ORIGIN AND SPREAD OF THE DISEASE. 41 



The spread of the disease from plant to plant in the same house 

 seems to be accomplished mainly throut^h the soil. One rt'achcs this 

 conclusion from the fact that healthy calla plants jrrowino- in pots and 

 standing- near diseased callas are less likely to become infected than 

 when similar healthy plants are growing in a solid bed with diseased 

 corms. Furthermore, it is almost always the case that the disease, if 

 undisturbed, first attacks, the corm beneath or just at the surface of 

 the ground. 



Usually the first season that the disease appears onl}' a few of the 

 plants are actually destro3^ed, but the millions of organisms which are 

 produced during the process of decay remain in the soil, and some of 

 them reach corms that were perfectly healthy when planted. These 

 infections, as already indicated, often produce the hold-over cases, 

 which develop the following season. The organism may be carried 

 from plant to plant by stirring the soil after some of the corms have 

 become well rotted, or simply by walking about on the bed in cutting 

 the flowers. 



The nature of the soil apparently has much to do with the spread of 

 the disease in the bed. A soil that is rich in vegetable matter is a bet- 

 ter medium for the organism to grow and spread in than a soil that is 

 poor in such material. Furthermore, a soil filled with hunuis retains 

 the moisture better than one that is lacking in vegetable matter, a con- 

 dition that greatly aids the multiplication of the organism. It often 

 happens that the roots reach from corm to corm through the soil of 

 the solid bed. Usually the corms are placed about 12 inches apart 

 each way, and it is not uncommon for the plants to produce roots from 

 6 to 12 inches in length. Plate IX shows a small plant with a root 

 more than 6 inches long. The vvriter has frequently been able to fol- 

 low the progress of the disease through these roots from plant to plant. 

 The contents of a calla root afl'ected with this disease become soft, 

 while the epidermis remains intact. The diseased roots are also some- 

 what darker than the healthy ones, so that the}^ can be distinguished 

 readily by sight as well as by touch. These appear to be the princi- 

 pal methods by which this disease is spread from plant to plant in the 

 solid bed. 



The only insect that has been observed by the writer in connection 

 with the diseased plants is the so-called bulb-mite, but in no case has 

 this insect been found on an}^ part of a healthy plant and only on the 

 decayed part of the diseased plants. To determine whether or not 

 those insects were at all responsible for the spread of the disease a large 

 number of mites were placed in petri dishes containing pure cultures 

 of the calla organism. After the mites had come into contact with the 

 colonies of bacteria the}^ were transferred to healthy callas. Some 

 were placed on the corms, others on the leaves, and still others on the 

 flower stalks, but in no case did any of these plants develop the rot. 



