426 PLANTS AND MAN 



ings or by wounds. When the fungus is inside the fruit, it spreads 

 rapidly, causing a brown discoloration which increases with its 

 spread until the entire fruit is discolored. In the orchard some of 

 these rotted fruits hang on the trees, and gradually dry to produce 

 the so-called mummies. It is in these infected fruits that the brown 

 rot organism overwinters, renewing its growth in the early spring 

 to produce spore bearing structures which start the infection over 

 again. Many infected fruits likewise rot in storage or on the mar- 

 ket, causing shipping and marketing losses. If warm, wet weather 

 is prevalent during the ripening period, an entire crop of peaches 

 may be destroyed within a few days time. A second kind of spore 

 is produced during the growing season, and is spread by wind, 

 insects and rain. The ascospores which cause initial infections 

 from rotted fruits depend mostly upon wind for dispersal. Con- 

 trol therefore involves removal of such potential spore producing 

 tissues or infected fruit and twigs. Sprays applied during flowering 

 and fruit maturing periods will aid materially in preventing 

 infection by the brown rot fungus. 



The POW^DERY MILDEWS comprisc a group of Ascomycetes 

 most of which are external parasites living as masses of cobweb- 

 like hyphae on the surfaces of plant tissues, and sending food 

 absorbing organs — haustoria — into the plant tissues. The external 

 mycelium soon begins to produce thin walled, light colored spores 

 which give the whole leaf or twig surface a dusty or powdery 

 appearance, hence their name powdery mildews. They are mainly 

 parasites of leaves, but may grow upon stems, flowers, or fruits. 

 Ascospores are produced in addition to the thin walled spores 

 referred to above, though some of the fungi of this group will 

 produce asci only upon certain host plants or under certain 

 environmental conditions. The group is known to occur upon 

 many host plants, among them the apple, grape, strawberry, 

 clover, pea, cereals and grasses, cherry, lilac, pear, and quince. 

 It is especially destructive on the first two hosts mentioned. On 

 the apple, it causes a dwarfing and stunting of the young leaves, 

 twigs, flower clusters, and fruits. The grape also is aff'ected in this 

 way, and badly infected vines may become much dwarfed in size. 

 Powdery mildews overwinter in old leaves or in the soil, as well 

 as in the infected buds and stems or twigs. Control therefore 



