNA and selected plants were tested for activity toward G. pallida eggs and R. solani mycelium in soil in glasshouse trials. Glucosinolate profiles of the brassicas incorporated into soil were determined by HPLC. Brassica green manures had no consistent effect on the density of R. solani inoculum or on resulting disease incidence on bioassay potato plants. In contrast, several brassica green manures caused substantial mortality to G. pallida eggs, of which three B. juncea lines (Nemfix, Fumus and ISCI99) containing high concentrations of 2-propenyl glucosinolate were the most effective. These plants caused over 95 % mortality in polyethylene- covered soil and over 80 % mortality in uncovered soil. Toxicity in soil correlated with the concentration of isothiocyanate-producing glucosinolate but not total glucosinolate. The biosafety of the approach was examined by characterising the effects of biofumigation and conventional fumigation on the free-living soil nematode community. Both treatments had strong and lasting effects on free-living nematodes, reducing the abundance of sensitive taxa by almost 100 %. This study has shown that biofumigation with particular brassica varieties can potentially achieve levels of control that would make it commercially viable as part of integrated management of potato cyst nematodes.