Virus diseases in plants are numerous, with more being discovered every day, and they take many forms but usually include loss of
color by suppression of chlorophyll development. Foliage is mottled green
and yellow, mosaic, or has yellow rings, ring spot, or a rather uniform
yellowing, yellows, or plants are stunted or variously distorted. The
viruses described here are a fragmentary sampling of a few important
types. Viruses are transmitted from plant to plant by insects, by rubbing,
by vegetative propagation, and occasionally in seeds. Control lies in
obtaining healthy plants, roguing (removing and destroying) diseased
plants, and killing insect vectors.
Common on many ornamentals besides aster and on
some vegetables but not legumes. Asters are stunted with a stiff,
yellow growth, shortened internodes, many secondary shoots and green-
ish flowers or none. Transmission is by leafhoppers, not through seed.
Control. Asters are grown commercially under cheesecloth to keep out
leafhoppers; in gardens DDT does a fairly good job of reducing leaf-
hoppers and disease. Pull up and burn affected plants as soon as noticed.
A rather new virus disease that has haunted flor-
ists since 1945 and has now moved into gardens. Transmission is
by grafts, and the symptoms take months to show up. Leaves are small,
pale green, flowers are smaller and earlier than normal, plants only half
as tall as healthy plants. Greenhouse operators have been cleaning up
their stock making sure they have healthy strains, but not much work
has yet been done to ensure disease-free hardy garden strains.
This virus causes mottling, curling, dwarfing, warty
fruits on cucumber, squash, and melon, a "blight" or yellowing of
spinach, "shoestring" leaflets on tomatoes, and various symptoms on lilies
and other ornamentals. Transmission is by several species of aphids. Resistant varieties of cucumber and spinach are listed in seed catalogues.
Rogue diseased plants as soon as noticed; keep down weed hosts.
A western disease of commercial importance on sugar beets
and damaging to many other vegetables and ornamentals. Leaves
are curled or rolled, mottled, sometimes thickened and otherwise deformed, often brittle, plants stunted (Figure 58D). The same virus causes
western yellow blight of tomatoes, seedlings turning yellow and dying.
Transmission is by the beet leafhopper. DDT may reduce some leaf-
hopper vectors, but there is little satisfactory control for ornamentals;
infected plants should be destroyed.
ELM PHLOEM NECROSIS.
This disease has killed thousands and thousands
of American elms in Ohio, Illinois, and other central states. Trees
wilt and die suddenly, inside 3 or 4 weeks, or decline gradually over a
year or two. The phloem, seen when the bark is scraped off, is colored
butterscotch yellow, and there is a distinct odor of wintergreen. The
roots die first, then the phloem in lower trunk, with bark loosening and
falling away. The virus is spread by leafhoppers.