1879.] 



AND HORTICULTURIST. 



365 



Natural History and Science, 



COMMUNICA TIONS. 



BLACK RUST ON VERBENAS. 



BY PROF. T. J. BURRILL, CHAMPAIGN, ILLS. 



Read before the Illinois Horticultural Society. 



This is in every way different from the mould 

 or mildew found on the leaves of the same plant 

 and which was the subject of a communication to 

 this society some years ago. Yet different in 

 origin and appearance as the two diseases are, 

 they have been frequentl}' confounded and some- 

 times by those who could and should have 

 known better. The mildew gives the leaves the 

 appearance of having been dusted with a 

 white mealy powder, which in fact is very near- 

 ly tlie truth. The white material is composed 

 of the threads and conoidial spores of a fungus 

 named long ago by Schweinitz, Erysiphe verbe- 

 nse, but which is most probably Erysiphe commu- 

 nis, (Schlecht), found on very many other plants. 

 The black rust is found on house-grown plants, 

 almost exclusively on the youngest portions of the 

 vegetation — the terminar buds and rudimental 

 leaves. These have a purplish tint and a stunt- 

 ed appearance ; growth nearly ceases ; there is 

 very little deformity, but simply a dwarfing. 

 It has been considered contagious and beyond 

 remedy, other than the complete destruction of 

 the plants in order to exterminate their enemy. 

 Hundreds of dollars' worth have thus been sac- 

 rificed more than once in our country. Dissap- 

 pointment has intruded upon many a lively an- 

 ticipation of the brilliant effect to be produced 

 when the verbenas had grown larger and become 

 full of flowers. 



Peter Henderson long since asserted the dis- 

 ease to be due to a mite which creeps over the 

 surfaces, and by puncturing and sucking, causes 

 the difficulty. He rudely figures the mite in his 

 book on Practical Floriculture. Since this was 

 published much discussion, pro and con., has 

 been indulged in on the subject. Charles Hen- 

 derson, son of the former, repeated the mite 

 theory in the Gardener's Monthly last year. 

 Others were skeptical, and to convince them, 

 Henderson, junior, sent specimens of the affected 

 leaves to several persons, including Drs. Farlow 



and Riley and the editor of the Gardener's 

 Monthly. However, it seems no one found 

 the microscopic mischief-makers. Perhaps 

 they instinctively eluded the intensely scientific 

 eyes turned towards them. Again the two dis- 

 eases of the plant were confounded and an- 

 swers returned accordingly. But Henderson is 

 correct, though his figure is not. The black 

 rust of the verbena is assuredly caused by a low- 

 ly organized member of the animal world, be- 

 longing to the order Acarina, family Acaridse 

 and according to some systematists' classification 

 to the sub-family Tyroghyphidae or cheese-mite 

 group. Its generic (perhaps) and its specific 

 names are yet unheralded, but will be announced 

 some time soon in an appropriate place and man- 

 ner. After careful examination I am led to be- 

 lieve that Henderson did not clearly identify the 

 particular species causing the disease in question, 

 but probably saw them as well as others, which 

 may or may not have been injurious. At any rate 

 this is the only explanation I can make of his 

 statements, as the figure alluded to is widely dif- 

 ferent from a true representation of the real ver- 

 bena pest. There is no other account of them, 

 as far as my information goes. The following is 

 from personal observations during the year 

 now closing : 



The verbena mite is too small to be seen with- 

 out a magnifier, being one-hundredth of an inch 

 long by one two-hundredth wide across the mid- 

 dle and widest part of the body ; it is pale-yel- 

 lowish white and sluggish in its motions ; the 

 legs and feet are unlike any described species 

 and differ among themselves. The first pair 

 are terminated by a single claw and double vesi- 

 cles, while the second and third pairs have 

 double claws and single vesicles, and the fourth 

 pair have neither claws nor vesicles, but taper 

 into a long bristle. They propagate their kind 

 by eggs laid on the leaves, and most probably 

 pass their whole life in this situation. They in- 

 fest abundantly, plants growing in the open 

 sround throughout the summer and in suchsitua- 

 tions work upon older full-sized leaves as well as 

 the tender extremities of the plant ; they rare- 

 ly, however, are met with on the upper sides of 



