DISEASES OF CULTIVATED PLANTS 521 



Buckskin. On fruits affected with buckskin the rind usually 

 presents a somewhat scurfy, grayish, slightly roughened, appear- 

 ance. This most frequently covers the greater part of the surface 

 of the fruits and quite often the entire surface. The rind often be- 

 comes abnormally thick, and is pliable on maturity. The fruit is 

 usually stunted in growth ; or if of normal size, becomes light with a 

 small amount of juice. Buckskin is usually not so rough, hard, or 

 rusty as melanose or rust-mite russeting. It is not in definite tracts 

 and irregular spots and lines, as are thrips marks; and it is usually 

 much lighter in color than the tear-staining or withertip russeting. 

 Buckskin appears to be most common in grapefruits on the interior 

 of the tree and lower branches in shaded places. Buckskin is 

 thought to be due to the combined effect of mites and a surface- 

 growing fungus. It appears to have its start in an attack of mites 

 when the fruits are very small. A fungus (which has a Coniothe- 

 cium-like spore) follows the injuries in the epidermis made by the 

 mites, and appears to be responsible for the further development of 

 the buckskin. No recommendations based on experimental evidence 

 are at hand, but it would seem that lime-sulphur sprays applied as 

 soon as the mites make their appearance ought to remedy the 

 trouble. 



Thrips Marks and Silver Scurf. Markings on the fruit known 

 as thrips marks, also called silver scurf, are caused in most cases by 

 thrips, but may also be caused when the fruit is quite young by 

 other slight injuries, such as scratches or abrasions. The markings 

 are grayish to silvery irregular patches, over which is a delicate 

 scurf made up of minute pieces of the outer epidermis under which 

 a new set of cells has been formed. These markings, which become 

 quite noticeable at picking time, usually have their beginning when 

 the fruit is very small, just after the bloom has shed. The scurfed 

 patches are at first small, but as the fruit enlarges the areas become 

 larger also, and a new epidermis is formed under the scurf. Some- 

 times the markings are so large and deep that the fruits become mis- 

 shapen. Similar marks are caused by slight injuries when fruits 

 are quite young. Spots with similar scurf are sometimes caused by 

 a too strong spraying solution when the fruits are quite small and 

 tender. 



Those of the markings due to thrips may be prevented to a large 

 extent by spraying to kill the thrips. As a result of experience and 

 experiment in California (Bui. No. 99, Part 1, U. S. Dept. of Agr. 

 Bur. Ent.) lime-sulphur with the addition of blackleaf tobacco ex- 

 tract, or blackleaf, "40," was found to be an efficient spray for thrips. 

 The formulae are as follows : 



1. Lime-sulphur (33 Baume), 1 gallon to 75 gallons of 

 water. To this add blackleaf tobacco extract (2% per cent, nicotin) 

 at the rate of 1 gallon to 100 gallons of the spraying solution. 



2. Lime-sulphur, 1 gallon to 75 gallons of water. To this add 

 blackleaf "40" at the rate of 1 gallon to 1,800 gallons of the spray- 

 ing solution. 



