38 MISC. PUBLICATION 49 8, U. S. DEPT. OF AGRICULTURE 



short tufts in depressions or breaks in the skin. Decayed tissue is col- 

 lapsed and watery but is held together by the tough mycelial threads 

 of the fungus. These can be detected with the tongue when the tissue 

 is tasted. Affected fruits usually become cracked and exude an 

 amber-colored liquid. 



Rhizopus nigricans, the causal organism, also produces soft rots in 

 peaches, strawberries, and a number of other fruits and vegetables. 

 In avocado fruits it enters at skin breaks or at the stem scar if the 

 fruit has been pulled from the tree. The fungus develops most rapidly 

 at temperatures between 70° and 85° F. At 50° there is little danger 

 of new infections being initiated and at about 45° development ceases 

 altogether. Control measures consist of careful picking and handling 

 of the fruit to avoid injuries to the skin and of cooling of the ripe 

 fruit to 50° or lower. 



(See 67.) 



SCAB 



(Sphaceloma perseae Jenkins) 



OCCURRENCE. SYMPTOMS, AND EFFECTS 



Scab is one of the most important diseases of the fruit and foliage 

 of the avocado in Florida, Mexico, and the West Indies but has not 

 been found in California or in the Hawaiian Islands. It is appar- 

 ently able to attack most of the commercially grown varieties, the 

 Fuerte, Lulu, Trapp, and Taylor being especially susceptible. 



The disease is very common on foliage of young plants in the 

 nursery. Spots on the leaves are small, circular to irregular in out- 

 line, and purplish brown to almost black. On the twigs, leaf petioles. 

 and young shoots, the spots are oval and appear darker and more 

 elevated than those on the leaves. The disease on the fruit occurs as 

 oval, raised spots like those on the twigs. Frequently the spots 

 coalesce into a large russeted or corky tan area with irregular bound- 

 aries (pi. 19, A). As the fruit enlarges these areas crack into angular 

 divisions suggesting somewhat the pattern on a turtle's back. During 

 the first month of their development the lesions on the fruit are cov- 

 ered with a dense velvety dark-olive conidial growth. This covering 

 gradually weathers away until on a mature fruit only a narrow 

 border of the fungus is present in an occasional lesion. The presence 

 of the dark border of fungus on some of the spots and the irregularity 

 of the edges in the coalesced spots are helpful in distinguishing scab 

 from the corky areas produced by limb rubs. 



Severely affected fruits are usually undersized and misshapen, but 

 otherwise scab occurs only as a surface blemish of the fruit and rarely 

 is the means of entry for decay-producing organisms. It does not 

 develop or spread in transit and storage. 



CAUSAL FACTORS 



The causal fungus Sphaceloma perseae for many years was con- 

 sidered identical with the citrus scab organism, but eventually they 

 were recognized as distinct. The critical period for infection seems to 

 be from the time the bloom drops until the fruit is 6 or 8 weeks old. 



