June, 1922 



SCIENTIFIC AGRICULTURE 



327 



Doidge more recently in South Africa. Mc- 

 Alpine named it "Black dot disease"^ an 

 excellent descriptive name. 



In the late summer and autumn of 1921 

 I discovered what appears to be the same 

 disease at Macdonald College in Quebec. 

 This is the first time its appearance is noted 

 on this continent and a full description will 

 be given later elsewhere. 



Symptoms 



The first symptom is a slight yellowing 

 of the foliage which begins at the tips of 

 leaflets and gradually involves the whole 

 leaves. It may or may not be accompanied 

 by dwarfing of the plant depending upon 

 the earliness and severity of the infection. 

 The yellowing will show early in the season 

 on plants heavily infected but usually it is 

 a midsummer symptom. The yellowing is 

 followed by browning and withering of the 

 leaves and if moist conditions prevail minute 

 black sclerotia will develop on the surface. 

 At this time the lower stem parts are covered 

 with sclerotia and gradually the stem dark- 

 ens and becomes brittle. On opening the 

 stem the pith is seen to be disorganized and 

 sclerotia line the inside walls of the vas- 

 cular cylinder. Figure 13 indicates the size 

 and number of the sclerotia both on the in- 

 side and outside of the stem. Roots and 

 rhizomes are attacked and the tubers are cov- 

 ered with the minute sclerotia. It apparently 

 does not cause serious damage to the tuber 

 since the mycelium is confined to the super- 

 ficial tissues and the sclerotia are developed 

 on the surface or are slightly erumpent. 



Mycelium is to be found throughout the 

 stem and leaf tissues of yellowed plants 

 which have sclerotia on the leaves. 



The org^anism 



Ducomet describes the sclerotia as astomic 

 pycnidia 75 — 150 microns in diameter with 

 setae 100 to 130 microns long and at least 

 biseptate. The spores are sliglitly curved 

 and 18-22 microns by 2.5 to 3 microns. In my 

 cultures the spores are slighth' smaller and 

 the setae longer and no pycnidia have been 

 found. McAlpine also states that he has 

 found no pycnidia in Australia. More detail- 

 ed discussion will appear elsewhere. 



Figure 14 illustrates the development of 

 sclerotia in a Petri dish culture on dextrose 

 agar. 



Other Hosts 

 Ducomet states that this organism is para- 

 sitic on Tomato and Physalis peruviana. I 

 have artificially infected tomato but have 

 not yet tried Physalis sp. 



Control 



This disease is serious only in that it 

 weakens plants and therefore reduces the 

 crop. Exactly to what extent it is of econ- 

 omic importance it is as yet impossible to 

 state. 



McAlpine advocates burning the' haulms of 

 diseased plants and selecting clean tubers 

 for planting. 



L. S. KLINCK 

 President of the University of British Colum- 

 bia and retiring President of the Canadian 

 Society of Technical Agriculturists,, who will 

 deliver the Presidential Address at the forth- 

 coming Convention of the Society at Macdonald 

 .. . College, on the evening of June 26th 



