24 



Letters on the Diseases of Plants. 



10. Induce as many as possible of your potato-growing neighbours to adopt 

 tbe above precautions. If tbey adopt them you will also be benefited. 



Caution. — There is a moth, Lita solanella, whose grub attacks potatoes, 

 and causes appearances which sometimes resemble scab. These deceptive 

 appearances, due to the attacks of the potato moth, have given rise to trouble 

 in the following manner, The grower, mistaking the moths' ravages for 

 scab, treats his seed-potatoes with corrosive sublimate, with no benefit, and 

 thereupon denounces the remedy. This is an unfortunate mistake, because 

 although the corrosive sublimate solution is almost harmless to the potato- 

 moth, it is very effective indeed against potato-scab. The ravages of the 

 moth are easily detected by cutting open a few potatoes. The passages 

 eaten by the grub will be seen under the skin, and even going right through 

 the potato from side to side. The grubs are also easily found. Scab does 

 not penetrate the potato as the grubs do. 



V. Diseases of the Orange. 



The Australian diseases of the Citrus fruits have never yet been properly 

 investigated, and the following notes on this subject are only such as 

 my recent letters contain. They may, however, serve as an introduction to a 

 more complete report which is in preparation. 



1. Melanose. (?) 



This disease is very well illustrated by the photographs I have taken at 

 various times from specimens sent me or gathered for the purpose during visits 

 to various orchards. (See figs. 28 and 29.) I have little doubt that this disease, 

 as it occur.s in Australia, is identical with that described bv Messrs. Webber 



/ 



^1 



-4 



Fig-. 24. — Healthy lireatliiug pore in the skin 

 of an orang-e as seen under the microscope. 

 The cells of the orange-skin are shown 

 containing bodies derived from the chloro- 

 phyll bodies, but now coloiired with an 

 orange pigment. The two large crescent- 

 shaped "guard-cells" are shown encircling 

 the elongated breathing pore. 



Pig. 25. — The beginning- of one of the numerous small 

 spots characteristic of the disease Melanose (?). 

 Each spot begins at a breathing i^ore. This illus- 

 tration should be compared with Fig. 24. It will 

 be seen that the interior cells of the skin of the 

 orange have begun to alter in colour. The guard- 

 cells have begun to disappear, and faint traces of 

 mycelium can be seen in the dark colouration near 

 the breathing pore. 



and Swingle as occurring in Florida. Still, my observations do not altogether 

 agree with theirs, and the two diseases may be difterent. Until a definite 

 difference can be pointed out, I prefer to adopt their melodious name. 



The differences I note between what I have seen and what the Florida 

 specialists have recorded in the only report of their w^ork that I have seen, are 

 briefly the following. The numerous small madder-brown spots, characteristic 

 of the disease, have a tendency on our varieties of orange to group themselves 

 in curved lines, somewhat like those charted archipelagoes, due to sunken 

 mountain chains, which one may observe at various places on a globe or 

 map of the world. This resemblance is so striking that before I had seen 

 the American report I had begun to call the disease the arcMpeligo disease, 



