FUNGI AND PLANT DISEASE 419 



would not be able to infect the red alder. The best control for 

 these parasites is to remove infected trees when the forest is being 

 logged. On shade or ornamental trees, the plants may be cut 

 or broken out of the trees. 



Plant Diseases Caused by the True Fungi 



Most of the true fungi, as we have seen in Chapter 4, are 

 grouped in three well defined classes; the Phycomycetes or algal 

 fungi, so called because of their similarity in body form and 

 reproductive habits, to some of the green algae; the Ascomycetes 

 or sac fungi, whose chief reproductive structure is a sac within 

 which the spores are produced; and the Basidiomycetes or club 

 fungi, which bear their spores upon a basidium. 



The Phycomycetes are the lowest group of true fungi, many of 

 their members still retaining an aquatic habit and living as 

 saprophytes or parasites upon water plants, fish, and insects. 

 Many others live as saprophytes or parasites in a terrestrial 

 habitat, and it is among these that some of man's most destructive 

 plant parasites are found. The main body, or mycelium, of the 

 Phycomycetes consists of a single cell, much branched to form 

 cylindrical filaments known as hyphae. Everyone is familiar with 

 the common bread mold (fig. 23) which has hyphae large 

 enough to be seen as tiny white "threads" with the naked eye. 

 These hyphae secrete enzymes which digest the food outside the 

 plant body, after which it is absorbed into the filaments. It seems 

 remarkable that these delicate hyphae can grow into seemingly 

 hard substances such as the skins of various fruits and vegetables, 

 woods, and even living green leaves of plants. Many of these 

 hyphae become specialized, not only for absorbing food, but 

 for both sexual and asexual reproduction. After the bread mold 

 has grown for a few days, countless black dots may be observed 

 at the tips of the hyphae. These are spore cases bearing numerous 

 asexual spores which, if alighting on food materials, may grow 

 into a new mold plant. If two diff'erent sexes or strains of the 

 mold are allowed to grow into contact with each other, special- 

 ized reproductive cells are formed by each. These fuse and 

 develop into a heavy walled resting spore which eventually 

 germinates to produce not a new mold plant directly, but an 



