118 HORTICULTURAL MANUAL. 



Western States and cow-peas in the South. In south 

 California the common field-pea makes a surprising 

 growth in late fall and early winter in orange orchards 

 and so far is a favorite cover-crop to supply the soil with 

 needed humus and nitrogen. 



127. Cover-crops and Blight. Beyond all doubt what 

 is known as fire-blight of the apple, pear, and quince is 

 caused by bacterial growth in the cell-structure of the 

 leaf, blo'ssom, and outer wood. Yet long-continued 

 observation has demonstrated that the color of the soil, 

 elevation, exposure, and soil-covering have much to do 

 with its occurrence and spread in a given climate. Other 

 things being equal, it is found that soil-shading during 

 the heated term by a cover-crop of buckwheat, vetch, 

 rape, cow-peas, or soy-beans, will show less blight with 

 given varieties than the orchard with bare soil during 

 July and August. 



It is also true in the prairie States that a given variety 

 of the apple, such as Yellow Transparent, will be free from 

 blight on light-colored ridge soil without shelter from 

 winds, while on a lower level with darker soil and sheltered 

 from winds it will be blackened with the disease. In the 

 same line it is also true that in cooler moister climates like 

 that of west Europe, from whence most of our fruits were 

 derived, fire-blight is not known. 



In these cases, and others now well known, we have 

 reason to believe that abnormal heat of soil and air have 

 much to do with the growth and development of this 

 bacterial disease. Professor M. Fremy, of France, is a 

 recognized scientist and a close observer. He says that 

 fire-blight of the apple and pear only occurs in France 

 when the trees are trained on south Avails where subjected 

 to abnormal heat. His opinion founded on chemical inves- 

 tigation was decided that ferment of the pectose preceded 



