Causal agent(s) and transmission
Tobacco rattle virus (TRV) is transmitted by nematodes of the genera Trichodorus and Paratrichodorus (Trichodoridae), which are polyphagous ectoparasites moving freely in the rhizosphere. There is some evidence of specificity between virus strain and vector species. 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. There is no evidence for the multiplication of the virus in the vector and it is probably not transmitted through nematode eggs.
Sandy and loamy light soils and high soil content favour the vector activity but TRV and its vector have been found in very diverse types of soil.
The nematodes can be readily detected by soil analysis although they may be patchily distributed within a field. The virus in its infectious stage can remain in the nematode for several months and in an infested plot for several years, or even several decades, in host plants.
TRV is also disseminated by (botanical) seeds of numerous weed species, grasses in particular. The roots of these plants serve as useful virus reservoirs for the nematodes, which transmit the virus to the potato tubers.
pHowever, necrosis-carrying tubers are generally regarded as poor virus sources for non- viruliferous nematodes.
Symptoms on foliage
The most typical symptom on the foliage consists of bright yellow or necrotic chevrons (photo 1), although it is rarely observed in field conditions.
Different forms of mosaic symptoms and mottling turning into necrotic spots may also be observed (photo 2), quite often on leaves with a single stem (hence the name “stem mottle”).
With some cultivars, notably Santé and José (Wilja), the infection may become totally systemic and associated with the presence of necrotic leaf spots, sometimes with a halo (photo 3), and sometimes yellowing of the entire plant.
Symptoms on tubers
The most typical symptoms of TRV are usually found inside the tubers, in the flesh (photos 4 and 5). These are quite marked brown necrotic arcs or more irregular wavy lines (photos 6 to 11), which can widen out to form spraing or corky spots sometimes extending over a large part of the tuber.
Superficial necroses can sometimes be found (photos 12 and 13). These can form rings centred on the lenticels. They are generally not typical enough to allow a definite diagnosis. When cut, the tuber shows that these lesions are not always related to internal necroses and that they may remain localised at the periderm.
TRV disease is recurrent in many potato producing countries and losses can be high in some years in some production areas. Internal tuber necroses, also known as spraing, are often due to TRV, but they may also be associated with Potato mop-top virus (PMTV) or following abiotic stress (physiological rust).
Risk factors
Sandy and loamy light soils, high soil content in organic matter and soil temperature around 15°C – 20°C favour the activity of the nematodes but TRV and its vector have been found in very diverse types of soil.
TRV may be retained in trichodorid nematods for many months but the virus may also survive in many weeds or plant seeds.
The potato is not a good host plant and the level of susceptibility of the cultivar is another important factor in the occurrence and severity of the spraing symptoms. However, although some potato cultivars are resistant to infection, others are merely tolerant, i.e. they may be systemically infected but produce few or no tuber symptoms.
TRV has one of the widest known host range of any plant virus. More than 400 species in more than 50 dicotyledonous and monocotyledonous families can be infected experimentally; in many cases the infection does not become systemic.
Host range includes many cultivated plants, including ornementals (bulbs such as tulip, narcissus, and hyacinth and also asters or hydrangeas), potato, beet, lettuce, spinach, artichoke and tobacco.
TRV can infect many plants from the Gramineae, for example Poa, and many weed species from various genera (Amaranthus, Capsella, Merculiaris, Polygonum, Solanum, Stellaria, Vicia,..).
Control
Soil cultivation and effective weed-control throughout the rotation are ways of reducing nematode populations in the soil and thus the risk of disease. Growing cereals for several years before the potato may help to reduce the virus but not the nematodes.
Knowledge of the history of the disease in a given area or farm can be useful for crop management and the choice of potato cultivars. The most susceptible cultivars should be planted in fields with no record of TRV whereas resistant or tolerant cultivars should be planted in infected fields.
Soil treatments used to deal with other types of nematode are effective in reducing the nematode populations and therefore the dissemination of the Tobacco rattle virus (TRV). However, they are expensive and harmful to the users and the environment.
Detection methods
Whilst there are numerous Elisa TRV detection kits, they are not reliable on potatoes because, in both tubers and foliage, TRV is often in the form of free nucleic acid and is thus undetectable with serology. A number of molecular methods have been described but none of them are fully reliable.
Diagnosis of the TRV involves (when it is feasible) several operations:
- soil testing with the identification of Trichodorus and Paratrichodorus nematodes;
- baiting the virus in the soil by means of baiting plants such as Physalis floridana or Nicotiana tabacum var. White Burley;
- molecular tests like PCR.
Importance économique
The economic significance of TRV is mainly related to the presence of necrotic symptoms on tubers which may lower the value and even lead to the downgrading or rejection of the affected potato lots.
TRV is a soil-borne disease and its transmission by seed tubers is not considered as important. Seed tubers with symptoms usually produce healthy plants (self-elimination), and, very rarely, infected plants producing symptom-free daughter tubers.
TRV may, however, be transmitted by seed tubers (from one generation to the next) in some cultivars in which the infection is symptom-free.