Tomatoes & cucurbits are not only affected by tobamo- and begomoviruses like ToBRFV, ToLCNDV and TYLCV, but by many other plant viruses as well. Our new blog series Plant virus snapshots shines a light on other key plant viruses commonly found in tomato & cucurbit crops. We start off with the tobamovirus Tomato mosaic virus (ToMV), one of the oldest described plant viruses.
What is the Tomato mosaic virus?
ToMV is a tobamovirus with worldwide distribution, which has been around since more than one hundred years. In the USA for example, it was first reported in 1909. It is one of the most common tomato viruses, but also affects many other fruits and vegetables such as peppers, eggplants, apples, pears and cherries. ToMV can even infect flowers. While closely related to and almost indistinguishable from another well-known tobamovirus, the Tobacco mosaic virus (TMV), ToMV is a distinct viral species. Not only do they differ genetically, but their hosts of choice are also different from another. For example, ToMV occurs more frequently on tomatoes and peppers than TMV, both in field and greenhouse crops. ToMV is present on every continent.
Virus structure
This plant virus is a positive-sense, single-stranded RNA virus with a rod-shaped structure. The ToMV genome encodes four different proteins: 180kDa/RNA dependent RNA polymerase, 130kDa/Methyltransferase/Helicase, 30kDa/Movement protein (MP) and 18kDa/Coat protein (CP). The 130KDa and 180KDa proteins are encoded by the genomic RNA and involved in viral replication. The MP and CP proteins are translated from the respective sub-genomic mRNAs and participate in the viral package and movement. As ToMV particles are very stable, they can remain infectious for many years after extraction.
Transmission
As a tobamovirus, ToMV spreads extremely easy via mechanical transmission. ToMV is seedborne and thus spreads mainly by seed, but can also be distributed through artificial grafting and human handling (e.g. workers’ contaminated hands, tools and clothing). Only a small number of seedlings need to be infected for ToMV to spread rapidly. ToMV can also be carried on the mouthparts of certain chewing insects or animals. However, there is no true insect vector for ToMV, and seeds are the main factor in its long-distance transmission. ToMV can survive for several years in soil or other substrates, especially in disinfected remains of leaves and roots. This plant virus also uses perennial (weedy) hosts as a reservoir.
Symptoms
The symptoms of ToMV are quite varied. ToMV may attack plants at any stage of growth and all plant parts can be infected. Different infected parts may show distinct symptoms. The intensity of these symptoms depend on factors such as the nature of the strains, cultivar, age of the infected plant, temperature, intensity of light, and nitrogen and boron soil content. ToMV causes the ToMV disease which includes a wide range of symptoms. These notably include abnormally shaped fruit, fruit lesions, reduced fruit size, distorted growing points, abnormal color (often yellowing), form, and patterns on leaves, distorted stems, and distortion and dwarfing of the plant as a whole.
Oftentimes, ToMV causes a general mottling or mosaic appearance on foliage. When a tomato plant is severely affected, leaves may look akin to ferns with raised dark green regions. Leaves may also become stunted. There can be dark brown patches on the stem and tendrils, and the discoloration is only limited to the epidermal tissue, but not deep into the stem. Symptoms may be inhibited during cool temperatures. Consequently, infected seedlings may not display symptoms until moved to a warm environment. The below images show the varied symptoms caused by ToMV in tomato plants:
Virus management and control
There is no chemical control method for ToMV. However, a range of mitigation methods exist to manage and control ToMV in tomatoes. A key mitigation method, as for other tobamoviruses, is to reduce and eliminate the virus sources. It is crucial to remove infected plants immediately, as well as practice thorough sanitation, disinfection and decontamination protocols. Thermotherapy for 24 hours is also a proven method to eradicate ToMV, and causes less damage to the seed physiological quality. Additionally, it is crucial to use only certified virus-free seeds and plant antiviral varieties, i.e. tomatoes which contain the Tm-22 gene. To detect ToMV, classical molecular/biological methods (e.g. ELISA PCR testing) and the use of indicator plants have proven to be effective.
Current status
Thanks to the widespread availability of resistant tomato varieties, ToMV has lost much of its initial potential for devastation of tomato crops. Moreover, unlike ToBRFV for example, the Tm-22 gene bred into commercial tomato varieties is still very much effective against ToMV. However, recent experiments of introducing non-resistant crop varieties have shown that the very stable ToMV virus is still widely present in both fields and greenhouses worldwide.
