VARIATION IN THE PATHOGENICITY OF PYTHIUM SPECIES ON CARROTS
F ROUXEL1 and D BRETON2
1 INRA, Station de Pathologie Vegetale, BP 29, 35653 Le Rheu cedex, France; 2SILEBAN-INRA, Gatteville-Phare, 50760 Barfleur, France
Background and objectives
During the last 20 ;years, many papers have reported on the involvement of Pythium spp. in carrot root disorders, often compromising seriously yield and quality. The major disease caused by this soilbome pathogen is undeniably cavity spot, considering its wide distribution and high economical incidence. However, a number of other less well known occurrences in the crop are also attributed to Pythium spp., some of them remaining a matter of speculation about symptoms, aetiology and conditions of occurrence. The aim of this paper is to synthesize the actual knowledge on the main carrot root diseases associated with Pythium spp., and to propose molecular criteria to identify the main species involved.
Results and conclusions
Despite extensive work devoted to it, root dieback, also named rusty root or browning, remains confusing because the diversity of symptom evolutions: in severe cases, the seedling may die, but after lower attacks on root apex, the taproot may fail to elongate and instead result in stubbing, or may proliferate in hairy roots to form forking, unsaleable mature roots . The Pythium species related to this disorder are P. ultimum, P. sulcatum, P. violae, P. irregulans, P. sylvaticum and P. aphanidermatum. Pythium spp. and cavity spot
Typical symptoms of this disease are characterized by the presence of small sunken, translucid, sometimes brownish-black spots, appearing single or numerous on the different parts of mature roots. In cases of early infection, epidermal tissues inside the cavity may crack, resulting in splitting which can be particularly destructive. The biological origin of this world-wide disorder has only recently been established and the list of the Pythium species involved is still probably incomplete. Surprisingly, according to the more recent literature , the main causal species are P. violae and P. sulcatum in Europe, P.violae and P. ultimum in North America, P. sulcatum and P. coloratum in Australia. This diversity in causal species, linked to slight differences in symptoms (lesions more or less large, brown, water-soaked), continues to call into question the exact definition of the disease.
Speck disease is a new root disorder that seems to develop specifically in the polder soils of Mont Saint Michel area in France. Symptoms, which can be seen on all parts of mature roots, are small, brown, often very numerous spots. Ultimately, the epidermis takes on a dried, corky appearance, sometimes accompanied by micro-cracks, making some plots unharvestable. Recent work  has shown that two main Pythium species, P. sulcatum and P. sylvaticum, interact with specific polder soil characteristics (fine silts, pH 8.5) to establish this disease.
Molecular criteria were used to identify the main Pythium species involved in carrot diseases. The data presented were achieved after the ribosomal DNA internal transcribed spacer (ITS) regions were amplified by the PCR and digested with restriction enzymes. They show that the Pythium species involved in the different disorder types can be distinguished by RFLP analysis.
In conclusion, it appears that some Pythium species (P. sulcatum) are able to induce all types of disease symptoms on carrot, and similarly, a similar disease (cavity spot) may result from different Pythium species. These observations raise questions about the pathogenicity of the Pythium spp. (strains, inoculum potential) and its relationships with extrinsic factors (soil type, climate, plant physiology, carrot variety) as regulators of the disease expression.
1. Villeneuve F, Bosc JP, Breton D, Rouxel F, 1997. Journal of Applied Genetics 38A, 71-80.
2. Campion C, Massiot P, Rouxel F, 1997. European Journal of Plant Pathology 103, 725-735.
3. Breton D, Bosc JP, Letouzd P, Villeneuve F, Rouxel F, 1997. Annals Fifth CIMA, Tours, pp.585-592.