712 FUNGI 



The protoplasm of the parasite, after reaching a certain stage 

 of growth, secretes a thick covering for itself and divides into a 

 large number of zoospores, which may escape during the summer 

 and carry on infection in other parts of the potato on which they 

 are grown. Similar sporocysts when produced late in the season 

 remain dormant during the winter. In the early part of the 

 growing season the protoplasm of the parasite often divides into 

 two to five portions, round each of which a thin wall is secreted 

 (Fig. 237A, 2). 



These portions become sporangia, inside which are developed 

 hundreds of small zoospores slightly smaller in size than those 

 present in the thick-walled sporocysts. 

 Such sporangia germinate during the 

 summer and the escaping zoospores 

 spread the disease. 



The disease is most prevalent on light sandy 

 soils, and is likely to become one of the worst 

 enemies of the potato grower unless stringent 

 measures are taken to stamp it out. 



No effective remedies are known, and when 

 'sy«Ji:yMum^^mfii-oace established in a field or garden it is diffi- 

 iwtzcum. a, motae;cult to deal with the pest. 



i, amoeboid. ,_„ *" . . 



I he sporocysts or the parasite m some form 

 may remain in uncropped soil for two or three years and be able 

 to infect potatoes planted there after that period has elapsed. 

 ^\'here the disease has occurred, the land should not be cropped 

 with potatoes for five or six years, and tubers coming from 

 infected districts should not be used for seed. 



5. The fungi belonging to the genera Plasmopara, Bremia and 

 Peronospora attack and destroy the tissues of plants in a similar 

 manner to Phytophthora infestans. The distinguishing morpho- 

 logical characters of the genera cannot be here discussed ; it may, 

 however, be noted that the hyphae bearing the asexual repro- 

 ductive organs are variously branched, and make their exit in 

 tufts through the stomata of their hosts ; they are produced in 

 such abundance that the affected parts appear covered with 

 patches of white 'mildew.' 



Button-shaped and branched haustoria are met with on the 

 intercellular mycelia of these fungi. The conidia and sporangia, 

 by means of which the fungi are rapidly reproduced, are oval or 



