Ring rot

Causal agent(s) and transmission

The bacterium Clavibacter sepedonicus (formerly named michiganensis subsp. sepedonicus and Corynebacterium sepedonicum) is the causal agent for bacterial wilt in growing crops and for ring rot on tubers, from the Microbacteriaceae family. This is Gram-positive, non motile bacterium, its thermal optimum is between 20 and 23°C.

This bacterium is typical of areas with a cool climate: Scandinavia, North Europe, North Asia, North America and temperate zones in South America. However, it has been discovered more recently in countries with a warmer climate such as Greece and Spain. The natural host of the bacterium is the potato, although it can also be transmitted to the aubergine or the tomato by artificial contamination.

The bacterium penetrates the plant’s vascular system through wounds. It multiplies actively in the plant and the accumulation of the bacteria and the products of degradation of the plant tissues cause occlusion of the xylem vessels. Then the circulation of the sap is disturbed and consequently the plant wilts.

Ring rot is a quarantine disease of the potato that is listed in the European Plant Health Directive and which may be very destructive to the potato crop (wilting, yield losses up to 50%, tuber rot) when cutting seed tubers at the time of planting. As the bacterium is highly contagious and persistent in plant and soil debris, this disease is submitted to strict regulation to avoid its dissemination by plant transport.

La bactérie pénètre dans le système vasculaire (xylème) de l’hôte par des blessures naturelles ou artificielles et va ensuite se propager dans la tige, les feuilles, les racines et les stolons de la plante. La multiplication des bactéries ainsi que l’accumulation des produits de dégradation des tissus végétaux provoquent l’occlusion des vaisseaux du xylème et perturbent la circulation de la sève, provoquant alors le flétrissement de la plante.

L’hôte naturel de cette bactérie est la pomme de terre bien que des contaminations naturelles sur plantes de tomates aient été observées en Belgique. Les espèces Beta vulgaris (betterave) et Solanum sarrachoides (morelle sp.) ont été décrites comme hôtes naturels de C. sepedonicus mais sans que ces observations aient été confirmées. Clavibacter sepedonicus est toutefois capable de provoquer des symptômes typiques sur tomate, aubergine et Solanum rostratum et de se maintenir sur des cultures de maïs, haricot, pois et colza ainsi que sur diverses adventices (Poa annua, Elymus repens, Taraxacum officinale, Urtica dioica et Veronica chameaedrys) après inoculation artificielle ; aucune contamination naturelle de ces espèces par la bactérie n’a été observée.

Les tubercules infectés constituent une des principales sources d’inoculum. Ils peuvent être porteurs d’infections latentes sur plusieurs générations avant que le taux d’inoculum ne devienne suffisamment important pour provoquer les symptômes caractéristiques de la maladie.

Les blessures provoquées lors de la manipulation des tubercules, les contacts entre tubercules contaminés et tubercules sains, l’utilisation d’équipement, de matériel agricole ou de locaux de stockage contaminés constituent également des voies de contamination.

Certains insectes comme le doryphore (Leptinotarsa decemlineata) et le puceron vert du pêcher (Myzus persicae) peuvent être vecteurs de la bactérie.

Découverte en premier lieu dans le nord de l’Europe, Clavibacter sepedonicus a été signalée dans plusieurs pays d’Europe, en Amérique du Nord et en Asie.

C’est une bactérie présente majoritairement dans les régions tempérées froides de l’hémisphère nord. Des signalements ont également été faits dans des pays à climat plus chaud comme la Grèce, le Mexique, le Pakistan et Taiwan mais de façon sporadique.

La bactérie peut causer de graves dommages aux cultures de pomme de terre et être responsable de pertes de rendements importantes (jusqu’à 50 %), même en l’absence de symptômes.

Le Règlement Santé des Végétaux 2016/2031 de l’Union Européenne classe la bactérie Clavibacter sepedonicus comme organisme de quarantaine pour laquelle aucune tolérance n’est admise et qui fait l’objet de contrôles importants en production et à l’importation. Des mesures sanitaires sont appliquées rapidement en cas de découverte de foyers, telles que la destruction des cultures et/ou des tubercules contaminés ainsi que la désinfection du matériel agricole et de stockage.

Symptoms on foliage

The symptoms are not always very apparent during the growing season. Indeed, they appear quite late in the season, often at the same time as the senescence of the plants.

The first symptoms are characterised by wilting of the basal leaves, which curl up and turn yellow. The symptoms then spread rapidly to the apex (photos 1 and 2).

The wilting is not always accompanied by the discolouration of all the leaves. Sometimes, only discolouration located between the veins can be observed.

Some cultivars develop forms of dwarfism or internodes with malformed terminal leaves.

Symptoms on tubers

The expression of the disease on tubers is characterised by a yellowish to brown colouration of the vascular ring often starting from the stolon-end. If pressure is applied to the tuber, the vascular ring may exude a cream-coloured ooze formed by the bacteria and the decomposed plant cells (photo 3).

In an extreme phase the tuber is cracked on the surface and dark marks appear under the periderm. It then decomposes without any particular smell and a cavity may form inside (photo 4).

Risk factors

Environmental conditions play an important part in the appearance of the disease. A soil temperature of around 25°C will encourage wilting, whereas very few symptoms will be seen at 16°C.

Survival of the bacterium in a bare soil appears to be low, while haulms and tubers left in the field provide habitats that enable the bacterium to survive for several years in the fields. Cool and moist soils encourage the survival of this bacterium.

Infected tubers are one of the main inoculum sources, as they can carry latent infections for several generations before the amount of bacteria becomes sufficient to produce the characteristic symptoms of the disease (wilt and ring rot). The bacterium may also be disseminated mechanically, through contact of healthy tubers with infected tubers or surfaces having been in contact with the infected tubers (farm equipment, storage facilities, etc.).

Some insects, such as the Colorado potato beetle (Leptinotarsa decemlineata) and the green peach aphid (Myzus persicae), can also carry this bacterium.

Control

Clavibacter michiganensis sepedonicus is classified as a quarantine parasite (zero tolerance) in most countries, including the European Union, and is therefore subject to mandatory control measures to avoid its dissemination and to eradicate the disease when and where an outbreak is notified.

Integrated pest management and prophylactic measures are the most effective ways to reduce the consequences linked to the presence of the disease and to prevent the introduction of the bacteria into disease free areas:

• the use of certified seed potatoes inspected in the field and tested in the laboratory is still the best way to avoid the spread of the disease;
• the cultivation in a healthy environment: a bacteria-free soil, the effective elimination of weeds and groundkeepers, and drying;
• the use of clean and disinfected agricultural equipment for both cultivation and storage.