Commercial potato growers are aware of the serious threat posed by two well- known pathogens, potato mop-top virus (PMTV) and tobacco rattle virus (TRV). According to Huimin Xu, research scientist with the Canadian Food Inspection Agency in Charlottetown and an expert on PMTV and TRV, these pathogens can cause potato tuber necrosis of sensitive potato varieties (the brown coloured streaking is necrosis of the potato flesh and is generally known as spraing).
Symptoms on the tubers are similar to those caused by the potato tuber necrotic strain of potato virus Y, says Xu. “In addition to spraing, TRV and PMTV can cause a wide range of symptoms on potato foliage, including stunting, yellow mottles, chevrons and yellow blotches or rings. The leaf symptoms can be easily confused with those induced by potato aucuba mosaic virus (PAMV, a Potexvirus) and alfalfa mosaic virus (AMV, an Alfamovirus).”
PMTV is not naturally transmissible by aphids or other vectors, but it may be artificially transmitted to some hosts by grafting. In the document entitled Potato Mop-Top Virus, Steven B. Johnson, crops specialist at the University of Maine Cooperative Extension, points out that PMTV is vectored by the same pathogen that causes powdery scab disease in potatoes, Spongospora subterranea f. sp. subterranea, which has affected potato crops throughout the world.
“Soil can be infested with S. subterranea without powdery scab occurring if the environmental conditions required for disease development are not met. Powdery scab can occur without PMTV being transmitted,” states Johnson. “Powdery scab and PMTV are generally associated with cool and wet soils, regions, and seasons. Wet soil conditions are particularly favourable to disease development during early tuber initiation, when potatoes are most susceptible to powdery scab. Precipitation at this stage is a good predictor of PMTV incidence.”
Initial results indicate that resistance to powdery scab and to PMTV development are independent. Johnson further notes that no varieties are immune to powdery scab or PMTV.
Vectors of TRV are plant-parasitic nematodes belonging to the Trichodorus and Paratrichodorus genera. More than 400 species in more than 50 plant families can become infected by TRV, and the virus retains infectivity in numerous hosts.
Seed tubers infected with TRV are the major source of the virus in the field. “For now, growers in most of the major potato-growing areas should not worry too much about TRV,” Xu says. “But growers should remove infected seed potato (necrotic) tubers as soon as it is detected.”
PMTV and TRV Across Canada
Recently, PMTV has been detected in commercial seed potato lots in most Canadian provinces, with the exception of British Columbia and Saskatchewan, according to Xu. “The incidence was 4.3 per cent, according to a PMTV survey we did a few years ago,” he says.
Seed potato tubers are a major source of infection in the field and are also responsible for the movement of the virus from region to region. The soil-borne fungus of potato powdery scab can spread PMTV only a limited distance in the field, but it is important to note that this type of fungus can stay in soils in the form of “fungal stores” for many years.
PMTV has a self-eliminating tendency in the field due to its poor distribution and limited mobility in the infected plants. Nevertheless, CFIA seed potato regulation programs require that potato nuclear stock must be screened for PMTV. According to Xu, the CFIA keeps a close eye on the fungal vector of PMTV and at the first signs of the disease becoming more widespread will be aware of the problem.
Meanwhile, Xu says that powdery scab is not widespread in Canada. “Growers need not worry too much about this virus for now. But growers should surely remove and destroy infected potato plants and seed tubers right away. Usually the symptoms of PMTV on potato leaves and tubers can be identified fairly easily.”
Although TRV has not yet become established across Canada, the virus was recently detected in commercial potato (not seed) lots in the Fraser Valley in British Columbia and also in private gardens in Alberta, according to Xu. “It seems that to date this virus [has not affected] Canadian seed potatoes,” he notes. “This virus is transmitted by several species of soil-borne nematodes that can survive in the soil for a long time—several years—and it will be very difficult to eradicate this virus if it is introduced to a region.”
Managing the Problem
Tuber necrosis is relatively easy to spot and to identify. Growers are encouraged to submit tubers that show necrotic symptoms to CFIA inspectors for further identification and testing.
Managing PMTV and TRV by way of controlling vectors in the soil or crop rotations is extremely difficult, according to Xu. The natural host range of PMTV is very narrow while the host range of TRV is extremely wide.
Efficient management of these viruses mainly relies on the use of virus-free seed tubers and resistant varieties. According to Xu, enzyme-linked immunosorbent assay tests and bioassays, which are currently used for the diagnosis of these two viruses, are either not reliable due to the weak immunogenic properties of the viruses, and/or because of irregular virus distribution and the possible absence of viral coat proteins in infected potato plants, particularly in tubers.
However, Xu says that protocols of reverse transcription-polymerase chain reaction and restriction fragment length polymorphism analysis have been developed at the CFIA in Charlottetown for the detection and identification of PMTV and TRV in various potato tissues, and these protocols have recently been successfully employed for the diagnosis of these viruses.