EXPERIMENT STATION BULLETINS. 579 



ON CROWN AND ROOTS OF PLANTS. 



The chief loss to the celery crop from this disease comes from attacks on 

 the roots and crown of the celery plant. In the incipient stages of the dis- 

 ease, there is usually a bluish-green discoloration of diseased parts; which, 

 as invasion continues, gives rise to a black, scurfy surface. A bluish-green 

 border, more marked on the leaf stalks than on any other part, is usually 

 found around these blackened areas. The fungus may confine its ravages 

 to the outer part of the crown and produce a black ring of diseased tissue. 

 This kind of an attack, by killing the epidermal and neighboring cells, often 

 leads to large cracks in the crowai and bases of the leaf petiole, as the plant 

 continues to grow and expand from within. The course of the disease after 

 infection depends entirely on environmental conditions. The plant may 

 attain normal size and reach maturity, with only a black ring around the 

 crown to indicate the presence of the disease. In other cases, the plant may 

 rot off at the crown or the roots may be attacked and destroyed, leaving the 

 plant to fall over, or to maintain itself by means of new roots which may be 

 shot out from the diseased base. Plants of this latter class are usually stunted 

 and are easilj' pulled out of the soil. They exhibit a black, ragged, cone- 

 shaped butt which is invariably studded -with black fruiting bodies of the 

 pathogen. 



In some cases, when the plants are deeply set, the attack may be confined 

 to the leaf stalks in the region where they come in contact with the first 

 inch or two of surface soil. In such instances, a killing of the outer leaves 

 usually occurs. In most cases, the disease spreads to the crowTi and roots 

 where the work of destruction is continued. 



Root attacks are most severe in close proximity to the crown. There is 

 rarely a general attack on the entire root system. Typically, infection takes 

 place at some point near the base of the leaves, and spreads down and arouad 

 the basal portion of the plant, invohdng the roots as it progresses. Those 

 roots which are near the surface are more subject to attacks along their 

 entire length than are those which extend down deep in to the soil. The 

 disease on the roots is characterized by a broAvnish discoloration. The 

 black surface, so tj'pical of the disease on other parts of the plant, is also 

 noted on the larger roots; small roots usually disintegrate before this color 

 appears. 



Etiology. 



MORPHOLOGY OF THE CASUAL ORGANISM. 



Mycelium 



Within the tissue of the host plant, the mycelium is composed of septate 

 threads, sparingly branched, and comparatively small; averaging between 

 3-4 microns in diameter. In young hyphae, cross walls are rare and the 

 threads are very uniform throughout. With age, the hyphae sometimes 

 increase in diameter, and the cells may bulge slightly as the cell walls thicken, 

 and as the septa become more numerous. Very young hyphae are hyaline 

 and the cell contents finely granular; older threads are vacuolate and often 

 densely granular within. 



In pure culture, the organism is subject to great variation in size of threads 

 in thickness and color of cell walls, and in structure of cell contents. New 

 hyphae vary from 2-4 microns in diameter. These become very closely 



