Violet Diseases 359 



Spot Disease 



Caused by Alternaria viola Gal. and Dors. 



Spot disease is often known under the names of 

 leaf spot, blight, smallpox, and rust. The disease 

 constitutes a serious drawback to indoor violet cul- 

 ture. In 1900, the violet industry of Alexandria, 

 Va., had been practically abandoned because of leaf 

 spot. From five to eight years before the appear- 

 ance of the trouble the glass area devoted to violet 

 in that vicinity was estimated at from 50,000 to 

 75,000 square feet of glass. The term "violet dis- 

 ease" is the one generally applied by the average 

 grower. 



Symptoms. The disease attacks violets in prac- 

 tically every stage of their development. Even cut- 

 tings in the propagating bed are not immune from it. 

 Generally plants which make rapid growth, but 

 which are soft and succulent, are most subject to 

 leaf spot. On the leaves the spots are at first small, 

 but definite, usually circular, greenish, or whitish 

 resembling the sting of some insect. The spots soon 

 enlarge and the light central portion becomes sur- 

 roimded by a narrow ring of discolored tissue, which 

 is black or brown at first, but which fades as the spots 

 become older (fig. 75, a.). Young spots are water 

 soaked and semitransparent. In a few days, how- 

 ever, the spots become dry, bleached, yellowish gray 

 to white or pure white. Occasionally young spots 

 fail to develop, dry and fall out, leaving a shot-hole 

 appearance. Usually there are several spots on a 



