Ring bacteriosis is a bacterial quarantine potato disease. Its occurrence causes serious losses in cultivation, caused by quarantine procedures, pathogen control programs and applicable regulations. For seed potatoes, there is a zero tolerance for ring rot culprits.

Seed potatoes are the main source of the pathogen, although the bacteria can survive from season to season not only in seed tubers, but also in self-seeding, waste heaps or infected plant debris left in the soil. The pathogen spreads to the daughter tubers directly from the seed potato with the use of bacterial mucus contaminated with tools, rooms, means of transport, boxes, bags, cut seed potatoes or remnants of diseased plants left undecomposed in the soil for the second year. Soil and by-hosts are of secondary and questionable importance as spreading factors.

In plants, the disease develops faster in drought, while in humid weather it develops slower. The optimal temperature for the development of the pathogen is 20-23oC, although it has a very wide range of occurrence (from 1-2 to 33oC).

Symptoms of paralysis

The development of disease symptoms usually begins during the flowering period, the first symptoms on plants may appear 60-70 days after planting. The severity of symptoms depends on the temperature and the potato variety. In hot weather all symptoms are visible, while in cooler conditions only some of them are visible or completely invisible. The first symptoms appear as pale yellow lightening of the lower leaves (between the veins), accompanied by a slight wilting of the leaves. Marginal necrosis of curling leaves and chlorosis between the veins are also observed (Photo 1). Leaf wilt progresses from the edges of the leaves and from the bottom of the plant upwards. Only the leaves on one side of the leaf or leaves on one side of the stem or leaves on parts of the plant can be infected (photo 2). Then the whole leaves dry up and the plants wilt. Wilting of the stems occurs only with a very strong infestation. In the advanced stage of the disease, a brown ring is visible on the cross-section of the stem, and cheese-like discharge from the vascular bundles.

Symptoms of the disease on tubers are more characteristic, although it happens that the outer part of even a heavily infested tuber may remain unchanged. In some cases, however, cracking of the tuber surface or bacterial leakage from the eyes is observed. The most characteristic symptoms can be seen in the tuber cross-section. There are discolorations of various lengths in the ring of the conducting bundles, from glassy to cream to yellow (photo 3). When the tubers are heavily infected, the entire flesh resembles a boiled potato (photo 4). Secondary infection of wet rot bacteria (Pectobacterium ssp.) May mask the typical symptoms of ring rot.

Plant infection – both in shoots and tubers – is often latent (asymptomatic), which makes it difficult to detect. The characteristic features of the disease that make its detection difficult are the slow worsening of symptoms. The first symptoms are often visible after the flowering of potatoes, on tubers at the end of storage, internal development of the disease in the plant, in vascular bundles. Plant symptoms are nonspecific, similar to blackleg, brown rot or mushroom wilt, and the use of certain defoliants.

Protection methods

There are no methods of direct chemical or biological control of the pathogen.

Prophylaxis and quarantine remain to be used. Prophylaxis in combating the disease consists in using primarily healthy, certified seed material for planting (also tested for latent forms) and not cutting seed potatoes.

The detection of disease symptoms in the field is tantamount to excluding the field from cultivation and imposing a 3-year quarantine (you should not grow potatoes and tomatoes, you have to fight self-seeding).

If a disease is found, the whole batch of infected tubers should be destined for fodder or industrial processing after steaming, the places in contact with diseased potatoes should be disinfected with substances containing ammonium ions, bleach, chlorine dioxide, iodine compounds and phenol groups.