152 Diseases of Greenhouse Crops 



Leaf Spot 



Caused by Septoria lactuccz Pass, and Septoria 

 consimilis E. and M. 



This disease is induced by two species of Septoria 

 fungi. The symptoms produced by both are so near- 

 ly alike that it is difficult to distinguish' one from • 

 the other, except by microscopic examination. Pale 

 brown discolored spots appear on the older leaves 

 with numerous black pycnidia in the center (fig. 27, 

 b-e.). The disease is of little economic importance, 

 as it usually occurs late in the season, on plants which 

 have nearly passed their usefulness. The Boston 

 variety is considered resistant, while the Salamander 

 and the Wonderful are more susceptible to leaf spot. 



Shot Hole 

 Caused by Marsonia perforans E. and E. 



The disease is of little economic importance. Af- 

 fected leaves are covered with dry spots which drop 

 out, leaving irregular perforations. Along the bor- 

 der of these holes, the causative fungus may be found 

 abundantly fruiting. The disease attacks the mid- 

 ribs of the leaves as well as the stem of the plants. 

 It seems to be more prevalent under conditions of 

 surface irrigation. 



With sub-irrigation, on the other hand, it is not 

 found to cause any damage. 



