a MISC. PUBLICATION 3 4 0, IT. S. DEPT. OF AGRICULTURE 



peaches; on strawberries, it frequently causes more loss than all other 

 decay organisms combined. 



On berries and grapes, Rhizopus causes a soft rot sometimes known 

 as leak, the name having originated from the fact that the fungus, as 

 it spreads through the fruit, breaks down the tissues and allows the 

 juice to escape. In late stages there is almost always an odor of fer- 

 mentation, but at no time does the fungus produce in berries and 

 grapes the marked browning that is one of the symptoms of the rot in 

 apples and peaches. 



CAUSAL FACTORS 



The cause of rhizopus rot of berries and grapes is here designated as 

 Rhizopus nigricans, although the possibility is recognized that more 

 than one variety or species of the genus is included under this name. 

 Under ordinary conditions of moisture and temperature the fungus is 

 characterized by a heavy growth of coarse white mycelium and small 

 spherical spore-bearing heads (sporangia) that are white and glisten- 

 ing when yoimg but which later become black and dull (pi. 10, A). 

 In cold storage or under refrigeration in transit only a scant growth of 

 mycelium is produced and the heads show as dense gray or black 

 masses close against the fruit. 



In warm wet weather, rhizopus rot is occasionally found on straw- 

 berries in the field; it sometimes occurs on mashed or overripe grapes, 

 strawberries, and other fruits in the field or aroimd the packing house. 

 But such cases give no warrant for calling rhizopus rot a field disease ; 

 they are too unusual, too closely connected with special conditions. 

 Most of the damage from this rot takes place in transit and storage 

 though even there the mere occurrence of such damage is no proof 

 that the fruit was generally diseased when it left the packing house. 

 There may have been a few slightly decayed fruits in the packages, 

 and a few so recently infected that they showed no discoloration or 

 marked softening; it is more probable, however, that the fruit bore on 

 its surface Rhizopus spores, which, because of conditions in storage or 

 in transit favorable for spore germination and for growth of the fungus, 

 started the rot at new places after the load was placed in the car or 

 the storage room. Favorable conditions, of course, hasten the growth 

 of the mold in fruits already attacked and its further spread to others 

 lying near or touching them. Wet weather in the field favors the 

 development of the disease in transit, by making the fruit more 

 sappy, more easily bruised, and more susceptible to attack by the 

 fungus. 



The development and spread of rhizopus rot is greatly favored by 

 water on the fruit, provided the temperature of the fruit is moderately 

 high (p. 23). Investigations in a number of States have shown, how- 

 ever, that there is a distinct advantage in picking strawberries in the 

 morning when they are cool even though they may be wet from rain 

 or dew. Such berries are 15° to 20° cooler than those picked a few 

 hours later, and when used in test shipments have always been found 

 to arrive on the market much freer from rot. 



The development and spread of rhizopus rot is also favored by skin 

 breaks resulting from careless handling prior to loading or in transit, 

 or from shifting and breakage due to improper stowing of the load. 

 There may be some danger of loss from rhizopus rot when injuries are 

 few, but it cannot be too often emphasized that the danger is much 



