TOMATO VIRUS REMINDER.

The New Zealand greenhouse tomato industry has had few virus worries over the past 25 years since the general adoption of tomato varieties resistant to tomato mosaic virus (TMV).

The recent identification of potato spindle tuber viroid in a several greenhouse crops in both the North and South Islands is a strong reminder to us of the susceptibility of tomatoes to virus diseases and the difficulties and problems that arise when tomatoes are infected by virus. Precautions against virus introduction and spread in tomato crops that were commonplace 25 years ago are now largely overlooked or forgotten.

Present Virus Situation

Practically all currently used glasshouse tomato varieties are resistant to the common strains of tomato mosaic virus, but these varieties have no resistance to other viruses. Spotted wilt virus still quite commonly infects glasshouse tomatoes, and new cultivars with resistance to spotted wilt virus are likely to become available in the near future. Cucumber mosaic virus is also relatively common in greenhouse tomato, but with usually with a low incidence, only a few plants per 1,000 are likely to be infected. The once dreaded double virus streak, caused by infection by both tomato mosaic virus and potato virus X, has virtually disappeared because of the use of TMV resistant varieties.

However there are many other viruses that can infect tomatoes, and it is probably only a matter of time before some other virus becomes as much a problem as TMV was in the past.

Effects of TMV

TMV can infect tomato plants at any time, in the seed tray, in the nursery or after planting out in the greenhouse, but all too often the symptoms seemed to suddenly appear on a very large percentage of the plants in a crop just after picking had started, when the plants were heavily loaded with fruit. Growth of infected plants slows down, so that they are shorter than virus free neighbours. The tops develop the typical yellow mottle virus symptom, fruit set would fall off, the growth rate would fall and fruit swelling would slow down. Mature fruit quality would decline with more blotchy ripening , pitting and bronzing than usual. TMV never killed its host plants. There would be some recovery of the plants later in the season, but yields were generally considered to be reduced by as much as 25%. This scenario was typical of nearly every tomato crop in NZ prior to the introduction of mild strain inoculation and then TMV resistant varieties.

Spread of TMV

TMV is a sap transmissible virus. The virus spreads throughout the host tomato plant and virus particles can be found in cells and sap throughout the plant. Sap containing the virus is highly infectious. Handling tomato plants leaves sap on the hands, and if this sap was from an infected plant, then the virus will be transferred via the sap on the hands to the next plant handled. The virus is extremely tough and can survive in dried up sap, in plant debris or in the soil for several years. It can infect the outside of tomato seeds but only infects a very very small number of seed internally.

Alternate hosts and sources of infection

TMV has a wide host range including most other members of the family Solanacea, including potato, capsicum, eggplant, pepino, tobacco, nightshade, and many ornamental plants and weeds. These plants may be host to many different strains of tobacco mosaic virus ( tomato mosaic virus can be regarded as a specific strain of tobacco mosaic virus, though many authorites now recognise these two viruses as being distinct), and may be symptomless or show severe symptoms.

Controlling TMV

1) Use clean seed. Tomato seed should always be extracted by acid extraction as this results in minimal risk of virus transfer on or in the seed. There used to be a quarantine requirement that all tomato seed imported into NZ be acid extracted or treated with acidulated mercuric chloride.

2) Raise virus free seedlings. Hands should be washed before pricking out tomatoes, especially if other crops or tobacco has been handled. Adhering seed coats should not be pulled off the cotyledons, since any virus present is more likely to be on the seed coat than in the seedling.

3) crops. Good clean up and hygiene practices between crops are required, and include,

a) removal off all debris from old crops and sweeping to make floors clean
b) sterilising all surfaces likely to be contaminated with sap, these include overhead crop wires, crop support cradles, layering hooks or bobbins, old strings, door handles, picking and pruning trolleys and floor surfaces.
c) ensuring that is no root debris left in the greenhouse between crops
d) ensuring that there are no weeds persisting from one crop to the next.

4) Train all staff to recognise virus symptoms, and to stop and clean their hands after inadvertently handing any suspected virus plant

5) Make sure workers or visitors do not bring virus into a crop. Change clothing or provide overalls if workers or visitors have been in other crops and wash hands before handling the crop.

Inactivating TMV

TMV is unusual in that the coat protein of the virus reacts with proteins in skim milk, and hence skim milk can be used as an inactivator of the virus. Skim milk is quite safe for hand washing.

Control of other viruses

TMV is a typical sap transmitted virus and measures used for TMV control should be equally effective against other sap transmitted viruses. The potato spindle tuber viroid is sap transmitted and extremely contagious, it can also be seed borne and may be aphid spread in conjunction with potato leaf roll virus. Control will require all the measures used for TMV control plus control of aphids.

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