136 



Jor market. It also causes some injury to sour orange trees (before 

 Ihey are budded) when grown for nursery stock. 



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

 the lime ( Citrus litnetta), and in some localities is often so severe as to 

 entirely destroy the crop. For example, a grove in tropical Florida, 

 capable of yielding 500 boxes of fruit annually, when attacked by this 

 disease produced for several years only a few hundred fruits, and the 

 majority of these fell off while still very young. In Louisiana it is said 

 to have attacked Satsuma oranges ; in Japan it causes considerable 

 damage to orange groves situated on low, moist land ; in Australia it 

 is reported as causing orange and lemon trees to lose their leaves and 

 to yield poor crops, badly affected trees often not setting a single fruit. 

 Probably the annual loss from scab in the United States is not far from 

 $50,000, most of the damage being done to the lemon in Florida. 



Symptoms. — The leaves and fruit of trees affected with this disease 

 show small, wart-like excrescences. These excrescences are of various 

 sizes, the diameter ranging from ^ mm. to 1 cm. (one-fiftieth to two- 

 fifths of an inch), but usually being from 1 to 5 mm. They sometimes 

 run together and cover a large portion of the leaf or fruit. In case the 

 fruit is attacked while still very young the tissues below the wart grow 

 more rapidly than normally. This causes tha fruit to become covered 

 with bumps, of irregular pyramidal shape. These grow proportion- 

 ately with the fruit and on the mature fruit may sometimes be 1 to 

 2 cm. across and project out nearly the same distance. At first the 

 warts look like small semi-translucent pimples, of a slightly lighter 

 shade of green than the surrounding healthy tissue. In a few days, if 

 the weather be favourable, the warts become prominent, assume a con- 

 spicuously light green colour, and look watery. After this the}" become 

 covered with a delicate fungus, which is at first gray, then dusky, and 

 at last black. Finally the infected tissue covering the tips of the warts 

 is cut off from the healthy tissue below by a formation of cork, and ulti- 

 mately the cork formation becomes so abundant ss to give a dinghy white 

 colour to the old warts. The appearance and development of the warts 

 are much the same on the leaves as on the fruits. There is no forma- 

 tion of a lump below the wart. When the leaves are attack«d while 

 still very young much the same effect is produced as in the case of the 

 fruit, the leaf surface bulging abruptly outward and causing the warts 

 to appear seated on hollow, conical protuberances. The leaf is often 

 considerably thickened where the wart is situated, and the persistence 

 of the leaves for at least a year in most cases enables the cork forma- 

 tion to proceed further than is usually the case on the fruit. 



Varieties of trees attacked. — Scab attacks only certain species of cit- 

 rus fruits, the sour orange (Citrus higaradia) being particularly subject 

 to its ravages. It was first noticed in the United States on this host. 

 Both leaves and fruits of affected trees are often severely injured. The 

 greatest loss, however, is caused by its disfiguring the lemon. It at- 

 tacks the fruits far more frequently than the leaves, and by causing 

 the lemons to become bumpy and warty renders them valueless or 

 •nearly so. On the foliage it is never abundant enough to do serious 

 harm. After the sour orange and lemon the Satsuma orange is most 



