POTATOES
Awareness – potato spraing causing quarantine pests with confusing symptoms André Wessels POTATO CERTIFICATION SERVICE andre@potatocertfication.co.za
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PRAING IS THE TERM used for a range of internal and external symptoms occurring in the potato tuber flesh, consisting of brown, necrotic rings, arcs and flecks or spots (Photo 1) (Fera Spraing Disease of Potato, 2020). Two viruses which do not occur in South Africa can cause these symptoms, namely Tobacco Rattle Virus (TRV) and Potato Mop Top Virus (PMTV). Both these viruses are soil borne with similar symptoms but transmitted through different modes of transmission. These viruses are of economic importance due to the presence of necrotic symptoms which may lead to downgrading or rejection of a seed lot and be unmarketable, especially if intended for processing. Tobacco Rattle Virus (TRV) Tobacco Rattle Virus (TRV) is transmitted by free living nematodes, Trichodorus and Paratrichodorus spp., which are migratory root ectoparasites, also named stubby-root nematodes. When present in large populations, they may damage the potato, either directly by feeding on the root system thus causing necrosis or indirectly by transmission of TRV (Stent, 2011:115). In the presence of virus-carrying nematode populations, susceptible cultivars to TRV may show symptoms. Adults and juveniles can transmit the virus when feeding on root cells and the virus can be retained for many months by non-feeding nematodes. Nematodes spread with surface water and favour light sandy soils, although TRV and its vector have been found in very diverse types of soil. Symptoms on foliage Certain plants can be healthy when infected seed potatoes are planted but in certain varieties the virus infection may cause mottled and/or distorted leaves. Different forms of mosaic symptoms and mottling, turning into necrotic spots (Photo 2) may be observed on leaves on one or two stems per plant which grows less vigorously. Progeny from infected plants are smaller and misshapen, with yield being affected (Wale et al., 2011:86).
Symptoms on tubers In potatoes, TRV causes Corky Ringspot Disease (CRS), also known as Spraing. Symptom expression varies greatly and is affected by cultivar and environmental conditions (Wale et al., 2011:85). The most typical symptoms of TRV are usually found in the flesh of the tubers with brown necrotic arcs or corky spots sometimes extending over a large part of the tuber (Photo 3 and 4). Symptoms are not always visible on the
Photo 1: Spraing symptoms on TRV infected tubers. Source – Plant Management Network.
Photo 2: Potato leaf necrosis and chlorosis caused by TRV. Source Ephytia.
tuber surface, but superficial necrosis (Photo 5) can sometimes be found, which can form rings around the lenticels (Photo 6) which, when cut, are not always related to internal necrosis and they remain localised, more or less discontinuous, underneath the skin (Photo 7). Symptoms do not increase during storage. Potato Mop Top Virus (PMTV) Potato Mop Top Virus (PMTV) is only known to be transmitted by the fungus Spongospora subterranea, the causal agent of powdery scab. PMTV is carried in the resting spores that can TO PAGE 18
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JANUARY • FEBRUARY 2021 | VEGETABLES & FRUIT
