of Tobacco Mosaic Virus on Flue-Cured Tobacco
Tobacco mosaic virus (TMV) results in losses in North Carolina of about 1 to 2 percent of the crop by reducing the yield and quality of flue-cured tobacco. The ideal way to control mosaic is by the use of resistant varieties, and several new hybrids, which have acceptable yield and quality, are now available. Strict sanitation procedures are necessary to prevent the virus from becoming established in the crop and to prevent spread of the virus if efforts to keep it out of the crop are unsuccessful. Crop rotation helps to keep losses minimal in fields where mosaic has occurred.
Mosaic is so common that most tobacco growers know the symptoms of the disease in the field. The most characteristic symptom is a "mottled" appearance of the leaf (alternate areas of light and dark green tissue). The tissue may be rough, and will burn on hot, sunny days. We know several other viruses that cause symptoms on flue-cured tobacco that look like mosaic. Symptoms on seedlings are much milder and easily overlooked. Stunting and mild mottling may be observed. The first step in controlling mosaic, therefore, is to be sure that the virus causing the mosaic symptoms is TMV. Growers who have a mosaic problem should get assistance from their County Extension Center if there is any doubt as to the identity of the causal virus.
Properties of the Virus
Tobacco mosaic virus, like other viruses, is a very small chemical particle that can multiply only in a living host and only can be seen with an electron microscope. It differs from other viruses that infect tobacco in two ways that are important in its control. First, TMV is very resistant to destruction. It will survive for at least 50 years in dead, dried tissue while other viruses become inactive when their host plant dies. Second, TMV is primarily transmitted mechanically while insects primarily transmit the other viruses.
Tobacco mosaic virus is transmitted mechanically by any means that results in the virus coming in contact with injured cells of a host plant. The primary mechanism for this is contaminated worker's hands or equipment that comes into contact with a healthy plant. Contaminated hands can be freed of the virus by washing with a detergent. The virus can be inactivated on equipment by scrubbing it with a brush using detergent or by steaming. Although the virus is transmitted primarily on worker's hands and equipment, anything that mechanically moves the virus from a source to a healthy plant can transmit it. Chewing insects, such as flea beetles and grasshoppers, are capable of transmitting the virus, but such transmission is very rare in nature. Seed may be infested with the virus, but seed transmission has not been proven.
Source of the Virus
Tobacco mosaic virus overwinters in a number of ways, and these must be understood for successful control through sanitation practices. Infection of tobacco from overwintering sources of the virus is known as "primary infection."
A. Tobacco Products - All forms of tobacco may carry TMV so it is advisable that these products not be used by workers, especially around greenhouses and during transplanting. Spraying plants with milk prior to pulling and transplanting will reduce the number of plants that become infected. It should always be used during transplanting in situations where tobacco use by workers cannot be prevented. Transplants should be sprayed until runoff with one pound of dried milk in a gallon of water immediately prior to pulling.
B. Tobacco Trash - Tobacco mosaic virus will survive for years in dried tobacco tissue, so anything that may be contaminated with pieces of leaf, stem, or root tissue should be cleaned prior to use in the crop. This includes float trays that may have infected roots in the walls of the tray.
C. Soil-Borne Virus - tobacco mosaic virus overwinters in infected stalk and root. Experimental data in North Carolina indicates that infection occurs at transplanting when a plant is pushed against a piece of virus-infected tissue. The number of transplants that become infected in this way will depend on quantity of overwintering tissue surviving. The virus will overwinter in dead as well as living plant tissue, but dead tissue contains less active virus than living tissue. Numerous studies during the past 40 years in North Carolina has shown that from 0.1 to 5.5% of the plants planted to a field that contained mosaic the previous year will become infected with TMV.
The problem of TMV carry-over in stalks and roots can be reduced by crop rotation or by using a mosaic resistant variety. Crop rotation for mosaic control consists of planting a crop that is not susceptible to mosaic in alternate years. Crops planted in North Carolina that are susceptible to TMV are tobacco, tomato, pepper, and eggplant. Tobacco varieties carrying mosaic resistance are essentially non-hosts of the virus and can be used as a rotation crop. Growers who do not want to plant their entire crop to mosaic resistant varieties might at least consider planting them in fields where a mosaic problem occurred the previous year.
Growers who find rotation unfeasible and who do not want to plant any of their crop to a mosaic resistant variety can reduce virus carry-over by doing a thorough job of stalk and root destruction. This will significantly reduce, and may in some situations eliminate, infection during transplanting from infested old crop debris. Plants that do become infected should be removed prior to the first cultivation to prevent spread of the virus.
D. Other Crops - Tomato, pepper, and eggplant are hosts of the TMV and in addition to not being used in rotation should not be handled prior to working in tobacco. Fruit from these crops also contain active virus and should not be handled while working in tobacco. There is, for example, enough active TMV in one infected tomato to infect every tobacco plant grown in North Carolina.
E. Weeds - A number of weeds are known to be hosts of TMV, but horsenettle is the only one found thus far to be infected in North Carolina. This weed is common where flue-cured tobacco is grown and frequently is found infected with TMV. The virus can be transmitted from infected horsenettle to tobacco mechanically, and this weed is suspected to be the source of TMV found in many tobacco fields. The only way horsenettle can be eliminated as a source of TMV is to eradicate it. Growers should consult with their County Extension Center for eradication procedures.
Most of the mosaic plants in heavily diseased fields were infected by virus that was spread from a few tobacco plants infected from overwintering sources of the virus. Secondary spread can be reduced by removing primarily infected plants from the field or by cultivating in a manner to prevent spread of the virus. In seedling production, clipping is a very effective means of spreading TMV. Seedlings should be scouted closely for TMV at each clipping.
There are a number of factors that must be considered when deciding if roguing in the field is feasible, so the final decision must be based on each situation. As a general guideline, however, the two most important factors are the time that primary infection occurred and the number of infected plants.
Although primary infection can occur anytime during the growing season, the most critical time is during transplanting. Plants infected at this time from any of the overwintering sources will show mosaic symptoms in 2 to 4 weeks. Generally, if fewer than 100 plants per acre are showing symptoms at this time it is feasible to remove them. Roguing should, of course, be done prior to cultivation. Plants used to replant rogued plants frequently become infected from virus in the roots left in the soil when plants are removed so replanting is not recommended. If too many plants are infected to make roguing feasible, secondary spread can be reduced by carrying out cultivation operations so that equipment does not touch the plants. Where this is possible, cultivation should be done when the plants are dry and preferably partially wilted because less virus transmission occurs under these conditions.
It is seldom feasible or of value to rogue plants after layby because most secondary spread occurs during or before this operation. Losses can be reduced by keeping a good supply of water to the crop because mosaic burn, the most damaging form of the disease, usually occurs only when infected plants come under water stress. Irrigation of a field containing a significant amount of mosaic may be worthwhile under conditions of moisture stress that would not be of value to rogue after layby if fewer than 10-20 plants are showing symptoms by the time the crop is knee-high. This will reduce losses on the current year's crop somewhat, and perhaps more importantly, prevent extensive spread to the remainder of the crop and thus overwintering of virus in stalks and roots.
Because TMV infects few crops other than tobacco and because the virus survives poorly in the absence of living tissue, crop rotation is a highly effective part of disease management. Virtually any crop grown in North Carolina, except tomatoes and peppers, can be used in a rotation to help control mosaic. Growers who have chronic problems with this disease should consider this principle carefully.
varieties have historically been lower in yield and quality than
non-resistant varieties. Some of the new releases compare favorably with
non-resistant varieties, however. Even when resistant varieties may not
perform as well as resistant varieties on a given farm, there are two
situations where they would be of benefit. The first is on problem mosaic
farms where losses to mosaic exceed the differential income potential between
a mosaic resistant and a susceptible variety. The other situation is where
monoculture is practiced and a mosaic resistant variety can be grown on a
problem field to break the cycle of virus carry-over.
Recommendations of specific chemicals are based upon information on the manufacturer's label and performance in a limited number of trials. Because environmental conditions and methods of application by growers may vary widely, performance of the chemical will not always conform to the safety and pest control standards indicated by experimental data. All recommendations for pesticide use were legal at the time of publication, but the status of registration and use patterns are subject to change by actions of state and federal regulatory agencies. Last printed 04/91
Revised March 2001 by Plant Disease and Insect Clinic