IN VITRO STUDIES OF SIX SPECIES OF PRATYLENCHUS (NEMATODA: ) ON FOUR CULTIVARS OF OILSEED RAPE (BRASSICA NAPUS VAR. OLEIFERA)

BY

ROBIN M. WEBB1) Department of Entomology and Nematology, AFRC Institute of Arable Crops Research, Rothamsted Experimental Station, Harpenden, Herts, AL5 2JQ U.K.

The roots of single and double low cultivars of oilseed rape were attacked by six species of Pratylenchusin in vitro culture. The degree of damage was dependent on the individual species and rape cultivar concerned and was independent of crucic acid or glucosinolate content of the rapeseed. The effects caused by each species of Pratylenchuswere not consistent, except that P. penetransdecreased root growth of all cultivars. Although it is unclear whether it can complete the life cycle outside the root, Pratylenchuscommonly feeds ectoparasitically on rape roots in vitro.The rape cultivars tested were tolerant of some Pratylenchusspecies and some may be resistant. Keywords:Pralylenchus, oilseed rape, Brassicanapus.

Oilseed rape (Brassica napus var oleifera) has become widely grown in the UK and Europe in the last 25 years but the status of the crop as a host for several nematode species and their potential economic importance are still uncertain. Root lesion (Pratylenchus spp.) are known pathogens of several crops under UK conditions including barley (Corbett, 1970), (Corbett et al., 1969) and field beans (McEwen et al., 1981). They are present in all soil types but are most numerous and damaging in light sandy soils. A significant propor- tion of the rape crop is grown on soils of this type in the UK. The importance of root lesion nematodes as pests of oilseed rape is not clear (Talatschian, 1974; Webb, 1978; Scotto La Massese, Cayrol et al., 1981; Scotto La Massese, Minot & Voisin, 1981; Evans & Spaull, 1984). These nematodes may damage roots of oilseed rape in vitro (Evans & Webb, 1989; Webb, 1990) and produce symptoms similar to those found in rape associated with large numbers of Pratylenchus in the field. This study investigates further the effects produced by six species of Pratylenchus, commonly found in UK arable soils, on the roots of oilseed rape cultivars which have a low level of erucic acid in their seeds (single low) or low levels of erucic acid and glucosinolate in their seeds (double low) in monoxenic in vitro culture and discusses the occurrence of tolerance and resistance in these cultivars. MATERIALS AND METHODS

Using the methods of Evans & Webb (1989) and 9 cm diam Petri dishes,

1) It is regretted that Robin Webb died on 10 July 1993. 90 excised root cultures of oilseed rape cultivars Bienvenu (single low), Ariana, Cobra and Libravo (double lows) and of a known good host, cv. Fronica, were prepared. Cultures of each cultivar were inoculated with about 500 individuals of each Pratylenchus species (P. neglectus, P. crenatus, P. thornei, P. pinguicaudatus, P. penetrans or P. fallax); every nematode and host combination was replicated ten times and uninoculated controls for each cultivar were also included. The inoculum contained both adult and juvenile stages in a standard length of infected maize root (Olowe & Corbett, 1976) and was obtained from single female-derived cultures maintained at Rothamsted. The plates were sealed with PVC adhesive tape and incubated at 22°C in the dark. Each day each plate was examined at x40 magnification and details of nematode attack were noted. Photographs of symptoms and nematodcs were taken using a Zeiss Universal photomicroscopc fitted with a Zeiss MC63 camera. After 13 weeks all plates were photocopied against a white ground, so providing an image of the root system from which measurements of length and number of tap and lateral roots were taken. The nematodes were then extractcd from all plates (Webb, 1990) and counted. RESULTS

All six Pratylenchus species penetrated the epidermis of the standard host plant, maize cv. Fronica. Soon after encountcring a root, nematodes entered the cortical parenchyma and fed endoparasitically. In contrast, the nematodes commonly fed ectoparasitically on all the cultivars of oilseed rape (Fig. lA). Eggs were often found on the agar around these sites but no nematodes were ever observed to moult outside the root. Early symptoms of nematode attack were frequently seen in areas where tracks in the agar indicated that nematodes had been present but had moved away. Such symptoms included a swelling of the root cortex just behind the root tip giving the root an 'Indian club' shape (Fig. IE). When heavy feeding halted the growth of the root tip, a lateral root emerged from the root hair region (Fig. 1 F). This latter observation was common where extensive ectoparasitic feeding had occurred but was not observed in maize. Penetration through the epidermis was observed in all rape cultivars by all species but not as frequently as in maize; in oilseed rape the favoured site of entry was at the point of lateral root emergence, where the epidermis was disrupted. All nematode species fed endoparasitically on the cortical cells of all rape cultivars but the numbers found differed between nematode species and rape cultivars. Nematodes moved longitudinally through the cortex and collected at sites of lateral root emergence (Fig. 1 B). When they collected and fed at the site of an unerupted lateral root bud they caused necrosis of the endodermal layer and throughout the cortex. Such lateral roots were often killed in situ but those that did erupt were often distorted and died soon after emergence (Fig. 1 C). Eggs were laid in the cortex, sometimes in sufficient numbers to spill out into the surrounding medium. Roots with