242 INTRODUCTION TO BOTANY 
Among the other alge-fungi which cause common destructive 
diseases are those which cause the cranberry gall; the brown 
rot of lemons and other citrous fruits; the white, or downy, 
mildew of shepherd’s-purse, the common radish, mustard, and 
turnip; and the downy mildew of cucumbers, pumpkins, 
watermelons, and lima beans. 
230. The sac fungi. Some of the more common sac fungi 
are the cup fungi, the morel, the yeasts, and the mildews; the 
latter often appear upon leaves of plantain, smartweed, lilac, 
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Fie. 188. Brown rot (Sclerotinia) growing upon old plums 
At the right are some of the fruiting cups; in the middle is a greatly magnified 
portion of the cup, showing the spore-bearing areas; and at the left is one of the 
spore-bearing threads still more magnified, so as to show the spores. After Duggar 
and oak. More fungi belong to this group than to any other, 
and since most of them are parasitic, it is evident that they 
are of great economic importance. They have wide variations 
in form and structure. Usually the parasitic sac fungi grow 
upon instead of within the host plant. From this superficial 
growth haustoria are sent into the tissues of the host plant. 
As illustrations of the damage that may be done by mem- 
bers of this group we may cite the brown rot of peaches, 
plums, cherries, and apricots. Old dried fruits are sometimes 
found lying on the ground or still clinging to the trees. These 
“mummies” are a result of the brown rot. Sometimes they 
produce little brownish cups which are the spore-producing 
