Field Observations on Development of Potato Blight 199 
A centre of infection—about 3 yards across, was found in a 
large plot of Sharp’s Express in a low-lying situation in Penzance, 
shewing the same symptons as noted previously. At the margin of 
the infected area there were a few spots of blight on the upper 
leaves, but beyond these all plants were healthy. 
At Madron a group of Royal Jersey potatoes—about 3 yards 
across, was affected by blight in the midst of a large plot of this 
variety. The symptoms were the same as before, but in this case 
opportunity was afforded of lifting some of the plants, with the 
result that a few tubers were found already affected by blight, 
Phytophthora subsequently developing on slices of these diseased 
tubers kept in Petri dishes. In these tubers the disease had not 
entered via the stolons. They may have been infected by spores 
which had fallen from the aerial parts, or by the fungus present in 
some form in situ in the soil. One of the upper leaflets of a plant 
about fifty yards away from the centre of infection was blighted, 
but otherwise the remaining plants were healthy. At this date 
neighbouring fields of early varieties were quite unaffected by 
blight. 
’June 2. One patch of blight, about three yards across, was 
found in a field of Sharp’s Express at Gulval, the disease being 
chiefly present on the lower parts of the stems and the lower leaves* 
with typical blight spots on the latter. The disease could not be 
traced downwards to parts of the stems below soil level. 
A group of the second early variety—British Queen—about 
three yards in diameter was affected in the middle of about four 
rods of this variety at Penzance. The symptoms and distribution 
of the disease were the same as before. On the circumference of 
the affected group, blight spots were found only on the upper 
leaves but in the centre of the disease area, the lower parts of the 
stem, the petioles,and midribs of the lower leaves were chiefly affect¬ 
ed. Here again the disease could not be traced from above soil level 
to the parts of the stems below the soil. Some of the diseased 
plants were lifted and blighted tubers were found, but these were only 
exceptionally affected at the heel end, thus showing that in most cases 
at any rate the disease did not enter via the stolons. Phytophthora 
subsequently developed on these tubers after incubation. 
June 23. The area last mentioned was again visited but there 
had been only a slight extension of blight since June 2, the weather 
having been dry in the meantime, but on a neighbouring plot there 
were a few blight spots on the upper leaves and also on a main crop 
