DISEASES OF CULTIVATED PLANTS 337 
such as agar or gelatin. This method has been worked out in 
Bulletin 203 and may often be applicable. The illustrations above 
will show the results in these cases as before referred to. With 
internal tuber infesting diseases as 
in the bacterial wilt disease of the 
potato, the Fusarium wilt or dry- 
rot fungus of the potato and the 
soil-rot of the sweet potato, we must 
go further than mere external 
examination. For the two named 
wilt diseases of potato, infection 
usually shows earliest at the stem 
end. Thin slices across this stem 
end of the tubers will show wheth- 
er or not there is discoloration in 
the vessels. In the absence of infec- 
tion there will be no discoloration 
with bacterial infection by Bacillus 
solanacearum, black areas or rings 
will be seen in these tissues while 
tubers infected with Fusarium 
oxysporum will show local areas of 
browned or blackened tissues. 
This infection applies usually to 
“harvest time. As the infect.on 
advances, one-half the length of the 
tuber or even more may become 
infected. In all cases sections from 
sterilized tubers may be used asa 
source of cultures in Petri dishes. 
The same applies to soil rot of 
sweet potato. These diseases are 
not reached by seed treatment. 
THE LIMITS IN SEED TREATMENT — 
Fig. 17. Pea stem showing lesion from 
It will be apparent that serious _ blight fungus, Ascociyta pisi, near surface 
eee iN of ground. Thisfungus came from the seed 
limits hold in regard to seed treat- pea. (Natural size). From Bul. 173. 
ments. Where the spores are 
external and simply adhering to the seed grain, treatment will 
destroy these spores if rightly adapted to the seed in question and 
the germination need not be much, ifany impaired. On the other 
hand where the seed infection is internal rather than external, grave 
doubts arise as to the possibility of successful seed treatment. 
