24 BULLETIN 903, U. S. DEPARTMENT OF AGRICULTURE. 



Their decay is expedited by a heavy rainfall and a high-water table. 

 Those formed during tlie spring and summer in a moist environment 

 rarely persist fresh beyond two months, and most of them decay 

 about one month after they arise. It has been repeatedly observed 

 how quickly a fresh tuberosity decays when it is placed against wet 

 sand, and if a stream of water finds its way down a root the tuberosi- 

 ties thereon start to decay immediately. On the other hand, they 

 are more capable of withstanding dry soil conditions than are the 

 nodosities, and under conditions approaching drought, which some- 

 times occur in late summer and autumn, may last for a considerable 

 time and even lignify, the dry environment having caused the insects 

 settled on them to seek more favorable conditions of moisture and 

 at the same time having kept in check decomposition. Tuberosities 

 withstand a considerably greater range in temperature than do 

 nodosities, and they are not affected by sudden changes in tempera- 

 ture in the same manner as are the nodosities. 



Tuberosities grow larger and more rapidly in proportion to the 

 soundness of the roots. On roots previously uninfested the growth 

 of the swellings is rapid and vigorous, and a root, after it has been 

 heavily phylloxerated for several months, becomes so greatly ex- 

 hausted that it can not respond to the punctures of the aphids by 

 developing new swellings, and the phylloxeras that are not gradually 

 driven away to seek more nutritious food develop on the root without 

 causing swellings. The decay of the tuberosities begins at the place 

 first punctured by the aphids, generally at about the center of the 

 swellings. The tuberosity forms around the insect, and decay is 

 first evident as a small, blackened spot, sometimes exuding a liquid. 

 The rapidity of decay of tuberosities is in proportion to the increasing 

 moisture content of their environment, and in an unusually dry 

 environment they frequently will lignify without causing the tissues 

 to rot. Under moist conditions the inflated cells rapidly break down 

 and decay usually spreads, and fungi and molds enter the tissues, es- 

 pecially in the case of large bulbous swellings. Decay finally drives 

 off the aphids, but through their stimulating action they are often 

 able to retain the freshness of a tuberosity for some time after it 

 has been surrounded by decayed tissues, and occasionally a fresh, 

 vigorous specimen is found on a root otherwise quite decayed. The 

 nutritious quality of these tuberous lesions provides for the produc- 

 tion of nymphs in great numbers. 



HOW ROOT LESIONS AFFECT THE HEALTH OF VINES. 



It has been shown in the foregoing pages that the nodosities are 

 those phylloxera lesions formed at the apex of growing fibrous root- 

 lets, whereas the tuberosities are lesions formed on all other parts 



