8 



INVESTIGATIONS OF THE CARNATION DISEASE BY ARTHUR 



AND BOLLEY. 



DESCRIPTION OF THE DISEASE. 



The trouble was reg-arcied by Arthur and BoUey ^ as one affecting 

 the carnation leaf mainly. They describe it as follows: 



It generally starts in the leaf when immature, and is best diagnosed in the 

 younger but full-sized leaves nearest the upper end of the stem. Taking such a leaf, 

 which on its surface presents no unusual appearance to the eye, and holding it 

 toward a strong light, small, pellucid dots may be detected scattered irregularly 

 through the leaf, sometimes having a faint yellowish color, which are the centers of 

 infection. The appearance of the dots has a close resemblance to those of the oil 

 glands in the leaves of the common St. John's wort {Hypericum perforatum) , a rather 

 abundant weed, or in the leaves of the false indigo {Amorpha fruticom) , a native 

 shrub, except that they have no regular disposition. Sometimes the surface of the 

 leaf is slightly raised over the dots, making watery pimples. 



After a time the surface of theleaf above the dots changes enough to indicate their 

 presence and finally shows a distinct spot. As the disease extends inside the leaf 

 the surface tissues dry, the internal tissues collapse, and whitish, sunken spots 

 appear. In some colored varieties of carnation the spots vary somewhat by being 

 more or less reddish or purplish. As the spots increase in size the leaves wither, 

 still clinging to the stem. Such spots never show distinct central darker-colored 

 specks and rarely any concentric circles, as do the spots made by parasitic fungi, 

 such as Septoria (spot disease) and Heterosporium (fairy ring) . 



Very badly diseased plants, especially when much crowded and growing in damp 

 atmosphere, have more yellowish green leaves than normal, of a more transparent 

 appearance, and usually smaller. The lower leaves of diseased plants in any atmos- 

 phere or soil die prematurely and the vitality of the plant is so lowered as to check 

 the growth and decrease the size and number of the flowers. 



In addition to the above description, these translucent dots may be 

 divided into three classes, representing three forms of the disease, 

 which differ slightly from each other in general appearance and behavior. 

 The first and most common form corresponds with the above description. 

 The translucent dots increase more or less rapidly, according to the age 

 of the leaf, until finally they reach a diameter of one to several milli- 

 meters, and often coalesce. The second form is characterized by the 

 appearance of very small spots, scarcely visible to the naked eye, but 

 easily detected when the leaf is held up to the light and examined with 

 an ordinary hand lens. These small spots are exceedingly numerous, 

 but in their further development, even on young leaves, scarcely ever 

 reach a diameter of more than half a millimeter. The third form 

 resembles the first in extent of development, but instead of being round, 

 the spots are elongated and irregular, and the affected plants are usually 

 more or less distorted. 



Dr. Arthur said that in his earlier study of the disease "no cause, 

 either parasitic or nonparasitic, could be found to account for the diffi- 

 culty. The eye, either unaided or by the assistance of the microscope, 



1 Arthur, J. 0., and Bolley, H. L., Purdue Univ. Exp. Sta. Bull. No. 59, 1896, p. 18. 

 All succeeding quotations from these authors are from this bulletin. 



