Spraing –Tobacco rattle virus (TRV) & Potato mop top virus (PMTV)

Symptoms   TRV causes stem mottle of potato foliage and necrotic arcs and rings in tubers. Some cultivars develop corky, ring spot symptoms, and tuber progeny are distorted, stunted, and mottled. The stem mottle is frequently confined to only one or a few leaves on some stems. The virus is spread wide, but the disease is more commonly associated with light, sandy soils. PMTV- symptoms vary with the cultivar and with the environmental conditions, but visual diagnosis to identify if plants have been infected by PMTV or TRV is difficult. PMTV is more readily identified when V-shaped yellow markings can be seen on the leaves. The necrotic arcs and rings found in TRV infected tubers is also typical of the symptoms caused by PMTV.

Sources and factors favouring developing

TRV- Primary infection of tubers follows nematode feeding directly on the tubers. Trichodorus and Paratrichodorus nematodes are the vectors of TRV that cause Spraing.  Infected seed tubers may serve to introduce TRV into fields; however TRV is not efficiently transmitted in tubers and tends to be self eliminating over time. Infected weed species can also act as hosts. PMTV- Infection from this form of spraing is vectored by the powdery scab fungus (Spongospora subterranea). Once established in a field it has been known to persist for up to 18 years. PMTV is more commonly associated with heavy, wet soils, (unlike TRV which is transmitted through light sandy soils) similar to the conditions that favour Powdery scab.

Control          TRV- It is possible to reduce TRV damage by means of rotation and limiting host weed populations. It is important to avoid introducing the virus into fields; growers should use certified seed free from uninfected seed farms or regions. Care should also be taken to avoid moving soil from field to field, which may contain nematodes on machinery. Some potato cultivars are resistant to infection. Nematicides can also be useful for controlling the nematode vectors of the disease.

PMTV- PMTV is naturally self eliminating in seed lots. When planted in soil free from the powdery scab fungus (S. subterranea) the virus was eliminated from partially infected seed stock after 3 years. The distinct foliar markings caused by PMTV enable rouging out of infected plants, reducing the incidence and spread of the disease. Currently the option of using cultivars as a means of resistance to PMTV infection is unreliable, however sourcing certified seed, free from Powdery scab can help reduce the spread and the persistence of the virus.