Harvest finally allows us to gauge the quality of the new, incoming crop. Any tuber abnormalities are critically inspected and may need additional diagnostics to better understand what may be causing the symptoms. On the watch list this year are three virus-induced internal spots, rings, or arcs and/or external lesions that can look similar to each other, but are caused by different viruses and vectored in vastly different ways. Often, these virus-induced tuber symptoms can look like physiological disorders such as heat necrosis or internal brown spot. It is important to know which virus is causing the tuber symptom, since management recommendations can be polar opposites.
The first virus-induced tuber disorder to be on the watch for is caused by potato mop-top virus (PMTV), which has more recently moved from previously confined geographical areas to be a more industry-wide concern. The virus is vectored or transmitted by the powdery scab fungus. Alone, powdery scab can be difficult to manage since it is persistent as resting spores for years in the soil. The presence of PMTV makes the task of managing powdery scab even more cumbersome. Tuber symptom development can be nil or minimal in cooler growing regions, although the virus may still be present. Typical symptoms are dark brown arcs in the flesh of the tuber. Raised ring spots on the skin may be observed. These are classic symptoms, but exceptions to the norm are often seen, especially in varying growing regions and cultivars. That is why PMTV can be confused with tuber symptoms of tobacco rattle virus (TRV).
Tobacco rattle virus symptoms are often referred to as corky ring spot and may give classic tuber symptoms of the virus showing internal and/or external necrotic rings or arcs, brown spots scattered within the flesh or malformed tubers. However, as with PMTV, the timing of infection and cultivar can influence symptom development. Tobacco rattle virus is vectored by the stubby-root nematode. Options for corky ringspot management include suppressing the nematode and weed populations as well as rotation into non-hosts such as alfalfa.
The third virus of concern is potato virus Y (PVY). The “old” PVY seen in the industry was often the PVY strain “O.” But due to recombination of various strains of the virus, the industry is now dealing with multiple strains of PVY such as N, N:O, N-Wi, and NTN. Certain PVY strains in susceptible cultivars can cause tuber symptoms resembling the two previously described viruses.
External arcs or rings in the tuber or internal brown flecking are typical symptoms of the potato tuber necrotic ringspot disease (PTNRD). The NTN strain is most likely to cause PTNRD, although it is dependent upon cultivar and timing of infection. Sometimes the N-Wi strain can cause tuber symptoms. PVY is transmitted by aphids.
Seed tubers can be infected with all three of these described viruses. As potatoes are being cut open for quality determinations this fall, be on the watch for internal and external symptoms caused by one or more of these viruses.
Don’t hesitate to send in samples to identify the cause of the symptoms in order to develop a management plan for the stored crop or even for next year’s potato crop.