MARKET DISEASES OF FRUITS AND VEGETABLES 27 



are occasionally found on the markets in stock shipped directly after 

 harvesting, but this disease is seldom found in sweetpotatoes shipped 

 from storage. Apparently the infected sweetpotatoes that reach stor- 

 age develop extensive decay within a few weeks and are discarded 

 before they are marketed (pi. 18, A). 



The causal organisms are common soil inhabitants tha/t thrive par- 

 ticularly well in wet soils that contain a generous proportion of organic 

 matter. Infection of plants and developing potatoes takes place 

 chiefly through the young rootlets. However, wounded sweetpotatoes 

 are readily attacked. Sweetpotatoes inoculated in wounds and held 

 at moderate temperature in high humidity have been found to develop 

 pronounced decay within 36 to 48 hours and to rot completely in 

 3 to 7 days. 



The early stages of decay following natural infection of sweet- 

 potatoes in the field are characteristically small, sunken, grayish- 

 brown spots and areas about the point of attachment of the sec- 

 ondary roots. Sometimes the causal fungus penetrates deeply within 

 the sweetpotato without much surface discoloration or decay being 

 evident. If the temperature is comparatively low a soft, gray, cheesy 

 type of decay is produced, but under relatively high temperature 

 conditions mottled and marblelike grayish-brown to chocolate-colored 

 areas and pockets are formed throughout the sweetpotato (pi. 18, B 

 and C) . A third type of decay, known as ring rot, is sometimes pro- 

 duced. In this case the grayish-brown surface decay spreads in a 

 band around the circumference of the sweetpotato (pi. 18, D) . Under 

 the usual curing and storage conditions these bands or rings of dis- 

 eased tissue soon become dry and sunken. This dry, sunken ring 

 rot stage is somewhat like a type of ring rot caused by Rhizopus 

 (p. 28). 



The most common causal organism {Pythium ultimum) grows well 

 at moderately low temperatures. In experimental work it has been 

 found that the minimum temperature at which infection will take 

 place is near 36° F., the optimum between 54° and 60°, and the maxi- 

 mum about 95°. This temperature relation indicates that under the 

 usual warm temperatures prevailing in the South, mottle necrosis 

 should not ordinarily prove serious. 



The use of new soil each year in the hotbed and the practice of 

 crop rotation have been recommended for controlling this disease. 

 Infection at harvesttime may be avoided to some degree if it is found 

 possible to dig the sweetpotatoes during dry weather. 



Since the Yellow Jersey, Big-Stem Jersey, and Triumph varieties 

 usually are most severely affected, other varieties should be tried if 

 the soil is known to be heavily infested with Pythium. 



(See 20, 37,$, SI, 113.) 



Mucor Rot 



{Mucor racemosus Fres.) 



Mucor rot is a storage and transit rot that occurs in sweetpotatoes 

 only when they are exposed to high humidity at low temperatures. 

 Consequently it is of minor importance in good storage houses be- 

 cause the temperature is maintained too high for its development. 



