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frequently attacked. This variety, probably a form of Citrus nohtlis, 

 comes from Japan. On this host scab rarely causes serious damage to 

 the crop and is seldom seen on the foliage. In all probability the 

 disease was introduced from Japan into the United States on this 

 orange. In several localities in Florida the first appearance of scab on 

 the sour orange and lemon was shortly after the introduction of 

 Satsuma orange trees. The fact of its being usually inconspicuous on 

 this host explains why it was not noticed first on this species. 



Scab, or a disease closely allied to it, occurs on very young fruit of 

 the lime (Citrus limetta), causing the fruits to fall while still very 

 small. It has not been found on the older fruits or leaves of this species. 

 It also occurs on the Otaheite orange, a variety of unknown origin, but 

 apparently having some lemon ancestry. All the common sweet 

 oranges (Citrns Aurantimn), the Kumquat [C. japoniea), tangerine, 

 mandarin, and king oranges ((7. nohilis), grape fruit and shaddock 

 [C. deciimana) seem to be nearly if not quite exempt. Certainly no 

 appreciable damage is ever caused to these sorts by scab. In Florida, 

 however, it is said to occur occasionally on the tangerine and mandarin 

 or inges. In Japan it appears to affect the ordinar}^ cultivated oranges, 

 which are largely varieties of Citrus nohilis. In Australia the disease 

 is reported on orange leaves, but no information has been furnished as 

 to whether or not any other sorts are exempt. 



Cause. — Scab is caused by a minute parasitic fungus (Cladispiroum 

 sp.). So far as known this fungus is found only on the varieties of 

 citrus plants subject to the disease in question. It is a very minute 

 species and usually forms a delicate dusky coating, only a few hun- 

 dredths of an inch thick, on the surface of the wart. The colourless 

 mycelial threads of the parasite creep about in the superficial layers of 

 the warts and give rise to tufts of erect, brown, branched, and multi- 

 cellular fruiting branches or hyphae. On these the dusky, spindle- 

 shaped spores are produced. The spores are borne in abundance near 

 the ends of the hyphae, which often show numerous scars where spores 

 have been detached. The spores are usually one-celled, but occasion- 

 ally are two, and very rarely even three-celled. They are very minute, 

 being only about ■^q)j(j inch long and -g^o-^- inch wide. The spores 

 germinate by pushing out a slender thread from the side. 



Conditions governing infection of the host plant hy the parasite. — It has 

 been found that this fungus is remarkably sensitive to weather condi- 

 tions and can fruit abundantly and spread rapidly only where the air 

 is moist. In Florida the disease often makes no perceptible progress 

 for weeks during the dry spring, but if a few days of damp weather 

 come on, it suddenly spreads and develops very rapidly. Groves in 

 low, moist situations are more subject to scab than those on high 

 and dry lands. The fact that the fungus depends so closely on moist 

 weather for its proper development and spread doubtless explains its 

 almost total absence from California, where the atmosphere is much 

 drier during the growing season than it is in Florida. 



Another important condition governing infection is that the tissue 

 must still be growing to be subject to the invasion of the parasite. The 

 period of greatest danger to both leaves and fruits is when they are 

 young and tender and growing rapidly. This explains why old lemon 



