BULLETIN OF THE BUSSEY INSTITUTION. 337 
deep that the tubers may have less chance for being infected from 
spores which have fallen on the surface. Practically, this does not 
work well, but potatoes planted near the surface do best. However, 
the plan tried by some cultivators in England, with apparently good 
result, of hoeing the earth up over a good part of the tops as soon as 
the rot appears, is worthy a trial. Cutting the tops on the appearance 
of the disease apparently does no good; and, if it appears in a violent 
form, there is nothing to be done but to dig the remaining sound 
tubers which, if the variety planted was an early one, are large enough 
to be of use at the time when the disease is likely to make its appear- 
ance. 
Although the present article is written with especial reference to the 
potato rot, a word on the lettuce mould may not be out of place. This 
disease does considerable harm in the region about Boston where large 
quantities of early lettuce are raised for the market. Although said by 
the farmers of Watertown to have troubled them for four-or five years, 
the fungus was first brought to my notice last August, growing on a 
plant of Lactuca altissima, cultivated in the Botanic Garden at Cam- 
bridge. A few weeks later, I received some diseased lettuce leaves from 
Mr. Locke, of Watertown, who wrote that the fungus caused him a great 
deal of trouble, particularly on the plants cultivated in hot-beds in the 
spring. The present month, April, I received more of the leaves from 
the same gentleman, with the statement that the disease was worse than 
ever. The appearance of the mycelium, as it breaks through the 
breathing-pores and bears asexual spores, has been shown in Fig. 3, p. 
328. The disease, of course, is most marked on hot-bed plants, since 
a constantly moist and warm temperature is kept up, and the pecuniary 
loss to the gardener is greatest as the early lettuce brings a higher 
price than that which grows later in the season in the open air, and 
which is less likely to be affected by the mould. The belief of some 
farmers that the disease is caused by watering with well-water, is, of 
course, entirely without foundation. The disease may be diminished 
by not watering the hot-beds too much, and by opening the frames fre- 
quently to admit the outside air. It is a good plan, once in a while, say 
once a fortnight if the weather permits, to keep the frame open towards 
night, so that the plants may be exposed to a temperature near the 
freezing point. Freezing, of course, injures or kills the plant, but a 
temperature as near freezing as possible without serious injury to the 
VOL. I. 43 
