66 protectio:n of plants. 192,0-21 



PLANT DISEASES OF 1920-21 



B. T. Dickson, Professor of Botany, Macdonald College, P. Q. 



In giving this brief account of the commonest diseases of the year, I have 

 thought it best to deal with them on a crop basis where possible. 



Orchard. — During the season there was Uttle apple scab {Venturio inae- 

 qualis) but unfortunately an abundance of black rot {Physalos'pora cydoniae). 

 This latter appears to be in sequence with the excessive winter injury arising 

 from the severe winter of 1917-18. These two — winter injury and black rot — 

 constitute one of the important orchard problems in Quebec. 



There were sporadic cases of silver leaf (Stereiim purpureutn) . 



Field and Garden. — Pea Beans (P. vulgaris) were severely attacked by Ps. 

 -phaseoli causing blight, and both mosaic and anthracnose were present, the 

 latter only to a slight extent. Broad bean {Viciafaba) suffered to the extent of 

 50% with mosaic. 



Sweet peas likewise were affected by mosaic and distinct mottling was 

 observed in the flowers of the red purple varieties. 



Clovers generally developed mosaic to a considerable extent. 



Red Clover (T. pratense) in the experimental plots showed over fifty per 

 cent. In the field yellow sweet clover (M. officinalis) and white sweet clover 

 (M. alba) were severely affected. Black medick (M. Iupidi7ia) and Alsike clover 

 (T. hybridum) suffered from the same disease, but no percentage counts were 

 taken. Alfalfa {M. Saliva) exhibited a speckling similar to that of mosaic, 

 but it cannot yet be stated definitely that alfalfa also has its mosaic. 



Experimental work still in progress shows that aphids are albe to inoculate 

 and cross-inoculate T. pratense, T. hybridum and 3/. lupulina.. Indications are 

 favourable that the same aphids cause the above mentioned spotting on alfalfa. 

 I have not been able to cross-inoculate from the sweet clovers to other clovers 

 by aphids. 



Tomatoes, potatoes and tobacco all developed mosaic. In the tomatoes 

 there was 52% of infected plants and motthng of the fruit was quite common. 

 Streaking of light and dark green was observed in the case of tobacco plants 

 with mosaic. 



The potato seed was carefully selected, but in spite of this there was approx- 

 imately three per cent mosaic, five per cent true leafroll and eight per cent 



