598 
Journal of Agricultural Research 
Vol. XIV, No. 13 
ordinary seedling parasites, wilting probably occurs only after the ab¬ 
sorbing portions of the root are invaded by the parasite. 
The limitation of the whitespot lesions to the stem just above the soil 
surface in seedling C and in most of those observed in the nurseries, indi¬ 
cates that the combined radiation from the heated soil and from the sun 
direct ordinarily results in a temperature in the stem at -that point higher 
than the temperature in the surface soil. In an experiment not described 
above, in which the source of heat was directly above the seedling, the 
lesion was, as might be expected, just below the soil surface rather than 
above it. The pot containing seedling E was also so placed that the 
sun's rays were more nearly perpendicular to the soil surface than in 
level seed beds, with the same result. It is probable that in at least 
some cases heat lesions will occur in the nurseries partly or entirely 
below the soil surface, as in seedlings D and E. It will be impossible, by 
any ordinary method of field observation, to distinguish from damping-off 
cases such as that of seedling E. Both the angle of the sun's rays, and 
the absorbing, conducting, and radiating capacity of the soil and of the 
stem will, of course, help determine whether the stem will be hottest 
above or just below the soil surface. 
Munch (8, 9) has described the same type of injury to tree seedlings 
in Germany, attributing it positively to heat at the soil surface. With a 
thermometer having a thin, flat bulb he obtained very high tempera¬ 
tures in the surface soil (jo). Others have also reported surface tempera¬ 
tures in unprotected soil from 55 0 to 68° C., or even higher (2, p. 55; 7, p. 
i$; 12; 17). Munch made an incubator test in which coniferous seed¬ 
lings survived for two to three hours at temperatures not exceeding 52°, 
but were killed by maxima of 54 0 to 55 0 C. This seems in general to 
agree with the temperatures reported as being fatal to most growing 
plants. 
Typical whitespot injury to seedlings has been found in several differ¬ 
ent States, though nowhere has it been observed to cause as heavy 
losses as at the Nebraska nursery, where it was first seen. Whitespot is 
not limited to conifers. In the vicinity of the Nebraska nursery an 
examination of fields of rye (Secale cereale) and cowpeas (Vigna sinensis) 
showed that both were affected in the seedling stage in much the same 
way as the pines. In the cowpeas the localization of the white, con¬ 
stricted lesions just above the soil surface was very marked. Plate 
cultures yielded no fungus suspected of parasitism. The rye seedlings 
were affected in the same way, though constriction was less in evidence 
than in the more fleshy plants. The relation between the disease and 
exposure to sun was very evident in the case of the rye. In the level 
portion of the field a moderate proportion, perhaps 5 per cent, of the 
shoots were affected. Where a dead furrow crossed the field from east 
to west this uniform distribution of disease was broken. On the wall of 
the dead furrow having a north exposure the disease was not noticeable. 
