TEAR-STAIN OF CITRUS FRUITS. 9 



HISTOLOGICAL EXAMINATION. 



To the unaided eye, rust-mite russet with its several patterns and 

 the so-called wither-tip russet or tear-stain intergrade imperceptibly. 

 Ordinarily the grower calls the streak tear-stain and the solid area 

 rust-mite russet. 



It was deemed important to make careful microscopic examina- 

 tions to determine whether distinctive features exist in the rind 

 tissue of the affected parts. Fruits were examined showing typical 

 patterns of rust-mite russet as well as those showing several degrees 

 of the so-called wither-tip tear-stain. Under the microscope there 

 appears to be no material difference between these types of injury. 

 The examination of the injured parts in both instances indicates 

 that the cuticle and epidermal cells appear to be punctured, and 

 beneath, depending upon the degree of russeting, one to three layers 

 of cells, together with their contents, are of a rusty brown color. 

 Quite frequently mycelial threads and spores of fungi are found 

 adhering to the affected parts. These fungi prove to be the types 

 commonly found on citrus, .such as species of Colletotrichum and 

 Cladosporium. 



The histological examinations have not revealed any feature that 

 would serve to distinguish between the rust-mite russet and the 

 so-called wither-tip tear-stain. 



The presence of punctures in the epidermal cells of the tear-stained 

 areas would strongly suggest the work of sucking parasites rather 

 than that of parasitic fungi. This suggestion is further substantiated 

 by the following observation: In July, 1919, by the aid of a hand 

 lens, numerous tear-stained immature grapefruits were examined 

 while still hanging on the tree in an unsprayed grove near Orlando, 

 Fla. The rust mites and their castings were more or less generally 

 distributed over the fruits, but were present in especially large 

 numbers over the tear-stained areas. This condition was very notice- 

 able early in July. By August 2 such a marked segregation of 

 mites in streaks was not particularly evident, and the mites them- 

 selves, as well as their castings, were nowhere present in very large 

 numbers, but the tear stains, presumably caused by rust mites, 

 were quite evident. 



INOCULATION EXPERIMENTS. 



An inoculation experiment was conducted the last week in June, 

 1919, on immature grapefruits which were about 2 inches in diameter 

 and so far as could be determined free from blemishes. Fifty fruits 

 were used in this experiment. The inoculum was derived in part 

 from dead sweet-orange twigs which had been held in moist chambers 

 and on which developed a copious growth of the wither-tip fungus 

 and in part from pure cultures of Colletotrichum gloeosporioides, 



