Causal agent(s) and transmission
TSWV is a virus transmitted by insects (thrips, including the western flower thrips Frankliniellia occidentalis, but also Thrips palmi, Thrips tabaci, etc.) in a persistent manner.
TSWV occurs worldwide because of its wide plant host range and the worldwide distribution of its vectors, different species of thrips.
More than 800 different plant species, from over 70 botanical families, including both monocotyledonous and dicotyledonous plants, have been reported to be susceptible to TSWV. The virus causes significant yield losses in a large number of economically important crops e.g. groundnut, lettuce, papaya, pea, potato, sweet pepper, tobacco, tomato and in many ornamental crops, including alstroemeria, begonia, chrysanthemum, cyclamen, dahlia, gerbera and impatiens. Many weeds can be a virus reservoir.
Outbreaks in potatoes are sporadic and related to the spread of vectors from other infected crops. In such cases, infected plants may have quite a low yield and produce necrotic or rotted tubers.
Symptoms on foliage
Symptoms of TSWV range from chlorosis, mottling, stunting and wilting to severe necrosis of leaf, stem and fruit tissues. However, symptoms may vary according to the condition and age of the plant, as well as environmental conditions.
Primary infections (i.e. occurring in the current year) may cause brown areas and necrotic spots, either round or in the form of concentric rings (photo 1). Necrotic lesions develop on petioles and stems and affected tissues dry up and then fall.
TSWV attack starts at the top of the plant, with symptoms ranging from discolouration or chlorosis of the top (photo 2). Stem necroses can also be observed.
When infected the previous year, the plants can suffer from leaf roll, leaf curl and necrotic spots (photo 3).
TSWV lesions sometimes have a necrotised appearance around a central green area.
Symptoms on tubers
Deformed tubers may be observed with splits, cracks and dark spots on the surface or under the skin. TSWV can also induce external or internal concentric brown necrotic arcs (photo 4), dark spots or internal necrosis and rot in the tuber flesh (photo 5).
Risk factors
As its vectors, the thrips, are more prevalent in warm climates, TSWV is widespread in tropical and subtropical climatic zones. However, in more temperate zones, it is often related to greenhouse crop production or warm areas.
The development of TSWV infection results from a combination of different sources of infection, the presence of susceptible crops in the vicinity, weak thrips’ management and poor weed control.
Controlling weeds within and around a susceptible crop can reduce the potential virus reservoirs and also the host plants for the survival and the spread of the vectors (thrips).
Control
This virus is classified as a quarantine organism in the European Union and is therefore subject to mandatory control measures such as inspection of tuber lots.
Preventive measures include:
- monitoring the immediate environment (avoid proximity to tomato or ornamental greenhouses);
- the use of certified seed potatoes resulting from rigorous certification schemes with field inspections and laboratory analysis.