42 
BULLETIN 879, U. S. DEPARTMENT OE AGRICULTURE. 
INFECTION BY WALKING ON PLANTS. 
In many cases the vines grow so thickly that a certain amount 
of injury from treading on the plants during picking is unavoidable, 
particularly where the rows are narrow. This probably has produced 
infection in many cases, one of which was noted by Mr. W. W. Gilbert 
in 1915 at Muscatine, Iowa. A party of several persons had crossed 
a field of seed cucumbers where the growth was rank and many 
mosaic plants were present. In so doing, vines were necessarily 
crushed under foot. A field of healthy plants in the neighborhood 
was visited soon after. The growth was heavy in this second field also, 
and further trampling of the plants occurred. Three weeks later 
the path taken through the second field was distinctly marked by 
mosaic infection, while the remainder of the field was still free from 
mosaic. This doubtless was an extreme case, but it is likely that 
some infection takes place in this way in many fields. 
INFECTION BY PICKING. 
In harvesting pickling cucumbers, they are picked every day or 
every other day, in order to secure small fruits. The fruits are 
broken off by hand in rapid succession and the juices, which soon 
cover the fingers, furnish an ideal and rapid means of mosaic trans- 
mission throughout the season. This has been demonstrated, as 
shown in Table XVI, by alternately picking fruits from diseased and 
healthy vines. The fruits were merely broken off with the thumb 
nail and no effort was made to produce any greater wounding or ad- 
herence of juices to the fingers than is common in field operations. 
Table XVI. — Experiments to prove mosaic infection of cucumber plants due to picking. 
Treatment. 
Number 
of fruits 
picked 
per 
plant. 
Number 
of plants 
inocu- 
lated. 
Results. 
Date. 
Number 
of mosaic 
plants. 
Date last 
observed. 
Aug. 5,1916 
DO 
Oct. 25,1916 
Do 
Nov. 24,1917 
Do 
Fruits picked from healthy plant after picking 
mosaic fruits. 
Fruits pickedf romhealthy plants only (control) . 
Fruits picked from healthy plant after picking 
mosaic fruits. 
Fruitspickedfromhealthyplantsonly (control). 
Fruits picked from healthy plant after picking 
mosaic fruits. 
Fruits pickedf romhealthy plants only (control) . 
2 
2 
2 
2 
1 
1 
4 
i 
5 
7 
7 
3 

3 

4 

Aug. 21,1916 
Do. 
Nov. 15,1917 
Do. 
Dec. 8,1917 
Do. 
WOUNDS NECESSARY FOR CONTACT INFECTION. 
In all the above work, the wound factor must be emphasized, 
since infection has never been known to occur where diseased and 
healthy vines were in undisturbed contact. Hundreds of cases have 
been observed where only a portion of the plants in a cage would 
