38 



orange and lemon was shortly aftei the introduction of Satsuma 

 orange trees. The fact of its being usually inconspicuous on 

 this host explains why it was not noticed at 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 (Citrus Aurantium), the 

 Kumquat (C. Japonica), tangerine, mandarin, and king 

 oranges (C. nobilis), grape fruit and shaddock (C. decumana) 

 seem to be nearly if not quite exempt. Certainly no appreciable 

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

 ever, it is said to occur occasionally on the tangerine and 

 mandarin oranges. In Japan it appears to affect the ordinary 

 cultivated oranges, which are largely varieties of Citrus 

 nobilis. 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 (Cla- 

 dosporium 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 veiy minute species and usually forms a delicate dusky 

 coating, only a few hundredths 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 waits and give lise 

 to tufts of erect, brown, branched, and multicellular fruiting 

 branches or hyphae, which often show numerous scars where 

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

 but occasionally are two, and very rarely even three-celled. 

 They are very minute, being only about -g^oo i ncn long and 

 B^ftjTS inch wide. The spores germinate by pushing out a 

 slender thread from the side. 



CONDITIONS GOVERNING THE INFECTION OF THE HOST PLANT BY THE 



PARASITE. 



It has been found that this fungus is remarkably sensitive 

 to weather corditions and can fruit abundantly and spread 

 rapidly only where the air is moist. In Florida the disease 

 often makes no peiceptible progress for weeks during the dry 

 spring, but if a few days of damp weather come on, it suddenly 

 spreads and developes 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 ite proper development and spread, doubtless 



