The most important naturally infected plant of virus Y is nightshade. It
is in this plant that the virus lives during the summer and it is from this plant
that the virus spreads into the crop during the fall and winter. Once the virus
has been established in the crop the importance of the nightshade host is
greatly lessened. Most of the spread after this time is from plant to plant in
the crop.

 The insect vectors of this disease are aphids. No other insects are known
to transmit potato virus Y and it appears very unlikely that any others do.
Certainly the spread of this virus in our area is very well correlated with
the numbers of winged aphids, particularly the green peach aphid, that are pre-
sent. It should be emphasized that so far as is known the virus is transmitted
only by aphids. It is not carried in the wind, in the soil, in the water or in
the seed. It is not spread by handling plants.

 We know a great deal about how the aphids transmit the virus. It takes
but a few seconds (10-15) of feeding for an aphid to acquire virus from a dis-
eased plant. Likewise it takes but a few seconds for the aphid to inoculate
virus into a healthy plant. Aphids that have fed for long periods of time
(hours) almost never are infective; thus the vector aphids are those that flit
from plant to plant. The reason that aphids lose their infectivity after several
hours of feeding is that the virus is concentrated in the outer part of the leaf
while in the center of the leaf where the aphid usually feeds there is but little
virus. Once an aphid has acquired virus it begins to lose it immediately. With-
in 5 minutes there is only half the chance of it causing an infection and within
10-mifate's .thereis only 'one*fourth-the probability of'it causing en infection
as compared to when the aphid first became infective. No aphid retains infec-
tive virus for more than one hour.

 How Far Can an Aphid Carry Virus?

 Practically all of the infections are caused by winged aphids. The
distance that a winged aphid can carry a virus has been the subject of much
investigation. We are certain that most infections come from flights of less
than 100 feet. Yet it is obvious that if an aphid can remain infective for an
hour and if it is caught in a 10 m.p.h. wind it can be blown for ten miles
before losing its infectivity. The points that have to be kept in mind are
these:

 1. The greater the distance between a diseased weed and a healthy plant
the less chance the aphid has of landing on the healthy plant.

 2. The greater the numbers of aphids the greater the chance of one of
them landing on a healthy plant.

 3. The greater the number of diseased weeds or crops the greater the
chance of an aphid picking up virus.

 4. Aphids normally fly short distances and spend several minutes on each
plant they land on.

 5. Aphids will not fly when the wind is over 5 m.p.h. and do most flying
during winds of 0-3 m.p.h.

 6. Aphids do not fly at night.

 7. Not every aphid acquires virs when it feeds on a diseased plant.

 8. Not every infective aphid transmits virus when it feed on a healthy
plant.