Aphids, hornworms and viruses in tobacco

Over the last few weeks, most Ontario tobacco growers have been busy with topping, sucker control, irrigation and recently harvest has started mostly on farms that hand prime.  Aphids, hornworms and viruses are still being observed.

Aphid pressure is increasing on Ontario tobacco. It is important to remember that aphid populations can increase extremely rapidly when temperatures are high.  Large populations of aphids can stunt plant growth and cause wilting of tobacco foliage, and honeydew secreted by aphids can promote the growth of a black, sooty mold on leaf surfaces.  Additionally, aphids can vector various virus diseases.

Aphids are typically found in colonies on the lower surface of leaves.  Initially aphid populations may be spotty and irregularly distributed within a tobacco field, but can ultimately infest entire fields.  Monitor tobacco fields carefully, especially near field edges.  Aphids can be found throughout the plant, but often congregate first on young tissue near the top of the plant.

When scouting for aphids, check for hornworms, which are still present in tobacco fields that have not been treated.  Treatment for hornworms is justified when the population averages 5 or more hornworms per 100 plants, while the treatment threshold for aphids occurs when at least 20 percent of the plants in an area have 5 or more groups of aphids on each upper leaf.  When selecting an aphid or hornworm control, remember to consult the product’s re-entry and pre-harvest intervals, particularly when you are nearing harvest, as these vary between products. Refer to Publication 298, Flue-Cured Tobacco Production Recommendations and consult product labels for more information and full application instructions.

Another case of Tobacco Mosaic Virus (TMV) was observed over the last week, as were some isolated cases of the tobacco veinal necrosis strain of Potato Virus Y (PVYn).  Symptoms of this disease include yellowing, mottling, and puckering of leaf tissue accompanied by brown or black necrotic veins (Figure 1 and Figure 2).  Partial to complete death of individual leaves may occur (Figure 3).  In severe cases, entire plants wither and die.

Early signs of tobacco veinal necrosis

This disease is transmitted by aphids from infected solanaceous plants such as potatoes and ground cherry.  The virus can only survive in living plant tissue, overwintering mainly in potato tubers and possibly ground cherry.  It is not transmitted through tobacco seed.

There is little that can be done at this point in the season to control the disease.  Isolated infected plants can be culled out.  In fields with more significant infection, growers should consider disinfecting harvesting equipment before moving into other fields or, if possible, harvest severely infected fields last.

 

For future years, this disease can be managed by avoiding growing potatoes near, or in rotation with, tobacco where possible.  Control ground cherry in surrounding fencerows and other uncultivated areas, and destroy volunteer potato growth from previous plantings in cull piles.  Tobacco growers with potatoes in home gardens should use only foundation or certified seed.