12 MISC. PUBLICATION 541, U. S. DEPT. OF AGRICULTURE 



velopment of the irregular, bluish-black to black resting bodies or 

 sclerotia of the fungus. 



CAUSAL FACTORS 



The fungus Sclerotinia sclerotiorum is the chief pathogen, but at 

 times 8. minor is responsible for the decay. These fungi are per- 

 petuated from crop to crop as sclerotia either in the soil or in refuse 

 from previously diseased plants. When favorable conditions of tem- 

 perature and moisture prevail, the mycelium grows from these into 

 the soil and attacks plants of the current crop. The pathogen is also 

 spread by means of innumerable spores produced in little cup-shaped 

 fruiting bodies (apothecia) that push up to the surface of the soil 

 from buried sclerotia. Here the spores are released and are carried 

 by air currents to healthy plants. 



Growth of the mycelium, germination of the sclerotia, production 

 of the apothecia, and infection of the plants all occur only under 

 moist conditions. Production and discharge of spores are favored 

 by moderately low temperatures and prevented by those above 80° F. 

 Each species of Sclerotinia has a wide temperature range for growth 

 and decay production. Infection may occur at temperatures as low 

 as 32° to 34° and as high as 82°. A range of approximately 70° to 

 78° is most favorable. 



The presence of fresh wounds, although not essential for entrance 

 of the pathogen, favors early infection. When temperatures become 

 high during transit or packages are held unusually long on the market 

 or in storage spread of the fungus from head to head may occur. 



CONTROL MEASURES 



The first step in controlling market losses from watery soft rot is 

 to reduce the disease in the field. Where feasible, it is advisable to 

 destroy diseased plants and plant trash in which the fungus may con- 

 tinue to develop and produce sclerotia. Studies have indicated that 

 after -harvest pasturing of lettuce fields with livestock, particularly 

 sheep, may be helpful in this connection. It has been shown, how- 

 ever, that livestock fed on diseased refuse from packing houses may, 

 through evacuation of viable sclerotia, spread the fungus to uninf ested 

 fields. A quarantine period of 4 days is therefore recommended for 

 sheep that have been pastured in infested fields or fed on diseased 

 lettuce trash. 



The disease can be controlled in seedbeds and greenhouses by 

 disinfecting the soil with formaldehyde. 



The maintenance of low temperatures, although not completely 

 checking the disease, constitutes the most important measure for 

 controlling watery soft rot in transit and on the market. 



(See 6, 11, 12, 59, 68, 115, 132, 150.) 



RHUBARB 



Rhubarb (Rheum rkaponticwm L.), sometimes called pieplant, is 

 grown for its thick, fleshy leafstalks. The marketability of these 

 depends largely upon tenderness and crispness, which are a matter 

 of age, and upon freedom from blemishes and decay. There are a 



