362 REPORT OF THE COMMISSIONER OF AGRICULTURE. 



siona unnecessary alarm* On the contrary, there is always a percep- 

 tible brightening as the fruit attains its full color, and oranges slightly 

 affected, or affected very early in the season, when fully ripe show but 

 little trace of rust. 



Is East a Fungus, or an Exudation of Gum f — The term " rust" is very 

 indefinitely applied to a great variety of plant diseases, some of which 

 are clearly due to the presence of fungi, and others are considered path- 

 ological conditions of the plant, attributable to, for the most part, un- 

 known or conjectural conditions of soil or climate. 



A good example of the first class is found in the common and very 

 destructive rust of the fig. Any one who will take the trouble to ex- 

 amine with a good glass the brown discoloration upon the surface of 

 the leaves, may easily detect the sacks, or asci, of the fungus, filled to 

 bursting with the spores, or pouring them out upou the surface. 



Nothing of this kind is seen upon the leaves or rusted fruit of the 

 orange. A microscopic examination of the fruit-rind reveals no forms 

 of fungus, but shows the oil cells to be more or less completely emptied 

 of their contents, and the outer layers, the epithelial cells, clogged with 

 brownish resin, or entirely broken up and divided by fissures, whicli 

 permit evaporation of the fluids from the underlying cells. The rind of 

 rusted fruit, therefore, shrinks and toughens, and loses by evapora- 

 tion or oxidation the greater part of its essential oil. 



THE ORIGIN OF RUST. 



Reasons for considering it the Work of a Mite. — If we examine critically 

 with a hand lens of considerable magnifying power the surface of a 

 rusted orange, we will find here and there in the depressions, groups of 

 minute white filaments adhering closely to the rind. Carefully trans- 

 ferring one of these filaments to the stage of a compound microscope, 

 and applying a power of several hundred diameters, the character of 

 the object is clearly shown. It is the cast skin of an insect. 



If the examination chance to be made in winter, when the fruit is 

 ripe, the number of these exuviae will not be strikingly great. But if 

 made in autumn or late summer, the surface of every orange showing 

 rust will be found thickly sprinkled with them, and we shall be forced to 

 conclude that we have before us the relics of a numerous colony, which 

 at some former period infested the fruit. 



Extending the examination to fruit that as yet shows no indication 

 of rust, we will, if the season is not too far advanced, obtain abundant 

 confirmation of this conclusion, and find these colonies in the full tide 

 of their existence. The former occupants of the cast skins rJrove to be 

 elongate mites, of honey-yellow color, too minute to be seen as indi- 

 viduals with the unassisted eye, but visible in the aggregate as a fine 

 golden dust upon the surface of the fruit. 



The Mite on the Leaves. — Having tracked the mite by meaus of its 

 tell-tale exuviae, and detected it at work upon the fruit, if we turn our 

 attention to the leaves it needs no prolonged search to discover it here 

 also, and in even greater abundance. In fact, it is evidently upon the 

 leaves that the mites exist and propagate throughout the year; for not 

 only are they found upon fruiting trees, but upon plants of all ages, in 

 the nursery as well as in the grove. 



Nothing resembling the rust of the fruit follows their attacks upon 

 the leaves. Each puncture of the mites gives rise to a minute pimple 

 or elevation, until the surface of the leaf becomes finely corrugated, 

 loses its gloss, and assumes a corroded and dusty appearance. 



