82 MASS. EXPERIMENT STATION BULLETIN 175. 



Later experiments with the roots of other diseased plants gave similar 

 low results. 



It is a very easy matter to infect seedlings at the time of transplanting, 

 and the writer has repeatedly seen many cases in the field which could 

 only have been brought about by such infection. It is only necessary to 

 get some of the juice from the diseased plant on to the hands to transmit 

 the disease by handling healthy plants, the causal agent gaining entrance 

 through the broken ends of roots, leaf hairs or broken and abraded leaf 

 areas. In some of the experiments conducted relative to this point, a 

 very high percentage of infection has been obtained. In one case where 

 the juice from a diseased plant was very thoroughly rubbed on the hands, 

 and 40 healthy seedlings immediately set, no care being used to guard 

 against bruising the leaves, etc., 31 plants developed the disease in two 

 weeks' time. In another experiment where 62 seedlings were subjected to 

 the same treatment, 30 plants developed the disease; in still another, 

 series of 28 seedUngs, 21 developed the disease. Controls planted at the 

 same time, handled with a hand rubbed with the juice of a healthy leaf 

 developed the mosaic in only a few isolated cases. From the above it 

 can easily be seen that great care should be exercised in the matter of 

 handling the seedlings, especially diseased seedlings. 



Contagious Nature of the Disease. 



In spite of the fact that it is held by some investigators that the mosaic 

 disease is contagious, the wTiter has never been able to satisfactorily dem- 

 onstrate that it is. Under carefully controlled conditions in the green- 

 house, guarding against accidental infection, it has been impossible to 

 demonstrate the contagious nature of the disease. In isolated instances, 

 indeed, apparent contagion has occurred, but it is believed that these 

 cases were clue to accidental infection, as the percentage was so low, — 

 less than 2 per cent., — and under the conditions the plants were subjected 

 to, such as contact, spraying of the juice on leaves, etc., the percentage 

 should have been much higher if contagion was to be held responsible. 



It is a fact that it is only necessary to break or rupture the trichomes or 

 hairs on the leaf, subsequently spraying with diseased juice, to obtain 

 infection, although this method does not give a very high percentage. 

 It can easily be seen that such a rupture may be very easily brought 

 about, and hence apparent contagion occur. As is stated elsewhere in 

 this paper, insect and other carriers may also play a part in these so- 

 called cases of contagion. 



