Turnip Mosaic Virus (TuMV) of Rutabaga
Table of Contents
Approximately 1,500 hectares of rutabaga are grown in Ontario, mainly in the southwestern counties of Huron and Middlesex. Production is based almost entirely on one cultivar, Laurentian, and a quality, purple-top cultivar resistant to turnip mosaic virus (TuMV) is currently not available.
The first indication of infection is often a premature yellowing and loss of older leaves of individual plants or patches of plants. In severe cases, the entire field will become infected and yellow all at once. Younger leaves produced after a plant becomes infected are stunted, wrinkled, and display a distinct mosaic pattern of yellow areas surrounded by normal green. Distinct symptoms on leaves of rutabaga develop approximately three weeks after plants become infected. The accelerated loss and replacement of leaves results in an elongated "goose necked" appearance of the roots. Infected rutabagas become stunted, and loss of leaves makes mechanical harvesting difficult. Roots of plants infected early in the season do not reach normal size.
Figure 1. Patch of TuMV in a rutabaga field showing premature yellowing of older leaves.
Figure 2. Rutabaga plant infected several weeks earlier with TuMV. Note the older yellow leaves and younger deformed leaves with mosaic pattern of yellow and green areas.
Figure 3. Virus-infected plant on the right has fewer and smaller leaves and a smaller root compared to the healthy plant on the left.
The virus overwinters only in living plant tissue. In Ontario, important
sources of infection early in the season include infected volunteer rutabagas
and infected rutabagas dumped from storage in the spring, winter rapeseed
fields and volunteer plants, and possibly a few species of wild plants.
Even small amounts of overwintering virus can be important, and severe
outbreaks of the disease often occur where volunteer rutabagas are growing
in fields of wheat, barley, or other crops in close proximity to rutabaga
fields. Strains of TuMV have been isolated in the spring from many wild
plants, such as dame's violet, yellow rocket, garlic mustard, and watercress,
but these strains generally do not infect rutabaga.
In the fall, aphids move from rutabaga fields to winter rapeseed, and fields of winter rapeseed planted within two or three kilometers of rutabagas often become highly infected (80 to 100%) with TuMV. Leaves of rapeseed infected with the virus display a distinct yellow and green mottling (mosaic); infected plants become stunted and yellow prematurely; and seed pods become shrunken and twisted. Infection with TuMV decreases yield of winter rapeseed and may reduce winter hardiness.
Figure 4. Volunteer winter rapeseed infected with turnip mosaic virus.
The virus is spread only by aphids. Water, soil, seed, machinery, and other insects do not spread the disease. Many species of aphids are able to spread (vector) the virus, and aphids that do not live on rutabaga are as important in the spread of the disease as aphids that colonize rutabaga plants. In Ontario, the corn leaf aphid and the green peach aphid have been shown to be the most important vectors of TuMV. Aphids require less than one minute of feeding to pick up the virus from infected plants, and an equally brief time to transfer the virus to a healthy plant. Because infection occurs so rapidly, insecticides are not effective for controlling the spread of TuMV. Large numbers of winged aphids move into the rutabaga crop daily, and several hours are required before they are killed by insecticides.
Figure 5. Green peach aphids on the undersurface of a rutabaga leaf. This common aphid is one of many species able to spread the disease. The virus is spread only by aphids.
A rapid increase in infection usually begins in early July when large numbers of winged aphids become active. Aphid flight continues from this time until the end of the growing season. Ideally, the main crop of rutabagas is planted during the final two weeks of June. Rutabagas planted at this time size during the cool fall weather, which improves the quality of the roots and prevents growth cracks and oversized roots. Losses from TuMV are greatly reduced, however, if the crop is sown no later than the third week of June and preferably prior to mid-June. Rutabagas planted after these dates are infected at a young age, and crop loss is often severe.
Listed below is a summary of recommended cultural control measures:
Figure 6. Healthy field of rutabaga sprayed weekly with a 1% oil solution to prevent infection with turnip mosaic virus.
Figure 7. Close-up of a leaf from a rutabaga infected with TuMV. Note the mosaic pattern of green areas surrounded by yellow and the puckering of the leaf.
Figure 8. Small misshapen (grose-necked) root of a 14 week old rutabaga plant infected in early July.
Follow label recommendations. Refer to the current issue of OMAFRA Publication 363, Vegetable Production Recommendations for recommended insecticides.
For more information:
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