357 



roots and are covered with many black protuberances or sclerotia. The 

 disease is caused l),v a soil fungus which penetrates tlie epidermis of the 

 root, attacking and lirealcing dnwn the tissue, wliidi is replai-ed by a 

 tangled compact mass of mycelial threads. The fungus is apparently able 

 to gain entrance into any part of the root, as some infections were found 

 which had started at the crown while others seemed to have originated 

 in the smaller roots. The outer tissue is first attacked, the mycelium 

 gi'adually turning black and giving the root its characteristic appearance. 

 At this stage the center of the root still retains its natural color, but in- 

 stead of being compact and brittle is ratlier soft an<l watery, while the 

 whole root is tough and pliable. Infected roots which have lain in the 

 soil two or three years gradually become black throughout and flimlly 

 decay. 



One of the ]iecnliar things about this fungus is that its period of 

 attack is during the winter. Healthy roots with well-formed buds, when 

 set in the fall in infected soil, fail to send up shoots the following spring, 

 and on examination are found diseased with Black Rot, the blackening 

 by this time usually extending one-fourth of the way to the center. After 

 the plants come up in the spring, with the return of warm weather, there 

 is no further spread of the disease until the next winter. In working 

 with the fungus in pure culture in the sunnner, an ice-box is necessary, 

 as it will not grow at tlie ordinary temperature. 



The organism causing this disease is a new species of fungus belonging 

 to the genus Sclerotinia. Tlie mycelium is septate, branching, and when 

 eld becomes more or less blackened. In pure culture it grows luxuriantly 

 on almost any medium if kept at a temperature of 40° Fahr. On nutrient 

 agar or potato agar, sclerotia are produced in three to six days. The 

 sclerotia are at first white compact masses of tangled mycelium, which 

 soon become black on the outside. They are for the purpose of producing 

 the perfect stage and carrying the fungus over unfavorable periods for 

 growth, being able to withstand submersion in boiling water for three 

 minutes without having their germinating power destroyed. Under favor- 

 able conditions of moisture and temperature, these sclerotia send out 

 germ tubes .iust as do spores. Under other conditions they may give rise 

 1o the perfect stage, although this has never been obtained in pure cul- 

 tures. However, last spring, (1910), the perfect stage^ was found in one 



1 Note,- — A technical description of this fungus is to bo published in an early 

 number of Phytopathology by Mr. W. II. Rankin. 



