AGRICULTURAL SCIENCE DIGEST
vol.9, Jan. - Feb., 1967	PURDUE UNIVERSITY

MAIZE DWARF MOSAIC DISEASE

Maize dwarf mosaic disease has caused
serious losses in a few cornfields in southern
Indiana since its discovery in 1963, and the
disease still constitutes a hazard to Indiana
farmers.

Past studies show that maize dwarf mo-
. saic disease is caused by a virus, attacking
not only corn but also several species of
weeds. The virus is transmitted either by
several species of aphids or by direct me-
chanical means. The virus is not seedborne.

Transmission of the virus to corn was re-
cently studied by Purdue scientists. It is trans -
mitted by aphids which become infective 10
to 20 seconds after feeding on diseased plants
and retain the virus for a period of 15 to 20
j minutes. It takes only about one minute for
the aphids to transmit the virus to a healthy
corn plant. This presents a control problem
since there is no phytotoxic (not toxic to
plants) insecticide with an effect rapid enough
to kill an aphid within one minute. If the kill-
ing effect of the insecticide occurs after that
time, the virus would already be transmitted.
The only advantage of spraying would be to
prevent the aphid from infecting another
healthy corn plant.

The scientists also investigated weeds
which were possible sources of the virus.
Of the 15 species of weeds used in the study,
Japanese chess was the most infective source,
followed by Witch grass and Johnsongrass.
Johnsongrass is particularly important be-
cause it is a perennial plant and can harbor
the virus in its roots and underground stems
during the winter and remain infective until

the next planting season. This explains re-
ports from different parts of the United States
associating the disease with Johnsongrass.

Controlling the insect vector may not be
feasible under practical conditions unless
there is a large aphid population. The proper
disposal of diseased corn plants and keeping
the cornfield weed-free seems to be the most
practical method of controlling the maize
dwarf mosaic disease at this time.

Journal Article 2596, J.B. Bancroft,
A.J. Ullstrup, M. Messieha, C.E. Bracker,
T. E. Snazelle, Botany and Plant Pathology
and Extension Circular 536, May, 1965.
* * *

CANADA THISTLE COSTS AND CONTROL

How much loss does Canada thistle cause?
How can this weed be effectively controlled?

These questions were recently answered
by Purdue researchers in a 4-year study con-
ducted to secure data on losses and gains in
alfalfa production and utilization due to direct
competition with Canada thistle and its con-
trol.

Their data show that production loss
caused by Canada thistle stands of 2 plants
per square foot was 7.4 tons of alfalfa while
consumption was reduced 4.5 tons per acre.
Canada thistle progressively competes with
alfalfa growth from the first to the last graz-
ing within any year and takes about four years
to effect the first grazing. Production loss is .
not only caused by the competition from the
weeds but also by the unavailability and un-
palatability of ungrazed forage as weed den-
sity increases and as the season progresses.

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