26 DISEASES OF TROPICAL PLANTS OH 



areas of country. They are usually spoken of as spring 

 or summer spores. 



The sexual method of reproduction involves two parts 

 and thus gives the new plant a double parentage. The 

 spores are 



(a) Formed by the union of two similar parts which 

 unite and produce a zygospore (Fig. 18, Bread Mould). 



(b) Formed by the union of two dissimilar parts 

 producing an oospore (Fig. 19, Downy Mildew). 



(c) Formed by the union of two dissimilar parts 

 producing a sporocarp (Fig. 20, Powdery Mildew). 



The sexual spores, which are usually referred to in 



FIG. 18. Zygospore of 

 Rhizopus niffricans. 



FIG. 19. Oospore of 

 Albugo ca-ndiiiii. 



FIG. 20. A sporocarp of 

 powdery mildew. 



temperate climates as winter spores, are usually resting 

 spores and serve to carry the fungus over the winter in 

 cold climates, and over unfavourable seasons in warm 

 countries, and thus perpetuate its existence from year to 

 year. 



Distribution of Spores. Spores are frequently 

 carried for a great distance by wind currents. In this 

 way the spores of fungi that attack the crops may be 

 carried from field to field and from farm to farm. Also 

 in this way spores of fungi which attack forest trees 

 may be carried from tree to tree and from forest to 

 forest. Spores are also carried by the drainage water 

 for long distances. The water also affords a means 

 of distribution for swarm spores. 



