THE FUNGI WHICH CAUSE PLANT DISEASE 



71 



A 



Fig. 42. — A, section showing sporangia or 

 sporocysts; B, zoospores, ciliated and 

 amoeboid. After Percival. 



In summer the resting spores which average about 52 >* in diam- 

 eter are found in abundance in the host cells near the surface, few 



in the outer layer, more below 

 down to the sixth or eighth 

 row of cells. Each resting 

 spore contains several hun- 

 dred roundish zoospores which 

 measure 2-2.5 ix. In spring 

 the resting spores germinate, 

 freeing numerous pear-shaped 

 uniciliate zoospores, which at 

 first swim with a jerky motion 

 but soon become amcBboid. 

 The summer sporangia may 

 germinate without protracted 

 rest, and also give rise to zoospores. Another type of sporangium 

 consists of thin sacs, produced singly or two to five in a sorus, 

 each bearing numerous zoospores somewhat smaller than those 

 from the first type of sporangia. 



The zoospores, says Percival, enter the potato apparently in 

 the amoeboid state in bud tissue of rhizomes and in the "eyes" of 

 young tubers. Usually only one zoospore enters each cell but 

 occasionally more may do so. Crushed sporangia produced 

 characteristic warts in three to four days when placed on suscep- 

 tible parts. Successful inoculations were also made by Salmon and 

 Crompton.'* The cytology has been studied by Percival." The 

 full grown tumors vary in size from that of a 

 pea to a hen's egg, and represent metamor- 

 phosed branch systems. 



S. vaccinii Thomas ^■''^^ is the cause of a 

 disease of the cranberry and related hosts. It 

 forms numerous, small, reddish galls in which, 

 deeply embedded, are the sori. 



S. papillatum Farl.^" occurs on Alfilaria in 

 California. 



Other species of Synchytrium are found upon 

 dandelion, (Enothera, Geranium, Amphicarpa, Ornithogalum, clo- 

 ver, elm, etc., but as yet are not of economic importance in America. 



FiQ. 43.— GaU 

 vaccinii. 

 Shear. 



