Potato mop top virus (PMTV), which appears to be increasing in importance in the United States, is a soil-borne virus. Unlike many of the important potato viruses, PMTV is not transmitted by aphids or nematodes. Rather, it is transmitted by infected spores of the powdery scab pathogen, Spongospora subterranea. Once infected Spongospora is present in a field, it remains infectious for many years. PMTV is considered a quarantine pathogen in some countries and is therefore important in national and international trade. The virus is widespread in parts of Europe, especially Scandinavia, and also occurs in both North and South America. In the U.S., PMTV has been officially reported in Maine, North Dakota, Idaho and Washington.
Powdery Scab
Spongospora infections result in lesions on the surface of infected tubers. Most russet varieties are somewhat resistant to this type of tuber infection. However, Spongospora also produces galls on the roots of infected plants and reduces root growth. Both of these types of infections can reduce yields and quality of a potato crop.
Powdery scab lesions can resemble those of common scab. In order to determine which pathogen is present, there is a commercially available test for rapid identification of powdery scab in tuber lesions or root galls. Only a small amount of tissue is necessary to run the test, and the results are obtained in a few minutes.
Mop-Top Virus Infections
Now that we’ve seen the symptoms of powdery scab infections, what about symptoms of PMTV infection? After Spongospora has infected potato roots with mop top virus, several symptoms appear. Sometimes there will be yellow spots, arcs or chevron patterns on the leaves of infected plants. However, these foliar symptoms appear to be rare in North America. Tuber symptoms include various spots, rings and arcs. The most characteristic symptom of PMTV infection is stacked or concentric ring patterns.
The symptoms of PMTV infection can resemble those caused by another soil-borne virus, tobacco rattle virus. Tubers can also be infected by both PMTV and TRV at the same time.
Only certain, specific tests can confirm PMTV infections. The enzyme-linked immunoserbent assay, or ELISA, is based on antibodies specific to the virus. A RT-PCR test can detect the genetic material of the virus. Both methods are widely used for detection of PMTV in tuber samples, and both are quite reliable. The ELISA test requires little in the way of specific equipment and expertise. The RT-PCR test is more suited to the research laboratory.
Field Trials
In 2012 personnel in my laboratory assisted with a field trial being conducted by Chuck Brown. The field trial was designed to identify potato cultivars or breeding materials with resistance to PMTV. More that 8,000 tubers of about 60 lines and cultivars were cut and scored for internal symptoms. The infection level in the test field was quite high and a combined total of nine percent of the tubers had internal symptoms. About 70 percent of the symptomatic tubers tested positive for the virus, showing that a high proportion of the symptoms were indeed caused by PMTV. Also of note is the fact that 22 percent of symptomless tubers from this field also tested positive for the virus.
We learned that there were differences in the percentage of symptomatic tubers produced by the various cultivars. Also of interest is the high percentage of tubers that were infected with PMTV that failed to produce internal symptoms. Many of the same lines evaluated in 2012 were also included in a field trial in 2013 in order to confirm these differences in infection level among cultivars.
We also determined that PMTV can be transmitted from infected tubers to daughter plants. The transmission rate also appears to vary among potato cultivars. This finding has particular implications for the seed industry.
What Should I Do?
What about management options to minimize losses from mop top virus? Since fields containing infected Spongospora remain infectious for long periods, the main thing is to keep the virus out in the first place.
Purchase clean, certified seed. Clean equipment between fields to prevent movement of infested soil to new locations. Since the symptoms of PMTV can be variable, confirmation tests need to be conducted. Long rotations between potato crops can help reduce infection levels but may not be practical in all areas. Some weeds, especially nightshades, can serve as hosts of Spongospora, so good weed control may help reduce infection levels. Remember that there are currently no effective chemical control options for Spongospora. Finally, there are differences in cultivar susceptibility so planting less susceptible cultivars may be an option.