A Nagging Issue

The persistent problem of bacterial ring rot

Published in the March 2014 Issue Published online: Mar 16, 2014 Alan Westra and Phil Nolte
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Each year, Idaho produces in excess of 30,000 acres of certified seed potatoes. The well-earned reputation for quality that this crop enjoys is the result of stringent production practices on the part of Idaho seed producers and a rigorous inspection program conducted by the Idaho Crop Improvement Association. Recent discoveries of bacterial ring rot (BRR) in commercial potato fields have, understandably, raised concerns about the health of the state’s seed potato supply and have prompted the questions “Why is this happening?” and “What can be done about it?”

In order to answer those questions, some review of the disease and recommended management practices is warranted. BRR was first discovered in North America in 1931 and, at the time, threatened the viability of the entire potato industry. Subsequent research and understanding of the disease led to the adoption of “zero tolerance” policies by seed certification agencies in the 1940s. These policies remain in place to this day and are extremely stringent. Seed certification rules, for example, require the rejection of any seed lot in which BRR is found as well as a flushing out of the remaining seed lots on the farm; the seed producer is required to start over again with new seed stocks. The adoption of zero tolerance for BRR in seed certification has had a significant impact on the overall incidence of this disease. In contrast to the conditions that existed prior to adoption of zero tolerance, the normal situation now is for minimal or no loss in the commercial potato crop due to BRR. However, BRR has not been completely eradicated, and there are periodic flare-ups of the disease that can and do have significant economic impacts.

BRR persists in the potato industry largely because of the following characteristics of the causal organism, Clavibacter michiganensis subp. sepedonicus (CMS):

• CMS multiplies only in infected potatoes. It can, however, survive for long periods of time outside of the host in potato debris and in dried bacterial slime on potato handling and storage equipment.

• Very small amounts of CMS are required to infect potatoes.

• CMS can be present in infected plants in small amounts. It is entirely possible that the bacterial population in an infected seed lot may be below the detection limits of current technologies regardless of the sample size. It may take several generations after infection for populations to rise to detectable limits and for disease symptoms to be expressed.

• Symptom expression by infected plants is not consistent and is not necessarily related to the initial inoculum dose or the final population of CMS. Plants of susceptible varieties with high bacterial populations may show few or no foliar or tuber symptoms. Some potato varieties may be tolerant of CMS and rarely or never show symptoms.

•Foliar symptoms tend to be expressed later in the growing season and may be masked by other diseases and/or the natural senescence of the potato vines. Similarly, tuber symptom expression may be masked or obliterated by other, secondary invaders such as the soft rot bacterium.

•CMS enters healthy potatoes through wounds and is easily spread. Seed cutting is the single most efficient way of spreading this disease – a single infected tuber in a seed cutting operation may infect tens or hundreds of healthy tubers.

These characteristics of the pathogen help to explain why the zero tolerance strategy has not eradicated BRR. They also suggest some best management practices that are applicable to the production of both seed and commercial potatoes:

•Buy certified seed potatoes from a known and trusted source. Avoid saving seed, or have it inspected and tested. Commercial growers should request a copy of the North American Plant Health Certificate for seed lots they intend to purchase, as this documents the seed farm’s history for BRR.

•Consider testing seed lots for BRR. There are a number of labs, including Idaho Crop Improvement, with the capability of testing for CMS.

•A standard recommendation is to avoid cutting seed if possible. While not practical in most circumstances, the planting of whole seed will significantly reduce the risk of spreading BRR.

•Good record-keeping and the isolation of seed lots will assist in tracing a problem and in eliminating its recurrence.

•Practice good sanitation:

•Thoroughly clean and disinfect all equipment and storage areas prior to receiving and handling seed. Pay particular attention to the seed cutter.

•Treat each seed lot as suspect. Clean and disinfect cutting, handling and planting equipment between seed lots to prevent potential contamination of healthy seed.

•Limit access to storages and equipment to authorized personnel only. Trucks should be cleaned and disinfected prior to entering potato storage or handling areas. Avoid sharing equipment. Avoid communal or commercial seed cutting operations that do not practice good sanitation.

•If BRR is confirmed on a farm, flush out all potatoes and perform a thorough cleanup of storages and equipment to reduce the chance of contamination of new seed.

•Review and, if necessary, improve management of other storage diseases. The major impact of BRR in storage is that it provides an infection court for other diseases such as soft rot.

Traditionally, BRR has been viewed as a seed-only issue. Certainly, the impact of BRR has been mitigated through good seed production practices and seed certification. Additionally, the application of new seed testing technologies to the seed production and certification process may allow for further alleviation of the problem. The continued persistence of BRR suggests, however, that it may be more appropriate to view this disease as an industry-wide problem and that its eradication may ultimately require an industry-wide effort. Control of BRR currently is dependent upon the identification and elimination of weak points in seed production. Extending this concept to the entire production system in a manner that allows for the identification of sources of CMS could go a long way to further reducing the incidence and impact of BRR. Above all, if BRR is to be eventually eradicated, the proper management practices, i.e., sanitation, should be implemented at all levels of production.