July i, 1921 
Relation of Horse Nettle to Leaf spot of Tomato 
505 
Septoria pycnidia found on naturally infected horse nettle were prob¬ 
ably also those of S. lycopersici , since they were similar to them in size, 
form, and spore contents and developed on approximately the same per¬ 
centage of spots as the pycnidia on the artificially infected horse nettle 
plants. 
It is very likely that in the Middle Atlantic States most if not all the 
horse nettle leafspots resembling tomato leafspot are caused by Septoria 
lycospersici . The fact that 5 . lycopersici infects horse nettle easily 
and produces spots, pycnidia, and spores that can not be distinguished 
morphologically from those occurring on horse nettle in the field would 
tend to support this conclusion. Moreover, it would also seem to be 
strengthened by the failure of the other organisms isolated from these 
spots to reproduce the leafspot disease. 
The results of the inoculations with a pure culture of Septoria lyco¬ 
persici show that horse nettle is a very susceptible host for this parasite. 
It should therefore be kept out of prospective tomato fields to prevent 
its harboring the tomato leafspot fungus and thereby largely destroying 
the benefits that would otherwise be secured from rotation of crops. 
For the same reason it should be destroyed in fence rows, roadsides, and 
other waste places near tomato fields 
SUMMARY 
Horse nettle, a weed common in fields and waste places in the eastern 
half of the United States, usually bears in the Middle Atlantic States 
numerous spots on its leaves resembling leafspot of tomato. Inocula¬ 
tion of more than 500 horse nettle seedlings with a pure culture of the 
tomato leafspot fungus, Septoria lycopersici , caused infection of about 
90 per cent of the plants, while an equal number of controls remained 
free from the disease. The spots closely resembled those occurring 
naturally on tomato and horse nettle. 
Pycnidia developed rather freely on about 1 per cent of the spots on 
the inoculated horse nettle plants both in the greenhouse and outdoors. 
The pycnidia and spores were indistinguishable from those of Septoria 
lycopersici on tomato. 
A culture of Septoria reisolated from the artificially infected horse 
nettle plants produced on both horse nettle and tomato leaves spots, 
pycnidia, and spores typical for Septoria lycopersici. 
Pycnidia and spores of a Septoria identical in appearance with Septoria 
lycopersici were found on naturally infected horse nettle leaves late in 
the fall. 
A Phoma, an Altemaria, and two species of Cladosporium were found 
fruiting on horse nettle leafspots, but repeated efforts to reproduce 
the spots with pure cultures of these fungi ended in failure. It is therefore 
likely that in the Middle Atlantic States most, if not all, these spots 
resembling leafspot of tomato are due to Septoria lycopersici. 
